Question

At what distance from a concave lens of focal length 20 cm should a 6 cm tall object be placed so that it forms an image at 15 cm from the lens. Also, determine the size of the image formed... ​

Answer 1

Explanation:

f=-20, h=6,v=-15

1/f=1/v-1/u=1/-20=1/-15-1/u

-1/u=-1/20+1/15=1/60

u=-60

size of image=v/u=-15/-60=1/4

so height of image is 1/4th of height of object

Answer 2

Answer:

Size of the image = 1.5 cm

Explanation:

Given : f = 20 cm

hi = 6 cm ( height of an image )

v = 15 cm

u = ?

ho = ? ( height of an object)

we know , 1 / f = 1/u + 1/ v

1 / 20 = 1 / u + 1 / 15

1 / u = 1 / 20 - 1 / 15

1 / u = 15 - 20 / 20 × 15

1 / u = -5 / 20 × 15

u = 20 × 15 / -5

u = - 60 cm

we know

ho / hi = - v / u

ho / 6 = - 15 / - 60

ho = 1/ 4 × 6

ho = 1.5 cm