Light Class 8 Extra Questions and Answers

These AP 8th Class Physics Important Questions and 7th Lesson Light Class 8 Extra Questions will help students prepare well for the exams.

Class 8 Science Light Extra Questions

Light Class 8 Important Questions

Question 1.
Define incident ray.
Answer:
The light ray, which strikes any surface is called the incident ray.

Question 2.
Define reflected ray.
Answer:
The ray that comes back from the surface after reflection is known as the reflected ray.

Question 3.
What is normal ?
Answer:
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.

Question 4.
What is angle of incidence ?
Answer:
The angle between the normal and incident ray is called the angle of incidence (∠i).

Question 5.
What is the angle of reflection ?
Answer:
The angle between the normal and the reflected ray is known as the angle of reflection (∠r).

Question 6.
What is the relation between the angle of incidence (∠i) and the angle of reflection (∠r)?
(OR)
What is first law of reflection?
Answer:
The angle of incidence (∠i) = the angle of reflection (∠r).

Question 7.
What will happen if threw a light on the mirror along the normal ?
Answer:
It will reflect along the normal.

Question 8.
Write the second law of reflection.
Answer:
The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is second law of reflection.

Question 9.
What is a lateral inversion of an image ?
Answer:
In an image formed by a mirror, the left of the object appears on the right and the right appears on the left. This is known as lateral inversion.

Question 10.
What is diffused or irregular reflection ?
Answer:
When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection.

Question 11.
What is the cause to diffused or irregular reflection of light ?
Answer:
Diffused or irregular reflection of light is caused by the irregularities in the reflecting surface.

Question 12.
What is a regular reflection ?
(OR)
In which type of reflection does image form ?
Answer:
Reflection from a smooth surface like that of a mirror is called regular reflection. Images are formed by regular reflection.

Question 13.
Set the two mirrors parallel to each other. Find out how many images of a candle placed between them are formed ?
Answer:
Infinity of images formed.

Question 14.
What principle is used in the making of a kaleidoscope ?
Answer:
The principle used in a kaleidoscope is the number of images formed by mirrors placed at an angle to one another.

Question 15.
Write one application of the kaleidoscope.
(OR)
How the kaleidoscope is useful to designers ?
Answer:
Designers of wallpapers and fabrics and artists often use kaleidoscopes to get ideas for new patterns.

Question 16.
What is dispersion of light ? Give an example.
Answer:
Splitting of light into its colours is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.

Question 17.
What is cornea ?
Answer:
The transparent front part of an eye is called cornea.

Question 18.
What is a pupil ?
Answer:
The small opening in the iris is called the pupil.

Question 19.
When does pupil enlarge ?
Answer:
The pupil enlarges in dim light to allow excess light into the eye.

Question 20.
Suppose your friend closed his eyes for some time and opened. What changes do you observe in his eye ?
Answer:
The enlarged pupil may observe in his eye.

Question 21.
What is the functioning of eye lids ?
(OR)
How do you appreciate the eye lids as they protect our eyes ?
Answer:
Nature has provided eyes with eyelids to prevent any object from entering the eye. Eyelids also shut out light when not required.

Question 22.
What are the Non-optical aids ?
Answer:
Visual aids, tactual aids, auditory aids and electronic aids are called Non-optical aids.

Question 23.
What are optical aids ?
Answer:
Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.

Question 24.
James did an experiment with a large plane mirror. He placed a ball in front of the mirror and covered the part of the mirror which was directly in front of the ball with a black cloth. He made his three friends Jill, John and Jacob stand in front of the mirror as shown in the figure.

Who will be able to see the image of the ball in the mirror ?
Answer:
Jacob and Jill.

Light Extra Questions

Question 1.
Can you see an object in the dark? Why ?
(OR)
How can we see the objects ?
(OR)
Eyes alone cannot see any object. Is it correct ? Justify your answer.
Answer:

1. Eyes alone cannot see any object.
2. It is only when light from an object enters our eyes that we see the object.
3. The light may have been emitted by the object, or may have been reflected by it.

