Question

a concave mirror has a focul length of 20 cm. at what distance from the mirror shoul 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 the image formed.

Answer 1

f=-20

h=4

v=-30

u=?

h'=?

1/f=1/v-1/u

1/u=1/v-1/f

=1/-4 -1/-20

=-20+4/80

=-16/80

1/u=-1/5

u=5/-1

u=-5

m=h'/h=-v/u

h'/4=- (-30)/-5

h'/4=30/-5

h'=-6×4

h'=-24