# AP 10th Class Physics Important Questions Chapter 6 Refraction of Light at Curved Surfaces

These AP 10th Class Physical Science Chapter Wise Important Questions 6th Lesson Refraction of Light at Curved Surfaces will help students prepare well for the exams.

## AP State Syllabus 10th Class Physics 6th Lesson Important Questions and Answers Refraction of Light at Curved Surfaces

### 10th Class Physics 6th Lesson Refraction of Light at Curved Surfaces 1 Mark Important Questions and Answers

Question 1.
Suppose you are inside the water in a swimming pool. Your friend is standing on the edge. Do you find your friend taller or shorter than his actual height? Why? (AP June 2018)
Friend is seemed to be taller. Because of refraction of light.

Question 2.
What happens to the image, if a convex lens is made up of two different transparent materials as shown in figure? (TS March 2016)

The convex lens is made up of two different materials. So the refractive i these two materials will be different. Hence two images will be formed.

Question 3.
Write the list of materials required for the experiment to find the focal length of a convex lens. (TS June 2017)
Convex Lens, Scale, Piece of paper, Sunrays.
(OR)
Convex Lens, V-Stand, Candle, Match box, Screen, Scale.

Question 4.
Complete the following ray diagram. (TS March 2019)

The parallel rays coming with some angle to principal axis meet on focal plane.

Question 5.
If the object is placed between the focal point and the optical centre of a convex lens, what will be the characteristics of the image formed? (AP SA-1:2019-20)
Object is placed between F2 and optic centre P :

Nature :
Virtual, erect and magnified.

Position :
Same side of the lens where object is placed.

Question 6.
For a concave lens, what type of image will be formed if the object is placed at the centre of curvature? (AP SA-1:2019-20)
Same size of object, inverted and real image will be formed.

Question 7.
Write lens formula. (AP SA-I:2019-20)
$$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$$

Question 8.
What is a lens? (or) Define lens.
A lens is formed when a transparent material is bounded by two spherical surfaces.

Question 9.
What is a double convex lens?
The lens having two spherical surfaces bulging outwards is called double convex lens.

Question 10.
What about the thickness of double convex lens?
It is thick at the middle as compared to edges.

Question 11.
What is a double concave lens?
The lens having two spherical surfaces curved inward is called a double concave lens.

Question 12.
Write about the thickness of concave lens.
It is thin at the middle and thicker at the edges.

Question 13.
What is centre of curvature?
The centre of sphere which contains the part of curved surface is called centre of curvature.

Question 14.
The distance between the centre of curvature and curved surface is called radius of curvature.

Question 15.
What is the mid point of lens called?
The mid point of lens is called pole (or) optical centre.

Question 16.
What is a focus?
The point where rays converge or the point from which rays seem emanate is called focal point (or) focus.

Question 17.
What is the distance between pole and focal point called?
Focal length.

Question 18.
What happens if the ray passes through principal axis?
It will be undeviated.

Question 19.
Where do light rays travelling to principal axis converge?
They converge at focus.

Question 20.
What happens to light rays passing through focus?
The path of the rays is parallel to principal axis after refraction.

Question 21.
What is a focal plane?
A plane which is perpendicular to principal axis at the focus is called focal plane.

Question 22.
What is lens formula?
$$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$$

Question 23.
On what factor does focal length of a lens depend?
It depends on refractive index of the medium, object distance and image distance.

Question 24.
What is lens maker formula?
$$\frac{1}{\mathrm{f}}=(\mathrm{n}-1)\left(\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right)$$

Question 25.
What happens to be image formed by a convex lens if its lower part is blackened?
Every part of a lens forms a complete image. If the lower part of the lens is blackened the complete image will be formed but its intensity will be decreased.

Question 26.
From which point of lens are all the distances are measured?
The optical centre of lens.

Question 27.
Is it possible for a lens to Act as a convergent lens in are medium and a divergent lens in another?
Yes. A convergent lens is placed in a higher refractive index of medium the nature of the lens changes i.e., it acts as divergent lens.

Question 28.
What are paraxial rays?
The rays which move very close to the principal axis which can be treated as parallel are called paraxial rays.

Question 29.
What is absolute refractive index?
It is the ratio of speed of light in air to speed of light in any medium.

