LIGHT – REFLECTION AND REFRATION

Q1:- Define 1 dioptre of power of lens?
Ans: – One dioptre is the power of a lens of focal length one metre.
Q2:- Why do you prefer convex mirror as a rear-view mirror in vehicles?
Ans: – A convex mirror produces an erect and highly diminished image of the objects.Further, since it is curved outwards; it gives a wider field of view.
Q3:- The refractive index of diamond is 2.42.What is the meaning of the statement?
Ans: – It means that light travels 2.42 times faster in vacuum than in diamond.
Q4:- One half of a convex lens is covered with a black paper. Will this lens produce a complete image of that object? Verify your answer experimentally. Explain your observation.
Ans:- Yes ,it will produce a complete image of the object.
This can be verified experimentally by observing the image of the distant object like tree on a screen when lower half of the lens is covered with a black paper. However, the brightness of the image gets reduced.

Q5:- How can you distinguish between a convex mirror, a plane mirror and concave mirror?
Ans: – With the help of virtual image of three mirrors, we can distinguish between three mirrors-
a) Plane mirrors will produce virtual image of the same size.
b) Concave mirrors will produce magnified virtual image.
c) Convex mirror will produce a diminished virtual image.

Q6:- The magnification produced by a plane mirror is +1.What does this mean?
Ans:- m = h’/h=1
Therefore, h’ = h
i.e, the size of the image is equal to the size of the object. +ve sign of m indicates that the image is virtual.
Q7:- Calculate the distance at which an object should be place in front of a thin convex lens of focal length 10cm to obtain a virtual image of double its size?
Ans:- f = +10cm
m = +2 (as the image is virtual)

As, m = v/u = 2
Or, V = 2u
As, 1/v -1/u = 1/f
Or, 1/2u – 1/u = 1/10
Or, – 1/2u = 1/10
Therefore u = -5cm
The object should be place at a distance of -5cm from the lens.
Q8:- A concave mirror of focal length 20cm is to be used as a shaving mirror. Which of the following is the suitable object distance from the mirror? Justify your answer.
a) 10cm
b) 20cm
c) 30cm

Ans:- For using concave mirror as shaving mirror, u
Q9:-Light enters from air into diamond. Which has a refractive index of 2.42? Calculate the speed of light in diamond. (The speed of light in air is 3 x 108 m/s)
Ans:-
As, n = c/v
V = c/n
= 3 x (10)8/2.42
= 1.24 x 108 m/s
Q10:- A concave mirror of focal length 15cm forms an image of an object kept at a distance of 10cm from the mirror. Find the position, nature and size of the image formed by it.

Ans:- Here ,
f = -15cm
u=-10cm
v=?
1/v= 1/f -1/u (mirror formula)
1/v= 1/ (-15) -1/ (-10)
= 1/10 – 1/15
= 3/30 -2/30
=1/30
Therefore, v = 30cm
Thus, the image is formed at 30cm from the mirror.
As, v is positive, the image is virtual and erect
As, m = -v/u
= – 30/ (-10)
=3
Therefore, the size of the image is three times the size of the object.http://ping.fm/LUzYx