AP 8th Class Physics 7th Lesson Questions and Answers Light

AP State Board new syllabus 8th Class Physics Guide 7th Lesson Light Textbook Exercise Questions and Answers.

AP 8th Class Physical Science 7th Lesson Questions and Answers Light

8th Class Physics 7th Lesson Light Questions and Answers (Exercise)

Question 1.
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer:

• Everything we can see around is due to the reflected light from the object that entering into our eyes.
• If there is no light falling on the object, we cannot see it.
• We cannot see the objects in the dark room because there is no light to reflect.
• But we can see objects outside the room because light falls and reflects on the objects.

Question 2.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer:
1. Differences between regular and diffused reflection :

Regular reflection	Diffused reflection
1. Reflection from a smooth surface is called regular reflection.	1. When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection.
2. Eg : Reflection on a mirror.	2. Eg : Reflection on writing plank.
3. Images are formed by regular reflection.	3. Clear images do not form by regular reflection.

2. Diffused reflection does not mean the failure of the laws of reflection.

• Each and every ray follow the laws of reflection in diffused reflection.
• Since the surface is irregular all the incident angles are different.
• So, the angle of reflection is also different.
• But the rays follow’the law, ∠i=∠r
• For example, incident angles of the parallel rays l, m and n are 40°, 50° and 30°. Their angle reflections are 40°, 50° and 30°. These reflected rays are not parallel but followed ∠i=∠r

3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper
Answer:
a) Polished wooden table : Regular reflection takes place on the surface of the polished wooden table because the surface is polished and has very less irregularities.

b) Chalk powder : Diffused reflection takes place on the surface of the chalk powder because the surface has more irregularities.

c) Cardboard surface: Diffused reflection takes place on the surface of the Cardboard because the surface has more irregularities.

d) Marble floor with water spreac over it : Regular reflection takes place because the surface has less irregularities. Water filled the irregularities and make the marble floor plane surface like mirror.

e) Mirror : Regular reflection takes place on the surface of the mirror because the surface is plain.

f) Piece of paper: Diffused reflection takes place on the surface of the piece of paper because the surface has more irregularities.

Question 4.
State the laws of reflection.
Answer:
Laws of reflection :

• The angle of incidence is always equal to the angle of reflection.
• The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

Question 5.
Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane. (Activity – 2)
Answer:
Procedure:

• Take a sheet of stiff paper or a chart paper and place it on a table.
• Let the sheet project a little beyond the edge of the table.
• Take a comb and close all its openings except one in the middle.
• Hold the comb perpendicular to the sheet of paper.
• Throw light from a torch through the opening of the comb from one side.
• With a slight adjustment of the torch and the comb you will see a ray of light along the paper on the other side of the comb.
• Keep the comb and the torch steady.
• Place a strip of a plane mirror in the path of the light ray. You can observe a reflected ray.
• Cut the projecting portion of the sheet in the middle.
• Now look at the reflected ray.
• Make sure that the reflected ray extends to the projected portion of the paper.
• Bend that part of the projected portion on which the reflected ray falls.
• Now you cannot see the reflected ray.
• Bring the paper back to its original position.
• Then you can see the reflected ray again.

Inference :

• We can infer that when the whole sheet of paper is spread on the table, it represents one plane.
• The incident ray, the normal at the point of incidence and the reflected ray are all in this plane.
• When you bend the paper, you create a plane different from the plane in which the incident ray and the normal lie.
• Hence, you do not see the reflected ray.

Conclusion :
It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is the second law of reflection.

Question 6.
Fill in the blanks in the following.
a) A person 1 m in front of a plane mirror seems to be …………… m away from his image.
b) If you touch your …………… ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ……………
c) The size of the pupil becomes …………… when you see in dim light.
d) Night birds have …………… cones than rods in their eyes.
Answer:
a) 2
b) left, left
c) more
d) less number of

Choose the correct option in Questions (7-8) :

Question 7.
Angle of incidence is equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Answer:
(a) Always

Question 8.
Image formed by a plane mirror is
a) virtual, behind the mirror and enlarged.
b) virtual, behind the mirror and of the same size as the object.
c) real at the surface of the mirror and enlarged.
d) real, behind the mirror and of the same size as the object.
Answer:
b) virtual, behind the mirror and of the same size as the object.