Question 30.
Give mathematic expression for power lens and explain the terms in the formula.
Power (P) = $$\frac{1}{f}$$
where f is focal length of lens.

Question 31.
If the size of image is same as object through a convex lens, then where is the object placed?
The object is placed at centre of curvature.

Question 32.
How will you identify a concave lens by touching it?
A concave lens is thinner at centre and thicker at edges.

Question 33.
How will you identify a convex lens by touching it?
A convex lens is thicker at centre and thinner at edges.

Question 34.
Give the sign conventions for lenses with regard to the object and image distance.
The distance measured in the direction of incident ray is taken as positive.
The distance measured against the direction of incident ray is taken as negative.

Question 35.
Give the sign conventions for lenses with regard to the height of objects and images.
All the heights of objects and images above principal axis are positive and below the axis are negative.

Question 36.
When light of two colours A and B passes through a plane boundary, A is bent more than B. Which colour travels more slowly in the second medium?
Colour A travels slowly.

Question 37.
What type of lens behaviour will an air bubble inside water show?
It will act as a concave lens.

Question 38.
Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?
Yes. A lens is placed in a medium of a high refractive index than that of the lens then nature of lens changes (ML > Mg).

Question 39.
The image formed by a lens is always erect and diminished. What is the nature of lens?
Given that the lens is forming an image which is always erect and diminished. So it is virtual also. Such type of image is formed by concave lens.

Question 40.
If a student observed an image of same size with a convex lens of focal length 20 cm, then where should he keep the object in front of lens?
Because the student got image of same size the object should be placed at a distance of twice the focal length, i.e. 40 cm.

Question 41.
For an object placed at a distance of 20 cm in front of convex lens, the image formed is at a distance of 20 cm behind the lens. Find the focal length of lens.
The object distance and image distance are same. So the object is kept at twice the focal length. So the focal length of the convex lens is 10 cm.

Question 42.
A doctor suggested spectacles for a student which has negative focal length. Which type of lens is that?
Focal length negative indicates that it is a concave lens.

Question 43.
What happens to a light ray which passes through optical centre?
The light ray which passes through optical centre does not deviate.

Question 44.
When do you get image at infinity with a convex lens?
When the object is at the focal point.

Question 45.
When do you get a virtual image with a convex lens?
When the object is placed between focus and pole.

Question 46.
Is focal length of a lens zero? If not, why?
No, focal length of lens never equals to zero because it is the distance between focal point and optical centre.

Question 47.
A thin lens has a focal length of 12 cm. Is it a convex leps or a concave lens?
It is a convex lens, because f is positive.

Question 48.
Name the different apparatus where we are using the convex and concave lenses.
The magnifying lense, telescope, microscope.

Question 49.
Draw the given diagram in your answer book and complete it for the path of ray of light beyond the lens.

Question 50.
The diagram below shows two incident rays P and Q which emerge as parallel rays R and S respectively. The appropriate device used in the box is ……..

The rays are diverging and they produced the parallel from the device after refraction. So the device is concave lens.

Question 51.
The following figure shows the incident and refracted rays pass through a lens kept in the box. Draw the lens and complete the path of rays.

The incident rays 1 and 2 have converged after refraction. So the lens is convex.

### 10th Class Physics 6th Lesson Refraction of Light at Curved Surfaces 2 Marks Important Questions and Answers

Question 1.
A convex lens is made of five different materials as shown in the figure. How many images does it form? Why? (AP March 2017)

The given convex lens is made up of five different materials.
So they have different refractive indices / different focal lengths.
Hence they form five different images.

Question 2.
The focal length of a converging lens is 20cm. An object is 60cm from the lens. Where will be image be formed and write characteristics of the image. (AP March 2018)
Focal length = f = 20 cm (+ 20cm)
Object distance = u = 60 cm (- 60cm)
Image distance = v = ?

Here, (-) indicates inverted images.
m = $$\frac{1}{2}$$ < 1 indicates diminished image.
Image forms between F1 and 2F1.
Characteristics of the image :

1. real
2. inverted
3. diminished.

