Question

A concave mirror has a focal length 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 formed .
This is question of my sister board exam 2019 class 10th she want to know answer. ​

Answer 1

f = -20cm h = 4cm

v = 30cm

But mirror formula

1/v + 1/u = 1/f

1/u = 1/f - 1/v

= 1/-20 - 1/30

= -1/12

u = -12cm

m = -v/u = h'/h

-30/-12 = h'/4

10 = h'

h' = 10cm

Answer 2

f=  -20  and h of object = 4        v=  -30

find u and height of image

formula is   1/v+1/u=1/f

1/u= 1/-20-1/(-30)

    =1/-20+1/30

    = 3-2/60

    =1/60

as we know u is always negative

so u=  -60

magnification=-v/u

m=-(-30)/-60

m=  30/-60

m=-1/2

m=h of image /h of object

1/-2=h of image/4

4/-2=h of image

-2 =h of image