Question 9.
Describe the construction of a kaleidoscope.
(OR)
Answer:
How do you construct a kaleidoscope. (Activity – 6)
Answer:

• To make a kaleidoscope, get three rectangular mirror strips each about 15 cm long and 4 cm wide.
• Join them together to form a prism as shown in Figure.
• Fix this arrangement of mirrors in a circular cardboard tube or tube of a thick chart paper.
• Make sure that the tube is slightly longer than the mirror strips.
• Close one end of the tube with a cardboard disc having a hole in the centre, through which you can see.

• To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc.
• At the other end, touching the mirrors, fix a circular plane glass plate.
• Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles).
• Close this end of the tube with a ground glass plate.
• Allow enough space for the colour pieces to move around.
• Now the kaleidoscope is ready.
• When we see a variety of patterns in the tube, through the hole.

10.
Draw a labelled sketch of the human eye.
Answer:

Question 11.
Gurmit wanted to perform Activity – 8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Answer:

• The nerve cells in the retina are very sensitive to intensity of light.
• So, the high intense laser light should damage the retina leads to permanent blindness.
• So, her teacher advised her not to do so.

Question 12.
Explain how you can take care of your eyes.
Answer:
It is necessary that you take proper care of your eyes.
If there is any problem you should go to an eye specialist. Have a regular checkup –

• If advised, use suitable spectacles.
• Too little or too much light is bad for eyes. Insufficient light causes eye strain and headaches. Too much light, like that of the Sun, a powerful lamp or a laser torch can injure the retina.
• Do not look at the Sun or a powerful light directly.
• Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor.
• Always read at the normal distance for vision. Do not read by bringing the book too close to your eyes or keeping it too far.

Question 13.
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer:
45°

Question 14.
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm ?
Answer:
Object placed between two parallel plane mirrors form infinite images. Hence, infinite images of the candle will be formed.

Question 15.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in given figure. Draw the reflected ray from the second mirror.

Answer:

Question 16.
Boolbo stand at A just on the side of a plane mirror as shown In given figure. Can lie see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer:

Boojho cannot see himself: He can see image of P.

Question 17.
a) Find out the position of the image of an object situated at A in the plane mirror.
b) Can Paheli at B see this image ?
c) Can Boojho at C see this image ?
d) When Paheli moves from B to C, where does the image of A move?

Answer:
a) A’ is the image of A.
b) Paheli can see the image.
c) Boojho can also see the image.
d) The image doesn’t move even though Paheli moves from B to C.

Extended Learning – Activities and Projects

Question 1.
Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case do you see yourself better and why?
Answer:
(Do it yourself)
We can see ourselves better in the case we put the glass slab on white paper because white sheet reflects more light as compared to black sheet.

Question 2.
Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes.
Answer:
Do it yourself.

Question 3.
Meet an eye specialist. Get your eye sight checked and discuss how to take care of your eyes.
Answer:
Do it yourself.
Some precautions to follow for healthy vision :

• Don’t play with sharp objects.
• Wash your eyes with cold water.
• Use suitable spectacles if you have any eye problems like myopia or hypermetropia.
• Don’t read in dim light or too bright light.
• Don’t see the welding sparks directly.
• Don’t see the solar eclipse directly.
• Don’t rub your eyes when the dust is fell in your eyes.
• Don’t use mobiles continuously.
• Sleep for 8 hours daily.
• Eat vitamin A food.
• Go to regular check-ups.

Question 4.
Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children.
Answer:
Do it yourself.
Some causes for eye sights :

• Deficiency of vitamin A, iron, etc.,
• Heredity
• Accidents and burns
• Continuous usage of mobiles
• Play with sharp objects
• Explosions of crackers
• Usage of traditional medicines.

8th Class PS 7th Lesson Questions and Answers (Lab Activities)

Question 1.
How do you verify the first law of the reflection?
(OR)
What is the relation between the angle of incidence and angle of reflection? How do you prove it?
Answer:
The first law of reflection : The angle of incidence is always equal to the angle of reflection. (∠i=∠r)

• Fix a white sheet of paper on a drawing board or a table.
• Take a comb and close all its openings except one in the middle.
• Hold the comb perrpendicular to the sheet of paper.

• Throw light from a torch through the opening of the comb from one side.
• With slight adjustment of the torch and the comb you will see a ray of light along the paper on the other side of the comb.
• Keep the comb and the torch steady.
• Place a strip of plane mirror in the path of the light ray.
• We can observe the incident ray, reflected ray.