Question 3.
When a light rays enters a medium with refractive index n2 from a medium with refractive index n, at curved interface with radius of curvature R is given by
$$\frac{\mathbf{n}_{2}}{\mathbf{v}}-\frac{\mathbf{n}_{1}}{\mathbf{u}}=\frac{\mathbf{n}_{2}-\mathbf{n}_{1}}{\mathbf{R}}$$
Now assume that the surface is plane and rewrite the formula with suitable changes.
Assume that the interface is plane surface
Then R becomes infinity
R = ∞ (or) R = 1/0
Substitute the above value in the given equation

Question 4.
Two convex lenses of same focal length are fixed in a PVC pipe at a distance double to their focal length. What happens if a boy sees the moon with that arrangement? (TS March 2017)

• The rays coming from moon are parallel. The first lens converges the rays at focus.
• The converging point is the focus of second lens. So the second lens convert the diverging rays into parallel.
• Hence, in the rays of moon, there will be no change when we see moon with this arrangement or without this arrangement.

(OR)

This arrangement does not make any difference in the rays coming from moon.
The moon appears same if we see directly or with this arrangement.

Question 5.
Focal length of the lens depends on its surrounding medium. What happens, if we use a liquid as surrounding media of refractive index, equal to the refractive index of lens? (TS June 2018)

• When the refractive index of surrounding media is equal to the refractive index of lens, the lens looses its characteristics.
• Lens do not diverge or converge the light.
• Light do not get refracted when it passes through that lens.

Question 6.
Complete the ray diagram given below (TS March 2018)

Question 7.
The refractive index of convex lens material is 1.46. The refractive index of Benzene and water is 1.5 and 1.0 respectively. How does the lens behaves when it is kept in Benzene and water? Given and write. (TS March 2018)

• When the convex lens with refractive index 1.46 is kept in Benzene with refractive index 1.5, then the lens acts as a diverging lens.
• If the same lens is kept in water whose refractive index is 1, then it acts as a converging lens.

Question 8.
Write the applications of lenses in day to day life. (AP SCERT: 2019-20)
Uses of lenses in day to day life :

• Lenses are used for correcting eye defects.
• They are used as magnifying lenses.
• They are used in microscopes, telescopes, binoculars.
• They are used in cinema projectors and cameras.

Question 9.
Water lens is made of double convex lens of radius of curvature “R”. Write lens makers formula for water lens. (AP SA-I: 2019-20)
1) Radius of curvatures of water lens are R1 = R2 = R and n = 1.5.
2) Sign conversion R1 = + R1, R2 = -R2.
3) Lens makers formula

Question 10.
Find the focal length of plane convex dens if its radius of curvature is R and its refractive index is n.
Given lens is plano-convex lens; radius of curvature = R
Refractive index = n

Question 11.
In a classroom, four friends found out the focal length of a lens by conducting an experiment. The value came out to be 12.1cm, 12.2cm, 12.05 cm, 12.3 cm. The friends discussed the reasons for the differences or defects. Mention those reasons.
Students got different focal lengths.

1. By observing the values they got all positive values. This indicates they are given by convex lens.
2. All the students got exact interger but different decimal value.

Reasons :
The difference in values is due to least count errors, parallax errors, random errors and systematic errors, etc.

Question 12.
How will you decide whether a given piece of glass is a convex lens, concave lens or a plane plate?
Hold the given piece of glass over some printed matter.

1. If the letters appear magnified, then the given piece of glass is convex lens.
2. If the letters appear diminished, then the given piece of glass is concave lens.
3. If the letters appear to be same size, then it is a plane glass piece.

Question 13.
State the type of lens used as a magnifying glass. Draw a labelled ray diagram to show that the image of the object is magnified.

A single convex lens is used as a magnifying glass, i.e. for seeing small object magnified. When the object to be seen in between the focus and optical centre of the lens, a vertical, erect magnified image of the object is formed as shown and convex lens is and to act as magnifying glass AB’ is the magnified image of AB.

Question 14.
Give conventions used in lenses.
(OR)
Write the signs of convex and concave lens using in drawing ray diagrams.

• All distances are measured from optical centre of the lens.
• Distances measured along the direction of the incident light are taken as positive.
• The distances against the incident light are taken as negative.
• The heights measured vertically above the axis, are taken as positive.
• The heights measured vertically down the axis, are taken as negative.

