Question

The image of a needle placed 10 cm from a lens is formed on a wall 20 cm on the other side of the lens. Find focal length of the lens and size of image, formed, if the size of object needle is 2.5 cm.​

Answer 1

given v =20cm

u=10cm

h1=2.5cm

now by using lens formula

1/v-1/u=1/f

1/20+1/10=1/f

1/f=3/20

f=20/3=6.67cm.

focal length =6.67cm

now.....

v/u=h2/h1

20/10=h2/2.5

h2=2.5*20/10

h2=5cm.

so size of image is 5cm.