• Draw lines showing the position of the plane mirror, the incident ray and the reflected ray on the paper with the help of your friends.
• Remove the mirror and the comb.
• Draw a line making an angle of 90° to the line representing the mirror at the point where the incident ray strikes the mirror.
• This line is known as the normal to the reflecting surface at that point.
• Measure the angle of incidence and the angle of reflection.
• Repeat the activity several times by changing the angle of incidence.
• Enter the data in Table.

Angle of Incidence and Reflection

S.No.	Angle of Incidence (∠f)	Angle of Reflection (∠r)
1.
2.
3.
4.
5.

Activity – 3

Question 2.
Write the steps of drawing an image formed by the plane mirror.
Answer:
1) A source of light 0 is placed in front of a plane mirror PQ.
2) Two rays OA and OC are incident on it.
3) Draw normals to the surface of the; mirror PQ, at the points A and C.

4) Then draw the reflected rays at the points A and C .
5) Name the reflected rays AB and CD , respectively.
6) Extend them further.
7) If they meet, mark this point as I.
8) For a viewer’s eye at E. the reflected rays appear to come from the point $I$.
9) Since the reflected rays do not actually meet at I, but only appear to do so, we say that a virtual image of the point O is formed at I .

Activity – 4

Question 3.
What is diffused reflection? Are they parallel? How do you prove it?
Answer:

• Imagine that parallel rays are incident on an irregular surface as shown in Figure. Remember that the laws of reflection are valid at each point of the surface.
• Use these laws to construct reflected rays at various points.
• They are parallel to one another.
• You will find that these rays are reflected in different directions.

• When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection.

Activity – 5

Question 4.
Show the relation between angle of two plane mirrors and the formation of images through an activity.
Answer:

• Take two plane mirrors.
• Set them at right angles to each other with their edges touching.
• To hinge them you can use adhesive tape.
• Place a coin in between the mirrors.
• We can see three images of the coin.
• Now hinge the mirrors using the adhesive tape at different angles, say 45°, 60°, 120°, 180° etc.

7) Place some object (say a candle) in between them.
8) Note down the number of images of the object in each case.
9) Finally, set the two mirrors parallel to each other.
10) Find out how many images of a candle placed between them are formed.
11) We can observe that the number of images formed is \(\frac{360^{\circ}}{\text { angle betweenthemirrors }^{\circ}}\) – 1.

Activity – 7

Question 5.
Write an activity to show that sunlight consists of several colours.
(OR)
How do you split the white light into seven colours?
(OR)
What is dispersion? Explain with an activity.
Answer:
1) Get a plane mirror of a suitable size. Place it in a bowl (katori) as shown in Figure.
2) Fill the bowl with water.
3) Put this arrangement near a window in such a way that direct sunlight falls on the mirror.
4) Adjust the position of the bowl so that the reflected light from the mirror falls on a wall.
5) If the wall is not white, fix a sheet of white paper on it.


Activity – 8

Question 6.
Write an activity to show the working of pupil in our eye.
Answer:

• Look into your friend’s eye. Observe the size of the pupil.
• Throw light on her eye with a torch. Observe the pupil now.
• Switch off the torch and observe her pupil once again.
• We may notice that pupil enlarges in low light and contracts in more light.
• Pupil is larger when switch off the torch.
• It is because to allow more light when switched off the torch.
• When the light is dim, we need to allow more light in the eye.

Activity 9

Question 7.
How do you demonstrate the blind spot?
Answer:

• Make a round mark and a cross on a sheet of paper with the spot to the right of the cross.
• The distance between two marks may be 6-8 cm.
• Hold the sheet of paper at an arm’s length from the eye.

• Close your left eye. Look continuously at the cross.
• Move the sheet slowly towards you, keeping your eye on the cross.
• You can feel the round mark disappear at some point.
• Now close your right eye.
• Look at the round mark now and repeat the activity.
• Again, the cross disappears.
• The disappearance of the cross or the round mark shows that there is a point on the retina which cannot send messages to the brain when light falls on it.

Activity – 10

Question 8.
How do you demonstrate that the impression of an image does not vanish immediately from the retina?
Answer:

• Get a square piece of cardboard of size 6-8cm.
• Make two holes as shown in Figure.
• Thread a string through the two holes.

• Draw/ paste a cage on one side of the cardboard and a bird on the other side.
• Twist the string and make the card twirl rapidly.
• We can see the bird in the cage.
• We can say the impression of an image does not vanish immediately from the retina.