Question 15.
A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Yes, the statement is correct.
For magnified virtual image :
The object should be placed between optic centre (C) and focus (F) (< 20)

For magnified real image:
Placed between focus (F) and centre of curvature (2F) (20 – 40)

Question 16.
Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building ? What is the approximate focal length of this lens?
As the image is real, therefore the lens use is convex lens. The distance of the real image formed by a convex lens from the lens decreases as the object distance from the lens increases. Hence, the screen has to be moved towards the lens to obtain the sharp image of the building. Approximate focal length of the lens =15 cm as the rays of light coming the window pane are considered to come from infinity. These rays of light are focussed by the convex lens at its focus, (i.e. on the screen).

Question 17.
What do you see when your friend brings a sheet of paper on which arrow was drawn behind the empty cylindrical shaped transparent vessel? Why do you see a diminished image?
We will see a diminished image of the arrow.

When the vessel is empty, light from the arrow refracts at the curved interface, moves through the glass and enters air, then it again undergoes refraction curved surface of the vessel and comes out into the air. In this way, light travels in two media, comes out of the vessel and forms a diminished image.

Question 18.
Using the formula of refraction at curved surfaces, write the formula for plane surfaces.
For curved surfaces the formula for refraction is $$\frac{\mathrm{n}_{2}}{\mathrm{v}}-\frac{\mathrm{n}_{1}}{\mathrm{u}}=\frac{\left(\mathrm{n}_{2}-\mathrm{n}_{1}\right)}{\mathrm{R}}$$

For plane surface, the radius of curvature (R) approaches infinity. Hence 1/R becomes zero.
$$\frac{\mathrm{n}_{2}}{\mathrm{v}}-\frac{\mathrm{n}_{1}}{\mathrm{u}}=0 \Rightarrow \frac{\mathrm{n}_{2}}{\mathrm{v}}=\frac{\mathrm{n}_{1}}{\mathrm{u}}$$

Question 19.
Explain how a convex lens behaves on converging lens and diverging lens.
The convex lens behaves as a converging lens, if it is kept in a medium with refractive index less than the refractive index of the lens. It behaves like a diverging lens when it is kept in transparent medium with greater refractive index than that of lens.
e.g. : Air bubble in water behaves like a diverging lens.

Question 20.
When does Snell’s law fail?

• Snell’s law fails when light is incident normally on the surface of refracting medium.
• Both media have same refractive index.

Question 21.
If on applying sign convention for lens the image distance obtained is negative, state the significance of negative sign.

• Negative sign of image distance means the image is virtual and erect.
• It is formed on the same side of the object with respect to lens.

Question 22.
Magnification of lens is found to be +2. What type of lens is that?
Magnification +ve indicates the image is erect and virtual.
Magnification 2 indicates it is magnified.
Magnified virtual image is formed by only convex lens.

Question 23.
For same angle of incidence in media A, B and C the angle of refractions are 30°, 25° and 20° respectively. In which medium will the velocity of light be minimum?
In medium R the velocity of light is minimum because it has greater refractive index. Refractive index and velocity of light in a medium are inversely proportional. So in medium R the velocity is minimum.

Question 24.
The radius of curvature (twice the focal length) of a convex lens is 40 cm. A student wants to get various images of following types (a) enlarged virtual image, (b) enlarged real image, (c) image of same size, (d) diminished image.
In order to get these images where should the object should be kept in front of convex lens?
a) The object should be kept in less than 20 cm (i.e. less than focal length).
b) In order to get enlarged real image, object should be kept between 20 cm to 40 cm in front of lens.
c) In order to get image of same size object should be kept at a distance of 40 cm from the lens.
d) In order to get diminished image the object should be kept beyond 40 cm from the lens.

Question 25.
A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images.

• Yes, the statement is correct.
• A convex lens of focal length 20 cm will produce a magnified virtual image if object is placed at a distance less than 20 cm from the lens.
• A convex lens of focal length 20 cm will produce magnified real image if the object is placed at a distance more than 20 cm and less than 40 cm.

Question 26.
A concave mirror and a convex lensare held in water. What changes, if any, do you expect in their focal length?
The focal length of a concave mirror independent of the medium and A convex lens depends on medium when they are placed in water.
The focal length of the mirror – Does not change.
The focal length of the convex lens – Changes (means increases 4 times).

Question 27.
When does a convex lens behave like a diverging lens? Given example.
A Convex lens behaves like a diverging lens when it is kept in a tranparent medium with greater refractive index than that of the lens.
Eg : An air bubble in water behaves like a diverging lens.

Question 28.
A pond appears to be shallower than it really is when viewed obliquely. Why?

