Question

Answer thisa concave mirror has a focal length of 20 cm . at what distance from the mirror should a 4 cm tall object be placed so that it forms an image at a distance of 30 cm from the mirror . also calculate the size of image​

Answer 1

f= -20 cm

h= 4 cm

v=-30 cm

Applying mirror formula ,

1/f = 1/u + 1/v

1/-20 = 1/u + 1/-30

1/-20 = 1/u - 1/30

1/-20 + 1/30 = 1/u

On solving,

1/u= 1/-60

u= -60 cm

Size of image (h'),

h' / h = -v / u

h'/4 = -(-30)/-60

h' = - 2 cm

Therfore the object should be placed at a distance of 60 cm.

The size of image is -2 cm

Answer 2

f = -20 cm

h = 4 cm

v = -30 cm

u = ?

1/u + 1/v = 1/f

1/u = 1/f - 1/v

= 1/-20 - 1/-30

= 1/-20 + 1/30

= -1/60

u = -60 cm

m = h'/h = -v/u

h'/4 = - (30/60)

h' = -8 cm