DAV Class 8 Science Chapter 10 Question Answers – Refraction and Dispersion of Light
Drop a coin into a glass of water and it seems to float higher than it really is. A pencil in a glass looks bent. After the rain, a rainbow splits sunlight into colours. All of these happen because light does not always travel in a straight line. Chapter 10 of the DAV Class 8 Science book explains how light bends when it moves from one material to another, how a prism splits white light into colours, and how lenses form images. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the answer and write it in your own words.
The chapter in one minute
When light passes from one transparent medium to another, such as from air to water, its speed changes, and this makes its path bend. This bending is called refraction. The bending depends on how optically dense the medium is. In an optically denser medium, light travels slower. Here are the rules:
When light goes from a rarer medium into a denser one (air to water or glass), it bends towards the normal. When it goes from a denser medium into a rarer one, it bends away from the normal. If light hits the surface straight on, along the normal, it does not bend at all.
A prism splits white light into seven colours (violet, indigo, blue, green, yellow, orange and red), and this is called dispersion. Violet bends the most and red bends the least.
A lens is a piece of transparent material with curved surfaces. A convex lens is thicker in the middle and converges (brings together) parallel rays to a point called the focus. A concave lens is thinner in the middle and spreads parallel rays out so they seem to come from its focus. Three special rays are used to draw ray diagrams:
- A ray parallel to the principal axis passes through the focus (convex lens) or appears to come from the focus (concave lens) after refraction.
- A ray passing through the optical centre goes straight, without bending.
- A ray passing through the focus (convex lens) comes out parallel to the principal axis.
A. Fill in the blanks
1. A ray of light passing from one medium to another does not bend if its angle of incidence is 0 degrees.
2. The greater the optical density of a medium, the less the speed of light through it.
3. The splitting of white light into seven colours is called dispersion.
4. The point on the principal axis of a concave lens from which parallel rays appear to spread out is called its focus.
5. To get a real, inverted image of the same size with a convex lens, the object must be placed at 2F.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. The basic cause of refraction is the change in the speed of light between media. | True | The speed changes, so the path bends. |
| 2. In an optically denser medium, light travels faster than in vacuum. | False | Light is always fastest in vacuum, and slower in a denser medium. |
| 3. A slanting ray always bends away from the normal when it enters a new medium. | False | It bends towards the normal when entering a denser medium and away when entering a rarer one. |
| 4. A coin at the bottom of an empty cup seems to rise when water is poured in. | True | Refraction at the water surface makes the coin appear higher. |
| 5. In a prism, yellow light bends more than blue. | False | Blue bends more than yellow. Violet bends the most. |
C. Choose the correct option
1. The condition that is NOT necessary for light to change direction between two media is: the incident light must be only of blue colour. Any colour of light bends.
2. For a ray going from a denser medium to a rarer medium, the correct diagram is Option B, the one in which the ray bends away from the normal.
3. For a rectangular glass slab, the angle between the incident ray and the emergent ray is 0°. The emergent ray comes out parallel to the incident ray, only shifted sideways.
4. The correct figure for white light passing through a prism is Option C, which shows the light splitting into colours, with violet bending the most and red the least.
5. A ray passing through the optical centre of a concave lens will go straight without bending.
6. When an object moves from infinity towards the focus of a convex lens, its image keeps moving away from the lens.
7. Tanya’s grandmother should use a convex lens, held closer to the book than its focal length. A convex lens used this way works as a magnifying glass. The source page marks “concave”, which cannot magnify, so choose the convex option in your book.
D. Short answer questions
1. Light travels faster in Medium A than in Medium B.
(a) Medium B is optically denser, because light moves more slowly in it.
(b) The ray will not change direction if it falls on the surface along the normal, that is, at a right angle to the surface.
2. Why does a pencil look bent when partly dipped in water?
Light from the part of the pencil under water bends away from the normal as it comes out into air. To our eyes, these rays seem to come from a different, higher place, so the underwater part seems to be shifted. The pencil therefore looks bent at the water surface.
3. Trace the path of rays in the given ray diagrams.
The diagrams are given as pictures. Use the three special rays described at the start of this post: a ray parallel to the axis goes through the focus, a ray through the optical centre goes straight, and a ray through the focus comes out parallel to the axis.
4. A lens held towards the Sun burns a hole in black paper.
(a) The lens is a convex lens.
(b) The distance between the lens and the paper is its focal length.
(c) Black paper absorbs most of the light’s energy, while white paper reflects most of it. So black paper heats up and burns faster.
5. What kind of lens forms each of these images?
(a) Inverted and magnified: convex lens (object between F and 2F).
(b) Erect and magnified: convex lens (object between F and the lens).
(c) Inverted and diminished: convex lens (object beyond 2F).
(d) Erect and diminished: concave lens.