• Suppose two rays are originated from the bottom of the pond.
• As these rays get refracted into air, they bend away from the normal.
• When these two refracted rays produced backwards they seem to meet at a point higher than the bottom of the pond.
• This point gives the apparent position of the bottom of the pond.
• Thus the pond appears to be shallower.

Question 29.
What happens to the image formed by a convex lens if its lower part is blackened?

1. Every part of lens forms complete image.
2. If lower part of the lens is blackened, the complete image will be formed.
3. But its intensity will decrease.

Question 30.
Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

• Yes, the type of lens changes, if it is placed in medium having higher refractive index that of lens.
• For example, convex lens acts as converging lens when it is placed in a medium of lower refractive index otherwise it behaves like a diverging lens.

Question 31.
Draw the different types of convex and concave lens.

Question 32.
Complete the ray diagram and give reason.

The light ray which passes through optical centre does not undergo refraction. So it goes straight.

Question 33.
How do you appreciate the refraction at plane surfaces and at curved surfaces?
The refraction of curved surfaces are used in various aspects such as

1. In microscope to enlarge microscopic objects.
2. In telescopes to see celestial objects.
3. To correct eye defects like myopia, hypermetropia and presbyopia.

So refraction at curved surfaces is thoroughly appreciated.

Question 34.
An object is placed at a distance of 50 cm from a concave lens of focal length 20 cm. Find the nature and position of the image.

Image distance is negative that indicates it is a virtual and erect image.

Question 35.
A bird is flying at the height 3m above the river surface while a fish is 4 m below the surface. At what depth would the fish appear to the bird ? At what height the bird would appear to the fish? (given a/w = 4/3)
Given that refractive index of air / water = $$\frac{4}{3}$$
The height the bird would appear to fish = 4 + $$\frac{4}{3}$$ × 3 = 4 + 4 = 8m

### 10th Class Physics 6th Lesson Refraction of Light at Curved Surfaces 4 Marks Important Questions and Answers

Question 1.
Complete the ray diagram when an object is placed between F2 and 2F2. (AP June 2017)

Question 2.
An object is placed at the following distances from a convex lens of focal length 10 cm.
(a) 8 cm.
(b) 15 cm.
(c) 20 cm.
(d) 25 cm.
Which position of the object will produce……. (TS March 2015)
(i) a diminished, real and inverted image?
(ii) a magnified, real and inverted image?
(iii) a magnified, virtual and erect image?
(iv) an image of same size as the object?
i) d (or) 25 cm
Reason : Object placed between centre of curvature and focal point.

ii) b (or) 15 cm
Reason : Object placed beyond centre of curvature.

iii) a (or) 8cm
Reason : Object placed between focal point and optic centre.

iv) c (or) 20 cm
Reason :Object placed at centre of curvature.

Question 3.
The ray diagrams showing the image formed by a convex lens are given in the following table. From these diagrams complete the table. (TS June 2016)

Question 4.
Explain the behaviour of light rays in any four situations of their incidence on a convex lens. (TS March 2016)
1) A ray passing along the principal axis is undeviated.

2) Any ray passing through optic centre is also undeviated.

3) The rays passing parallel to principal axis converge at focus.

4) The rays passing through the focus will take a path parallel to principal axis.

Question 5.
Draw the ray diagrams to find the images when an object is placed in front of the lens (i) at a distance of 8 cm, and (ii) at a distance of 10 cm on the principal axis of a convex lens whose focal length is 4 cm. Write the characteristics of images in both the cases. (TS June 2017)
(i) Ray diagram :

Characteristics of Image :
i) Size of the image equal to the size of the object,
ii) Inverted image,
iii) Real image,
iv) Image formed at C.

(ii) Ray diagram

Characteristics of Image :
i) Image size is less than that of object size,
ii) Inverted image,
iii) Real image,
iv) Image is formed between F & C.

