Question

at what distance from a concave lens of focal length 20cm a 6cm tall object be placed so as to obtain it's image at 15cm from the lens also calculate the size of the image formed

Answer 1

F = 20. [ Sign convention]

Ho = 6cm

V = -15. [ Image formed by concave Mirror is Virtual]

USING LENS FORMULA,

1/F = -1/U+1/V

Put values and solve u will get the the answer.

Using,

M = Hi / Ho = V/U [ Hi is height of Image and Ho is height of object]

Hi/ Ho = V/U

Put VALUES AND SOLVE U WILL GET THE HEIGHT OF IMAGE.

Answer 2

Given:
f = 20cm
o = 6 cm
v = 15 cm (image distance)

1\v -1\u = 1\f
1\15 - 1\u = 1\20
1\u = 1\15- 1\20
1\u = 1\60

I\O = V\U
I = 15 × 6 \ 60
I = 1.5 cm

Hope it helps!!