28 September 2011

X Solved practice Problem-Light (Reflection and Refraction)

1. Out of convex mirror and concave mirror, whose focus is situated behind the mirror?
Ans: The focus of convex mirror is situated behind it.
2. For what position of an object, a concave mirror forms an enlarged virtual image?
Ans: Object between pole (P) and focus (F) of the concave mirror.
3. If the focal length of a concave mirror is 25cm. What is its radius of curvature?
Ans: Focal length = Radius of curvature / 2
25 = R / 2  => R = 50cm.
4. A ray of light falls on a mirror normally. What are the values of angle of incidence and the angle of reflection?
Ans: Both angle of incidence and the angle of reflection are zero.
5. What is the focal length of a plane mirror?
Ans: Infinite.
6. Which spherical mirror is called a divergent mirror?
Ans: A convex mirror is called a divergent mirror.
7. What is the angle of incidence, when a ray of light falls on the spherical mirror from its centre of curvature?
Ans: The angle of incidence is zero, when a ray of light falls on the spherical mirror from its centre of curvature.
8. Name the type of mirror that always forms a virtual image for a real object?
Ans: Convex Mirror.
9. State the relation between radius of curvature and focal length of spherical mirrors.
Ans: Radius of curvature = 2 X focal length.
R = 2f.
10. Which of the two is a diverging lens? Convex lens (or) Concave lens.
Ans: Concave lens.
11. If an object is placed at a distance of 10cm in front of a plane mirror, how far would it be from its image?
Ans: The object would be at a distance of 20cm from its image.
12. Name the physical quantity which remains the same when light goes from one medium to another?Ans: Frequency of light.
13. Define power of a lens.
Ans: The reciprocal of the focal length of the lens is called power of a lens. P = 1/f
14. Calculate the power of a convex lens of focal length 25cm?
Ans: Focal length f = 25cm.
Power = 1/.25 = 100/25 = +4D.
15. Name the phenomenon responsible for the following affect.
When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand.
Ans: Lateral Inversion.
16. Define Refractive Index:
Ans: It is the ratio of speed of light in vacuum to the speed of light in medium is called Refractive index.
17. Why is the convex lens also known as a converging lens?
Ans: A convex lens is also known as a converging lens because it converges a parallel beam of light raying passing through it.
18. Give the ratio of velocities of two light waves travelling is vacuum and having wave lengths 4000A0 and 8000A0.
Ans: In Vacuum, light of all the wave lengths travel with the same velocity i.e. 3 X 108 m/sec.
19. For what angle of incidence, the lateral shift produced by parallel sided glass plate is zero?
Ans: For i = 0.
20. What are the factors on which the lateral shift depends?
Ans: Thickness of the refracting medium, angle of incidence and its refractive index.
21. What is the power of a plane glass plate?
Ans: Zero.
22. An object is placed at the focus of a concave lens. Where will its image be formed?
Ans: The rays will appear to come from infinity. Therefore, a virtual image will be formed at infinity.
23. What is the unit of power of a lens?
Ans: Dioptre.
24. Refractive index of media A,B,C and D are
A 1.54
B 1.33
C 1.46
D 1.62
In which of the four media is the speed of light (i) Minimum (ii) Maximum.
Ans: (i) Speed of light is minimum in medium D (n = 1.62).
(ii) Speed of light is maximum is medium B (n = 1.33).
25. Why the stars appear to twinkle?
Ans: The twinkling of a star is due to the atmospheric refraction of star’s light.
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