Question 6.
A double concave lens with the refractive index (n) = 1.5 is kept in the air. Its two spherical surfaces have radii R1 = 20 cm and R2 = 60 cm. Find the focal length of the lens. Write the characteristics of the lens. (TS March 2017)

Hence f = – 30 cm (Here minus indicates that the lense is divergent)
Characteristics of the biconcave lens :

1. It is a diverging lens.
2. It is thin at the middle and thicker at the edges.

Question 7.
Draw ray diagrams for a double concave lens of focal length 4 cm, when objects are placed at 3 cm and 5 cm on principal axis. Write characteristics of images. (TS June 2018)
i)

ii)

Characteristics of images :

1. Image formed between P and F
2. Diminished image
3. Errected image
4. Virtual image

Question 8.
Write the characteristics of the images which are formed when objects are placed at 50cm and 75cm on the principle axis of a convex lens with focal length of 25 cm. (TS March 2018)
i) Object is placed at 50cm.

Characteristics of the image :

1. Image forms at 2Fp
2. Image is real,
3. Image is inverted,
4. Image is same size

ii) Object is placed at 75cm
f = +25cm; u = -75cm; v = ?

Characteristics of the image :

1. Image forms between F1 and 2F1
2. Image is real
3. Image is inverted
4. Image is diminished.

Question 9.
Write the role of lenses in our daily life. (AP March 2019)
The role of lenses in our daily life :

1. Used for correcting eye defects.
2. Used as magnifying lens.
3. Used in Microscopes.
4. Used in Telescopes.
5. Used in Binoculars.
6. Used in Cinema Projectors.
7. Used in Cameras.

Question 10.
Draw the ray diagrams for the following positions of objects in front of a convex lens mention the characteristics of the image. (AP SCERT: 2019-20)
a) Object is placed beyond 2F2.
b) Object is placed between focal point and opint center.
a)

b)

a) Characteristics of the Image :

1. real
2. inverted
3. diminished.

b) Characteristics of the Image :
If we place an object between focus and optic centre, we will get an image which is virtual, erect and magnified.

Question 11.
The focal length of a convex lens is 2 cm. Draw the ray diagram of an object placed on principal axis at the ‘C’ of lens and at a distance of 3 cm from its optic centre.
1) Object is placed on principal axis of a convex lens at ‘C’.

2) Object is placedon principal axis at a distance of 3 cm from its optic center.

Question 12.
Using biconvex lens, a point image is made on its principal axis S. Let us assume that we know optical centre P and its focus F. We also know PF > PS. Draw the ray diagram to identify the point source and give reasons.

Given lens is biconvex lens and given condition is PF > PS’ means image is formed between optic centre (P) and Focus (F).

According to Snell’s law this condition is possible when the object is also placed between P and F. Because reflected rays are divergent.

Question 13.
Write about the behaviour of light rays when they incident on a lens.
1) Situation I:
Ray passing through the principal axis.
⇒ It is not deviated.

2) Situation II:
Ray passing through the pole.
⇒ It is also undeviated.

3) Situation III:
Rays travelling parallel to the principal axis.
⇒ They converge at focus or diverge from the focus.

4) Situation IV :
Ray passing through focus.
⇒ It will take a path parallel to principal axis after refraction.

5) Situation V :
Parallel rays fall on a lens making some angle with principal axis.
⇒ They converge at a point or diverge from a point lying on a focal plane.

Question 14.
Write characteristics of image formed due to convex lens at various distances.

Question 15.
Write characteristics of image formed by a concave lens at various distances.

Question 16.
You are given a convex lens of focal length 10 cm. Where would you place an object to get a real inverted and highly enlarged image of the object? Draw a ray diagram.
If an object is placed at the focus of the lens it forms real, inverted and highly enlarged image. Thus, the distance of the object from the optical centre of the lens is equal to the focal length of the lens =10 cm.
The ray diagram is as shown

Question 17.
Derive the formula of image formation in refraction at curved surfaces.
1) Object at infinity :
The rays coming from the object at infinity are parallel to principal axis and converge to the focal point after refraction. So, a point-sized image is formed at the focal point.

2) Object placed beyond the centre of curvature on the principal axis :
When an object is placed beyond the centre of curvature 2F2, a real, inverted and diminished image is formed on the principal axis between F1 and 2F1

3) Object placed at the centre of curvature :
When an object is placed at the centre of curvature (2F2) on the principal axis, a real, inverted image is formed at 2F1 which is same size as that of the object.

4) Object placed between the centre of curvature and focal point:
When an object is placed between centre of curvature (2F2) and focus (F2), we will get an image which is real, inverted and magnified. This image will form beyond 2F1.

5) Object located at focal point:
When an object is placed at focus (F2), the image will be at infinity.

