## 28 October 2012

### X Refraction and Reflection :FA-Test Paper Solved

CBSE PHYSICS CLASS X        Very short Answer
1. What is radius of plane mirror?
Ans: Infinity
2. What is angle of reflection for a ray falling normally on a plane mirror?
Ans: 0
3. Why do we use convex surface for side view mirror?
Ans: Convex mirror always form virtual, smaller and upright image of object for vide range.
4. Can a virtual image be photographed?
Ans: yes.
5. What is the power of a plane mirror?
Ans: 0 because focal length is at infinity
6. When the angle of incidence is 90 , what is the angle of refraction?
Ans: 0 degree
7. What is the value of refractive index of air?
Ans: 1. {The ratio of the sines of the angles of incidence and refraction will be the same if the medium is the
same.}
8. What is the magnification produced by a plane mirror?
Ans: Plane mirror cannot magnify image and has magnification 1 because height of image formed is equal to
that of object.
9. Define power. Give its S.I unit?
Ans: power of a lens is d ability of a lens to converge or diverge an incident rays of light. it is defined as the
reciprocal of its focal length d = [1/f(m) ] Dioptre . Its S.I unit is Dioptre.
10. What is power of sun glass?
Ans: sun glasses do not converge or diverge rays of light hence have no power.
11. Between which two points related to a concave mirror should an object be placed to obtain on a screen
an image twice the size of the object? [2010]
Ans. Between centre of curvature and the principal focus.
12. What are the two factors on which the lateral displacement of an emergent ray from a glass  slab
depends ? [2011]
Ans. (i) Angle of incidence (ii) Thickness of glass slab
13.What is the minimum distance between an object and its real image in case of a concave mirror ? [2011]
Ans. Zero, when object is at centre of curvature, its real image is also formed at the same point.
14. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans. It is because, it covers a wide rear field and forms small, erect and virtual image, close to the eye of the
driver of the vehicle.
15.. Convex mirrors are commonly used as rear-view mirrors in vehicles. Suppose you are sitting in a parked car, you notice a jogger approaching towards you in the side view rnirror.The radius of Curvatureof the side view mirror is 2cm, lf the jogger is running at a speed of 5m/s,how fast the image of the jogger appears to move when the jogger is 9m away from the mirror?
Ans: Radius of curvature C =2cm ; f=c/2 = 1cm = .01m
speed of jogger=5m/s
dist of the jogger from the mirror ,u=9m
image distance at the begining=
1/v + 1/u = 1/f
1/u- 1/f = -1/v
1/v = 1/f - 1/u
=1/9 - 1/-.01
v =9/901m
image distance after 1 sec. the jogger would have moved 5 m.
u=9-5=4m
f=-.01m
1/v = 1/f - 1/u
=1/4 - 1/-.01
v = 4/401m
speed of the image of the jogger =distance / time
dist. = 9/901 - 4/401
t = 1s
speed = (9/901 - 4/401)m/s=0.00995 m/s=0.01 m/s
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