Short Questions Answer
1. What is the principal focus of a concave mirror?
Answer: The point on the principal axis where light rays parallel to it converge after reflection is called the principal focus of a concave mirror.
2. What is the relationship between focal length (f) and radius of curvature (R) of a spherical mirror?
Answer: R=2f
3. Name the mirror used as a rear-view mirror in vehicles. Why?
Answer: A convex mirror because it provides a wider field of view and gives an erect, diminished image.
4. Define the refractive index of a medium.
Answer: It is the ratio of the speed of light in vacuum to the speed of light in that medium.
5. Write the mirror formula and explain the terms.
Answer: \(\frac{1}{v}\) + \(\frac{1}{u}\) = \(\frac{1}{f}\)
6. What is meant by lateral inversion?
Answer: The phenomenon in which the left side of an object appears as the right side in its mirror image.
7. What type of image is formed by a convex mirror for any position of the object?
Answer: Always a virtual, erect, and diminished image.
8. What is the magnification produced by a plane mirror?
Answer: Magnification = +1 (image size = object size, erect image).
9. Why does a pencil partly immersed in water appear bent at the surface?
Answer: Due to refraction of light at the interface between air and water.
10. What is the nature of the image formed by a concave mirror when the object is placed between its pole and focus?
Answer: Virtual, erect, and enlarged.
11. Define optical centre of a lens.
Answer: The central point of a lens through which a light ray passes undeviated.
12. What happens to the speed of light when it passes from air to water?
Answer: It decreases because water is optically denser than air.
Long Questions Answer
1. Explain the laws of reflection of light. Draw a diagram to illustrate them.
Answer: (i) The angle of incidence equals the angle of reflection.
(ii) The incident ray, reflected ray, and normal all lie in the same plane.
2. State and explain Snell’s law of refraction.
Answer: (i) The incident ray, refracted ray, and normal lie in the same plane.
(ii) \(\frac{sini}{sinr}\) = constant.
This constant is the refractive index of one medium with respect to another.
3. Define power of a lens. Derive its relation with focal length and mention its unit.
Answer: Power (P) = \(\frac{1}{f}\), where f is in metres.
Unit = dioptre (D).
Positive for convex lenses, negative for concave lenses.
4. Derive the mirror formula.
Answer: From geometry of a concave mirror, using similar triangles, we derive
\(\frac{1}{v}\) + \(\frac{1}{u}\) = \(\frac{1}{f}\)
5. Explain refraction through a rectangular glass slab with a ray diagram.
Answer: When light passes through a glass slab, it bends towards the normal at entry and away at exit. The emergent ray is parallel to the incident ray but laterally displaced.
6. Differentiate between concave and convex mirrors (any four points).
Answer:
| Concave Mirror | Convex Mirror |
|---|---|
| Converging mirror | Diverging mirror |
| Can form real and virtual images | Always forms virtual images |
| Used in solar furnaces | Used in vehicles as rear-view mirrors |
| Focus is in front | Focus is behind the mirror |
7. Explain the concept of refractive index with an example.
Answer: n = \(\frac{c}{v}\), where c is speed of light in vacuum and v in medium.
Example: For water, n=1.33, meaning light is 1.33 times slower in water than in vacuum.
8. A convex lens forms a real image twice the size of the object at a distance of 60 cm from it. Find the focal length of the lens.
Answer: v = 60cm, m = 2, \(\frac{v}{u}\) = 2 ⇒ u = 30cm
Using lens formula: \(\frac{1}{f}\) = \(\frac{1}{v}\)− \(\frac{1}{u}\) = 1/60 − 1/30 = −1/60
⇒ 𝑓 = 20 𝑐𝑚
Hence, the focal length = 20 cm.
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