6) Object placed between focal point and optic centre :
If we place an object between focus and optic centre, we will get an image which in virtual, erect and magnified.

Question 18.
Distinguish between convex lens and concave lens.

 Convex lens Concave lens 1. Objects appear to be big in the lens. 1. Objects appear to be shrink in the lens. 2. It generally forms real image, (except object is placed between optical centre and focal point) 2. It always forms virtual image. 3. Light rays tend to converge after refraction from lens. 3. Light rays tend to diverge from lens after refraction. 4. The image due to lens may be enlarged or same size or diminished. 4. The image is always diminished. 5. The image due to lens may be inverted or erect. 5. The image is always erect. 6. It is used to correct hypermetropia. 6. It is used to correct myopia.

Question 19.
The ray diagram given below illustrates the experimental set up for the determination of the focal length of a converging lens using a plane mirror.

1) State the magnification of the image formed.
2) Write the characteristics of the ipiage formed.
3) What is the name given to the distance between the object and optical centre of the lens in the following diagram?

1. The magnification of the image formed is unity (or 1).
2. The image is a) real and b) inverted is at the same position of the object.
3. The distance between the object and the optical centre of the lens is called object distance.

Question 20.
A concave lens made of a material of refractive index n1 is kept in medium of refractive index n2. A parallel beam of light incident on the lens. Complete the path of rays of light emerging from the concave lens if
i) n1 > n2
ii) n1 = n2
iii) n1 < n2.
i) When n1 > n2, light goes from rarer to denser medium. Therefore, in passing through a concave lens it diverges.

ii) When n1 = n2, there is no change in medium. Therefore no bending or refraction occurs.

iii) When n1 < n2, light goes from a denser to rarer medium. Therefore, in passing through a concave lens it converges.

Question 21.
One half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

• Every part of a lens forms an image.
• For formation of image we require only two light rays to converge.
• Therefore, if the lower half of the lens is covered, it will still form a complete image.
• However the intensity of the image will be reduced.
• This can be verified experimentally by observing the image of distant object like tree on a screen, when lower half of the lens covered with a black paper.

Question 22.
Complete the following table if the object is placed at various positions in front of a convex lens.

 Position of object Position of image Nature of image 1. At infinity 2. Between F1 and 2F1 3. Same size, real and inverted 4. Seen in the lens

 Position of object Position of image Nature of image 1. At infinity On Fj (focal point) Highly diminished, real and inverted 2. Beyond 2F2 Between F1 and 2F1 Diminished, real and inverted 3. On 2F1 On 2F2 Same size, real and inverted 4. Between focus and optical centre Seen in the lens Enlarged, virtual and erect

Question 23.
A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations.

 Distance of flame from the lens (cm) Distance of the screen from the lens (cm) 1. 60 20 2. 40 24 3. 30 30 4. 24 40 5. 15 70

a) From the above table, find the focal length of lens without using lens formula.
b) Which set of observations is incorrect and why?
c) In which case the size of the object and image will be same? Give reason for your answer.
a) From the observations, it is clear that for u = 30, v = 30 cm. This means this value must be equal to twice the focal length of the convex lens.
∴ Focal length of convex lens = 30/2 = 15 cm

b) The observation (5) is not correct because if u = 15 cm i.e., the object is kept at focus so the image should be at infinity and not at 70 cm.

c) For twice the focal length we know size of object = size of image. So when object is kept at 30 cm the size of object and image are same.

Question 24.
Draw the ray diagrams when incident ray striking a convex surface or a concave surface moving from one medium to another medium.

Question 25.
Draw ray diagrams of image formed by a convex lens at various distances.
1) Object at infinity.

2) Object placed beyond the centre of curvature (2F2).

3) Object placed at the centre of curvature.

4) Object placed between 2F2 and F2.

5) Object at the focal point.

6) Object placed between F and P

Question 26.
Write about the focal length of the lens with diagram.

• A parallel beam of light incident on a lens converges to a point as shown in figure (a) or seems to emanate from a point on the principal axis as shown in figure (b).
• The point of convergence (or) the point from which rays seem to emanate is called focal point or focus (F).
• Every lens has two focal points.
• The distance between the focal point and optic centre is called the focal length of lens denoted by ‘f’
• To draw ray diagram for lenses, we need two more points in addition to focal points F1 and F2.
• These points are equidistant from centre of the lens and also equal to double the focal length. So we call them 2F1 and 2F2.
• For drawing ray diagrams related to lenses, we represent convex lens with a symbol £ and concave lens with J as shown in the figure c and d.

