Batches - PHONON

Class - XI

LENS

1.

A thin concavo-convex lens has two surfaces of radii of curvature R and 2R. The material of the lens has a refractive

index µ. When kept in air, the focal length of the lens :

(A) Will depend on the direction from which light is incident on it

(B*) Will be the same, irrespective of the direction from which light is incident on it

R

(C) Will be equal to µ 1

Sol.

From left to right

(D*) Will be equal to

R

µ 1

1

1

1

1 1

–

–

f L1 = (µ – 1) – R –2 R = (µ = 1) 2R R

µ

1

µ – 1

f L1 = – 2 R

From right to left

1

f L2 = (µ – 1)

=–

1

1

–

= (µ – 1)

2 R ( R )

convex

surface

2R

concave

surface

R

1 1

–

2R R

µ – 1

2R

1

1

2R

= f = – ( µ –1)

f L1

L2

Option (B) and (D) are correct.

2.

Sol.

An object is placed 10 cm away from a glass piece (n = 1.5) of length

20 cm bound by spherical surfaces of radii of curvature 10 cm. Find the

position of the final image formed after twice refractions.

Ans. [50 cm]

Refraction at first surface :

20 cm

air

air

B

object

A

10 cm ROC = 10cm

n = 1.5

ROC = 10cm

1.5

1

1.5 –1

– (–10) = (10) V1 = – 30 cm from A

V1

Refraction at second surface.

1

1.5

1 – 1.5

– (–50) = (–10) V2 = +50 cm from B

V2

Hence final image will be 50 cm from B.

3.

A concave mirror of radius R is kept on a horizontal table (figure). Water (refractive

index = ) is poured into it upto a height h. What should be distance of a point

object from surface along principal axis so that its final image is formed on itself.

Consider two cases.

(i) h 0

(ii) in terms of h

Page 1

( R h)

R

; (ii)

]

Ans. [(i)

Sol.

Object should appear to be at distance R from mirror.

µ(d) + h = R

if

Sol.

d=

h

h << R

4.

d

R–h

d=

µ

c

R

µ

A person's eye is at a height of 1.5 m. He stands infront of a 0.3 m long plane mirror which is 0.8 m above the ground,

The length of the image he sees of himself is :

(A) 1.5 m

(B) 1.0 m

(C) 0.8 m...