Class 10 Physics Light Chapter: 25 Most Important Questions and Answers (Reflection and Refraction)
Class 10 Physics Light Chapter: 25 Most Important Questions and Answers
In this post you will learn:
- The laws of reflection and the types of images
- Concave and convex mirrors and their uses
- Refraction, critical angle and total internal reflection
- Mirror and lens formulas
- Everyday phenomena such as twinkling stars, the blue sky and a bent pencil
Part 1: Reflection of Light
1. What are the laws of reflection?
- The angle of incidence is equal to the angle of reflection (∠i = ∠r).
- The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
2. What is a plane mirror?
A plane mirror has a flat, smooth reflecting surface. It forms a virtual, erect image of the same size as the object, located as far behind the mirror as the object is in front of it.
3. What is the magnification produced by a plane mirror?
The magnification of a plane mirror is always +1, which means the image is the same size as the object.
4. What is the difference between real and virtual images?
| Real Image | Virtual Image |
|---|---|
| Formed when light rays actually meet | Formed when light rays only appear to meet |
| Can be obtained on a screen | Cannot be obtained on a screen |
| Usually inverted | Always erect |
5. How is the image in a plane mirror different from that in a concave mirror?
A plane mirror always forms a virtual, erect image of the same size. A concave mirror can form real or virtual images of different sizes, depending on the position of the object.
Part 2: Spherical Mirrors
6. What is the principal focus of a concave mirror?
It is the point on the principal axis where rays parallel to the axis meet after reflection from the mirror.
7. What is focal length?
Focal length is the distance between the pole of a mirror (or the optical centre of a lens) and its principal focus. It shows how strongly the mirror or lens converges or diverges light. For a spherical mirror, f = R/2.
8. What image is formed by a concave mirror when the object is beyond C?
The image forms between C and F. It is real, inverted and diminished (smaller than the object).
9. Why is a concave mirror used in vehicle headlights?
The bulb is placed at the focus of the concave mirror. The reflected rays leave as a parallel beam, which lights up the road brightly and over a long distance.
10. What is a convex mirror and where is it used?
A convex mirror curves outward and diverges light rays. It always forms a virtual, erect and diminished image. It is used as a rear-view mirror in vehicles because it gives a wider field of view.
11. What is the difference between a concave and a convex mirror?
| Concave Mirror | Convex Mirror |
|---|---|
| Converging mirror | Diverging mirror |
| Forms real or virtual images | Always forms virtual images |
| Used in headlights and shaving mirrors | Used in rear-view mirrors |
Part 3: Refraction of Light
12. Define refraction of light.
Refraction is the bending of light when it passes from one transparent medium to another, caused by a change in the speed of light.
13. What is the difference between reflection and refraction?
- Reflection: light bounces back into the same medium.
- Refraction: light enters a second medium and changes its direction.
14. How do you find the speed of light in a medium?
Speed of light in a medium = Speed of light in vacuum ÷ Refractive index of the medium
That is, v = c / n.
15. What is the principal focus of a convex lens?
It is the point on the principal axis where rays parallel to the axis meet after refraction through the convex lens.
16. What happens when the object is at infinity in front of a convex lens?
Parallel rays converge at the principal focus (F) on the other side of the lens. The image is real, inverted and highly diminished (point-sized).
17. Why does a coin in water appear raised?
Light from the coin bends away from the normal as it passes from water (denser) to air (rarer). The coin therefore appears closer to the surface than it actually is.
18. Why does a pencil in water appear bent?
Light from the part of the pencil inside the water refracts at the water-air boundary. This makes the pencil look broken or bent at the surface.
Part 4: Total Internal Reflection
19. What is the critical angle?
The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.
20. What is total internal reflection (TIR)?
When light travels from a denser to a rarer medium and the angle of incidence is greater than the critical angle, the light is completely reflected back into the denser medium. Optical fibres and the sparkle of diamonds work on this principle.
Part 5: Natural Phenomena
21. What is dispersion of light?
Dispersion is the splitting of white light into its seven colours (VIBGYOR) when it passes through a glass prism.
22. Why does a prism disperse light?
Different colours have different wavelengths, so a prism bends each colour by a different angle. Violet bends the most and red bends the least.
23. Why do stars twinkle?
Starlight passes through layers of air of varying density. This atmospheric refraction keeps changing the apparent position and brightness of the star, so it seems to twinkle. Planets do not twinkle because they are closer and behave like extended sources.
24. Why is the sky blue?
Air molecules scatter shorter wavelengths (blue) of sunlight much more than longer wavelengths (red). This scattering of light is why the sky appears blue.
Part 6: Important Formulas
25. What are the formulas for mirrors and lenses?
| Formula | Expression |
|---|---|
| Mirror formula | 1/v + 1/u = 1/f |
| Lens formula | 1/v − 1/u = 1/f |
| Magnification (mirror) | m = h′/h = −v/u |
| Magnification (lens) | m = h′/h = v/u |
Tip: Always follow the New Cartesian Sign Convention while solving numerical.
Quick Revision Tips for Board Exams
- Practice ray diagrams daily, since they regularly carry 2-3 marks each.
- Learn the sign convention properly before attempting numerically.
- Remember: a concave mirror and a convex lens are converging, while a convex mirror and a concave lens are diverging.
- Solve the NCERT in-text and exercise questions at least twice.
Frequently Asked Questions (FAQ)
Q. Which topics are most important in the Class 10 Light chapter?
Ray diagrams for mirrors and lenses, the mirror and lens formulas, and numerical are asked most often.
Q. Is a concave mirror converging or diverging?
A concave mirror is a converging mirror.
Q. What is the difference between the mirror formula and the lens formula?
The mirror formula is 1/v + 1/u = 1/f, and the lens formula is 1/v − 1/u = 1/f.