Question 2.
Draw a diagram of incident and reflected ray at a plane surface.
Answer:

Question 3.
Draw the reflected ray to the given incident ray.

Answer:

Question 4.
Rithvik got some doubts while being observing his image in a plane mirror. What would be those doubts ?
Answer:

1. Was the image erect or upside down ?
2. Was it of the same size as the object ?
3. Did the image appear at the same distance behind the mirror as the object was in front of it ?
4. Could it be obtained on a screen ?

Question 5.
i) Was the image erect or upside down ?
ii) Was it of the same size as the object ?
iii) Did the image appear at the same distance behind the mirror as the object was in front of it ?
iv) Could it be obtained on a screen ?
(OR)
What are the properties of the image formed in a plane mirror ?
Answer:
i) The image is erected.
ii) The image is the same size as the object.
iii) The image appears at the same distance behind the mirror as the object was in front of it.
iv) It cannot obtain on a screen.
v) The image suffers lateral inversion.

Question 6.
What are called by illuminated objects? Give example.
(OR)
How do we see the moon ?
Answer:

1. The objects which shine in the light of other objects are called illuminated objects.
2. Moon, for example, receives light from the sun and reflects it.
3. That’s how we see the moon.

Question 7.
What are luminous objects? Give examples.
(OR)
Why do the sun is called a luminous object?
Answer:

1. The objects which emit their own light are known as luminous objects.
2. Examples : Sun. fire, flame of a candle and an electric lamp.
3. Sun is a luminous object, because it gives own light.

Question 8.
Can the reflected rays be further reflected if incident on another mirror? Where do you it finds?
Answer:
Yes. In saloons, we can find that the reflected rays be further reflected if incident on another mirror.

Question 9.
How is a periscope made? What is the use of periscope?
Answer:

1. The periscope is made by using two plane mirrors.
2. Periscopes are used in submarines, tanks and also by soldiers in bunkers to see things outside.

Question 10.
Can you explain how reflection from the two mirrors enables you to see objects which are not visible directly?
Answer:

1. In a periscope, two mirrors are placed at a distance oppositely.
2. The image of an object is formed first in the first mirror and it reflects again and forms the second image in the second mirror.
3. So, an observer can see the second image in the second mirror.

Question 11.
How many images are formed when the angle between the plane mirror is i) 30° ii) 90° ?
Answer:
i) \(\frac{360^{\circ}}{30^{\circ}}\) – 1 = 12 – 1 = 11 (∴ \(\frac{360^{\circ}}{\theta}\) – 1 = n)
Number of images form = 11

ii) \(\frac{360^{\circ}}{90^{\circ}}\) – 1 = 4 – 1 = 3
Number of images form = 3

Question 12.
What consists of a white light? From where it can obtain?
Answer:
The white light consists of seven colours. It can obtain from the sun.

Question 13.
How is outer coat of the eye ? How does it protect us ?
Answer:

1. The outer coat of the eye is white.
2. It is tough so that it can protect the interior of the eye from accidents.

Question 14.
What is an iris? Where is it located ?
Answer:
The dark muscular structure in the eye is called iris. It is located behind the cornea.

Question 15.
What is the role of the iris in our vision ?
(OR)
How does an iris work ?
(OR)
How do you appreciate the role of the iris in our vision ?
Answer:

1. The size of the pupil is controlled by the iris.
2. The iris is that part of eye which gives it its distinctive colour.
3. When we say that a person has green eyes, we refer actually to the colour of the iris.
4. The iris controls the amount of light entering into the eye.
   So, I appreciate the role of iris in our vision.

Question 16.
Why do you cannot see the surroundings when you enter a cinema theatre suddenly?
Answer:

1. The pupil will contract and allow less amount of light in the bright light when we are outside of the theatre.
2. When we suddenly enter the theatre, only less amount of light will enter the eye because the pupil is in the contract position still.
3. Then the iris starts to enlarge gradually to allow more amount of light.
4. After some time, it enlarges sufficiently and we can see the theatre clearly.