6. Show how the virtual images of a concave lens and a convex lens differ.
Draw both as ray diagrams. In words: a concave lens always forms a virtual, erect and smaller image, which lies between the lens and its focus on the same side as the object. A convex lens forms a virtual image only when the object is between the lens and its focus. That image is erect and bigger than the object, and it lies farther from the lens than the object, on the same side.
E. Long answer questions
1. A coin is at the bottom of an empty glass vessel, just out of the observer’s sight. Water is poured in slowly while she keeps her eyes in the same position.
(a) She will see the coin appear. Once there is enough water, the coin comes into view.
(b) The phenomenon is refraction of light.
(c) For the diagram, draw the vessel with water, the coin at the bottom and the eye at the side. Draw a ray from the coin going up to the water surface, and show it bending away from the normal as it enters the air and reaches the eye. Then show the apparent position of the coin, higher than the real one, where the extended ray seems to come from.
2. Identify the correct emergent ray in each figure and give reasons.
The figures are given as pictures, so match these rules to them. In figure (a), the correct ray is P, because the ray bends and comes out of a glass slab parallel to the incident ray. In figure (b), the correct ray is R, because light going from a rarer to a denser medium bends towards the normal. In figure (c), the correct ray is R, because light going from a denser to a rarer medium bends away from the normal.
3. White light falls on a glass prism. Draw a ray diagram of the emergent beam.
Draw a triangular prism and a narrow beam of white light falling on one face. Inside and after the prism, the beam splits into a band of seven colours. Violet is at the bottom, bending the most, and red is at the top, bending the least. The order from bottom to top is violet, indigo, blue, green, yellow, orange, red.
4. Explain with a diagram: (a) centres of curvature, (b) optical centre, (c) focus.
(a) Centres of curvature: A lens has two curved surfaces, and each is part of a sphere. The centres of these spheres are the centres of curvature, usually marked C1 and C2.
(b) Optical centre: It is the central point of the lens. A ray passing through it goes straight without bending.
(c) Focus: For a convex lens, it is the point where rays parallel to the principal axis meet after refraction. For a concave lens, it is the point from which such rays seem to spread out.
5. A convex lens has a focal length of 20 cm. Show the image formed when the object is at (a) 55 cm, (b) 35 cm, (c) 15 cm.
Draw the ray diagrams using a suitable scale, such as 1 cm for 10 cm. These are the results you should get, which I checked by calculation:
| Object distance | Image position | Nature | Size |
|---|---|---|---|
| 55 cm (beyond 2F) | Between F and 2F, at about 31 cm on the other side | Real and inverted | Smaller than the object |
| 35 cm (between F and 2F) | Beyond 2F, at about 47 cm on the other side | Real and inverted | Larger than the object |
| 15 cm (inside F) | On the same side as the object, at about 60 cm | Virtual and erect | Larger than the object |
6. A concave lens has a focal length of 15 cm. Show the image when the object is at (a) 30 cm, (b) 15 cm, (c) 10 cm.
In all three cases, the image is virtual, erect and smaller than the object. It forms on the same side as the object, between the lens and its focus. Using a scale of 1 cm for 10 cm, you should find:
| Object distance | Approximate image distance | Nature and size |
|---|---|---|
| 30 cm | About 10 cm from the lens | Virtual, erect, diminished |
| 15 cm | About 7.5 cm from the lens | Virtual, erect, diminished |
| 10 cm | About 6 cm from the lens | Virtual, erect, diminished |
7. What lens is used as a magnifying glass? Where is the object placed? Draw the ray diagram.
A convex lens is used as a magnifying glass. The object must be placed between the focus (F) and the optical centre (O) of the lens. The image formed is virtual, erect and magnified, and it is on the same side as the object. For the diagram, draw a ray from the top of the object going parallel to the axis and refracting through F, and a second ray passing straight through the optical centre. Extend both refracted rays backwards, and where they meet gives the top of the enlarged, upright image.
Mistakes to avoid in the exam
Students often mix up the bending directions. Remember: rarer to denser means towards the normal, and denser to rarer means away from the normal.
Another common slip is saying that yellow bends more than blue in a prism. Violet bends the most, and red the least.
Do not use a concave lens as a magnifying glass. Only a convex lens can magnify, and only when the object is closer than its focus.
Finally, in ray diagrams, always use a ruler, draw arrows on the rays, and mark F, 2F and O clearly. Marks are given for the neatness and correctness of each ray.
Frequently asked questions
Why does light bend when it enters water?
Light travels slower in water than in air. The change in speed makes it change direction at the surface, and this is refraction.
What is dispersion of light?
It is the splitting of white light into its seven colours when it passes through a prism. A rainbow is nature’s example of dispersion.
What is the difference between a convex and a concave lens?
A convex lens is thicker in the middle and brings parallel rays together. A concave lens is thinner in the middle and spreads them apart.
Which colour of light bends the most in a prism?
Violet bends the most, and red bends the least.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.