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