Question

a lens produces An erect image of size 6 cm when an object of size 3 cm is placed 20cm from optical centre .

a) identify the nature of the lens .

b) find the magnification produced .

c) indicate the position object with respect to focus of the lens to obtain an image of the type given .

d) calculate image distance and focal length .

e) find a power of this lens .​

Answer 1

Answer:

a) convex lens

b)3

c)

d)

Answer 2

Given :

- Size of image, Hi = 6 cm

- Size of object, Ho = 3 cm

- Object distance, u = 20 cm

To find :

a) identify the nature of the lens .

b) find the magnification produced .

c) indicate the position object with respect to focus of the lens to obtain an image of the type given .

d) calculate image distance and focal length .

e) find a power of this lens .

Solution :

a) Convex lens

b) magnification, m = Hi / Ho

= 6 cm / 3 cm

m = 2

c) Position of object is in between optical centre and focus.

d) We know that,

m = Hi / Ho = v / u

Hi / Ho = v / u

v = Hi×u / Ho

= 6×(-20) / 3

v = - 40 cm ( left side of the lens )

Also,

1/f = 1/v - 1/u

= 1/(-40) - 1/(-20)

= -20 + 40 / 800

= 1 / 40

f = 40 cm

e) Power, P = 1 / focal length

P = (1 / 40)×100 m

P = 2.5 D