## 28 October 2012

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

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? 
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 ? 
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 ? 
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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