Question

Mohan secured a concave mirror. He wanted to find its focal length more accurately. Suggest a method to Mohan to find its focal length accurately.

Answer 1

measure the length from pole

Answer 2

Keeping an object in the infinity will give you IMAGE at its FOCAL LENGTH.Thus focal length of mirror can be found.

◆FOCAL LENGTH of a concave mirror is positive .

◆MIRROR FORMULA ,

1/f = 1/v + 1/u .

◆v ,u - image , object distance respectively.

◆u is at infinity, 1/u is zero.

◆Thus, 1/f = 1/v

f= v.

◆Hence, Keeping an object in the infinity will give you image at its focal length .