Question 17.
How is an eye lens? What is its functioning?
Answer:

1. Eye lens is a convex lens that is thicker in the centre.
2. Eye lens focuses light on the back of the eye, on a layer called retina.

Question 18.
What is a retina? What is its functioning?
Answer:

1. The layer at the back of the eye is called retina.
2. It catches the light as a screen coming through the eye lens.
3. The retina contains several nerve cells.
4. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.

Question 19.
How many kinds of nerve cells are presented in the retina? What is their functioning?
(OR)
What are cones and rods?
Answer:

1. There are two kinds of cells present in the retina of the eye- (i) cones and (ii) rods.
2. Cones are sensitive to bright light and sense different colours.
3. Rods are sensitive to dim light.

Question 20.
What is the blind spot in the eye ? Why is it called so ?
Answer:

1. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.
2. It is called as blind spot because no vision is possible at that spot.

Question 21.
How can we see the moving objects ?
Answer:

1. The impression of an image does not vanish immediately from the retina.
2. It persists there for about l/16th of a second.
3. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.

Question 22.
What is the minimum distance at which the eye can see objects ? Does it vary ?
Answer:

1. The most comfortable distance at which one can read with a normal eye is about 25 cm.
2. The minimufn distance at which the eye can see objects distinctly varies with age.

Question 23.
What are the eye defects? How can be corrected them ?
Answer:

1. Some persons can see objects close to them clearly but cannot see distant objects so clearly. This type of eye defect is called myopia.
2. On the other hand, some persons cannot see objects nearby clearly but they can see distant objects quite well. This type of eye defect is called hypermetropia.
3. With suitable corrective lenses, these defects of the eye can be corrected.

Question 24.
How is visually impaired people learn the Braille system ?
Answer:

1. Visually impaired people learn the Braille system by beginning with letters, then special characters and letter combinations.
2. Methods depend upon recognition by touching.
3. Each character has to be memorised. Braille texts can be produced by hand or by machine.

Question 25.
In the adjacent figure, AO and OB are incident and reflected rays respectively angle AOB = 90°. Find the values of angle of incidence and angle of reflection.

Answer:
We know that angle of incidence is equal to angle of reflection.
i = r ……………… (1)
From the figure, ∠AOB indicates i + r = 90°
From (1) ⇒ i + i = 90°
⇒ 2i = 90° ⇒ i = 90/2 = 45° ⇒ i = r = 45°
Angle of incidence i = 45°; Angle of reflection r = 45°.

Question 26.
If the angle between the mirror and incident ray is 40°, then find the angle of reflection.
Answer:
Given that angle between incident ray and mirror = 40°.
Suppose angle of incidence = x.

∴ 40 + x = 90
x = 90 – 40 = 50°.
But we know angle of incidence = angle of reflection
∴ Angle of reflection = 50°.

Extra Questions of Light Class 8

Question 1.
Draw the diagram of regular and diffused reflections. A.
Answer:

Question 2.
How can we see strips of still pictures as a moving picture in the theatres ?
Answer:

1. The impression of an image does not vanish immediately from the retina.
2. It persists there for about 1 /16th of a second.
3. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.
4. The movies that we see are actually a number of separate pictures in proper sequence.
5. They are made to move across the eye usually at the rate of 24 pictures per second (faster than 16 per second).
6. So, we see a moving picture.

Question 3.
What is cataract ? Is it possible to treat it ? How ?
(OR)
Write about the eye defect cataract.
Answer:

1. Sometimes, particularly in old age, eyesight becomes foggy.
2. It is due to the eye lens becoming cloudy.
3. When it happens, persons are said to have cataract.
4. There is a loss of vision, sometimes extremely severe.
5. It is possible to treat this defect.
6. The opaque lens is removed and a new artificial lens is inserted.
7. Modern technology has made this procedure simpler and safer.

Question 4.
How are Non-optical and optical aids for visually impaired help them ?
Answer:
Non-optical aids :
1) Visual aids : Visual aids can magnify words, can provide suitable intensity of light and material at proper distances.

2) Tactual aids : Tactual aids including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.

3) Auditory aids : Auditory aids include cassettes, tape recorders, talking books and other such devices.

