Question

A marginal ray falls upon a concave mirror of radius of curvature 20 cm as shown in figure. Find distance CM (in cm).

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Answer 1

The distance of CM is 20 / √3.

Explanation:

We are given:

• The radius of curvature of concave mirror = 20 cm
• To Find: Distance of CM = ?

Solution:

C = the center of curvature

Now CP = Radius of curvature = 20 cm

By applying Sin rule

CT / Sin (120°) = CM / Sin (30°)

20 / Sin [ 180° - 60° ] = CM / ( 1 / 2 )

20 / Sin [ 60° ] = 2 x CM

20 / ( √ 3 / 2 )  = 2 x CM

CM = 20 x 2 /√3 x 2 cm

CM = 20 / √3

Thus the distance of CM is 20 / √3.