Question 27.
The diagram shows an object OA and its image IB formed by a lens. The image is same size as the object.
a) Complete the ray diagram and locate the focus of lens by labelling it as F.
b) State whether the lens is convex or concave.
Show it in the diagram.

a)

1. Optical centre goes undeviated, therefore to find the optical centre, join A to B to meet the line 01 at the point P which gives the position of optical centre of the lens.
2. Draw a line CP through P perpendicular to the line 01 to represent lens.
3. Draw another ray AC from the point A parallel to the principal axis 01 to meet the lens line CP at a point C.
4. This ray AC will reach the image point B while passing through the focus, therefore join C to B to meet line 01 at a point F which is the focus of the lens. The completed ray diagram is shown above.

b) Since the image is real and inverted, the lens is convex.

c) Since the size of object and image of equal, the object must be at a distance of twice the focal length, i.e., at 2F2.

Question 28.
The diagram shows an object AB placed on the principal axis of the lens L. The two foci of lens are F1 and F2. The image formed by the lens is erect, virtual, and diminished.

i) Draw the outline of the lens L used and name it.
ii) Draw a ray of light starting from B and passing through ‘O’. Show the same ray after refraction by the lens.
iii) Draw another ray from B which is incident and parallel to the principal axis. Show how it emerges after refraction from the lens.
iv) Locate the final image formed.
i) Since the image formed by the lens is erect, virtual, and diminished, the lens is concave.
ii) The light ray BO incident at the optical centre ‘O’ of the lens, passes undeviated as OC after refraction by the lens.
iii) The light ray BP is incident and parallel to the principal axis. It emerges as PQ after refraction which appears to diverge from the second focus F2 of the lens.
iv) The refracted rays OC and PQ do not actually meet, but they appear to diverge from a point B’ (i.e. they meet at a point B’ when they are produced backwards).
v) Thus, B’ is the complete image A’B’ is obtained by drawing perpendicular from B’ on the line F2OF1. The image is formed between the optical centre O and focus F2 of the lens.

Question 29.
Figure below shows the refracted ray BC through a concave lens and its foci marked as F1 and F2. Complete the diagram by drawing the corresponding incident ray and also give reason.

1. Figure shows the refracted ray parallel to the principal axis. Therefore, the incident ray must be travelling towards the focus F2.
2. Thus, to find incident ray, F2 is joined to the starting point B of the refracted ray and produced backward as BA.
3. Then AB is the required incident ray.
4. The completed diagram is shown below.

Question 30.
State the type of lens used as a simple magnifying glass. Draw a labelled diagram to show that the image of the object is magnified.

• A single convex lens is used as a magnifying glass, i.e. for seeing small object magnified.
• When the object to be seen between the focus and optical centre of the lens, a virtual, erect and enlarged image is formed.
• So a convex lens acts as magnifying glass for object AB as shown in the figure.

Question 31.
Radii of biconvex lens are equal. Let us keep an object at one of the centres of curvature. Refractive index of lens is ‘n’. Assume lens is in the air. Let us take R as the radius of the curvature.
a) How much is the focal length of the lens?
b) What is the image distance ?
c) Discuss the nature of the image.
Radii of curvatures (R) of biconvex lens are equal, so R1 = R2 = R

c) The nature of the image is inverted and v < u.

Question 32.
Refractive index of a lens is 1.5. When an object is placed at 30 cm, image is formed at 20 cm. Find its focal length. Which lens is it? If the radii of curvature are equal, then what is its value?
Object distance = u = – 30 cm (Infornt of the lens)
Image distance = v = 20 cm

Question 33.
A convex lens of focal length 10 cm is placed at a distance of 12 cm from a wall. How far from the lens should an object be placed so as to form its real image on the wall?

Question 34.
A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. If image distance is thrice the focal length, find object distance, image distance and nature of image.
Given that
Focal length of lens = + 20 cm
Object distance = -u
Image distance (v) = + 3u

So the object is between F2 and 2F2. So the image beyond 2F1 it is real, inverted, and magnified.

Question 35.
What are the various applications of lens?