4) Electronic aids : Electronic aids such as talking calculators and computers, are also available for performing many computational tasks. Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination. Nowadays, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.

Optical aids:

1. Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.
2. While the lens combinations are used to rectify visual limitations, telescopic aids are available to view chalkboard and class demonstrations.

Question 5.
What is Braille ? Who was developed it ?
(OR)
How do you appreciate Braille ?
Answer:

1. Braille is the most popular resource for visually challenged persons.
2. Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821.
3. There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system.
4. So, I appreciate Braille.

Question 6.
Name the great visually impaired Indians and write their achievements.
Answer:

• Some visually impaired Indians have great achievements to their credit.
• Diwakar, a child prodigy has given amazing performances as a singer.
• Ravindra Jain, born completely visually impaired, obtained his Sangeet Prabhakar degree from Allahabad. He had shown his excellence as a lyricist, singer and music composer.
• Lai Advani, himself visually impaired, established an Association for special education and rehabilitation of disabled in India. Besides this, he represented India on Braille problems in UNESCO.
• Helen A. Keller, an American author and lecturer, is perhaps the most well-known and inspiring visually challenged person. She lost her sight when she was only 18 months old. But because of her resolve and courage she could complete her graduation from a university. She wrote a number of books including The Story of my Life (1903).

Question 7.
Write about the “Situations using the plane mirrors.
Answer:
Plane mirrors have many uses.

• Periscopes: They are used in periscopes to see bends and corners. It is used for observing enemy movements from trenches without any danger of being seen. Sailors on submarines use periscopes to see things above the water level.
• Kaleidoscopes: Kaleidoscope is a toy that uses light and mirrors to reflect objects and create beautiful, fascinating repeating patterns.
• Security : Mirrors are used while looking for explosives underneath a vehicle. Even these mirrors are used in shops to keep an eye on the customers. Mirrors are also used in blind turns of busy roads to see the vehicles coming from the other side.
• Telescopes and Microscopes: Plane mirrors are used in many scientific applications like telescopes and microscopes.
• Dressing mirrors : Plane mirrors are used in dressing tables to see ourselves, while dressing, shaving, etc.
• Ophthalmic doctors : They use plane mirrors to increase the distance of the eye test chart while examine the eye of a patient.
• Docorating mirrors: Plane mirrors are used to decorate the building for elivations. Some shops also used plane mirrors to get multiple images of the items in their shops.

Question 8.
Observe the below figure. AB and BC are two plane mirrors arranged at 120°. A ray incidents at and angle 55° on AB. Find the value of ‘x’.

Answer:

Let us say, angles a, b, c, d as shown in the figure.
From the figure,
a = 55° [∵ i = r]
a + b = 90° [∵ Normal to the plane]
55° + b = 90° ⇒ b = 90° – 55° = 35°
120° + b + c = 180° [∵ Total of angles in a triangle]
120° + 35° + c = 180° ⇒ c = 180° – 155° = 25°
c + d = 90° [∵ Normal to the plane]
25° + d = 90° ⇒ d = 90° – 25° = 65°
d = x [∵ i = r]
∴ x = 65°

Question 9.
A thin beam of light falls on mirror 1 as shown in the diagram below. Four boxes P, Q, R and S are placed in front of mirror 1 and mirror 2.

Nanda wants to find out on which box in the diagram is the beam likely to fall after relfecting off mirror 1 and mirror 2.
List the steps that he should follow in the correct order.
Answer:

1. Draw a perpendicular line (normal) at the point of incidence on mirror 1.
2. Measure the angle of incidence on mirror 1, which is 30°.
3. Draw a reflected ray from the point of incidence on mirror 1, making an angle of 30°.
4. The reflected ray intersects with mirror 2.
5. The reflected ray becomes the incident ray on mirror 2.
6. Draw a perpendicular line (normal) at the point of incidence on mirror 2.
7. Measure the angle of incidence on mirror 2, which is 30°. ,
8. Draw a reflected ray from the point of incidence on mirror 2, making an angle of 30°.
9. This reflected ray intersects with box Q.