Question

A 4 cm heigh object is placed at a distance of 60cm from a concave lens of focal length 20cm.Find the size of the image ? ​

Answer 1

Given:-

• Object Height (ho) = 4cm

• Focal Length (f) = -20cm

• Object Distance (u) = -60cm

To Find:-

• Size of the Image (he)

Solution:-

By using lens formula

⟹ 1/v -1/u =1/f

Put the value, we get

⟹ 1/v = 1/u + 1/f

⟹ 1/v = 1/(-60) + 1/(-20)

⟹ 1/v = -1/15 cm

⟹ v = -15cm

The magnification

⟹ m = he/ho = v/u

⟹ m = he/ho = -15/-60

⟹ m = he/ho = 1/4

⟹ he = ho/4

⟹ he = 4/4

⟹ he = 1 cm.

∴ The image is 1 cm high . The positive sign shows that it is erect .