Question

A concave mirror of radius 30cm is placed in water. Its focal length in air & water differ by ​

Answer 1

Answer:  focal length in air f = r/2 = 15cm

i.e. (μ-1)(1/r-1/R) = 1/15....1

in water (μ$\\_{w}$ - 1) (1/r -1/R) == 1/f    ....2

dividing 1 by 2

f/15 = u-1 /(μ$\\_{w}$ - 1)

f = (u-1 ) 15 /(μ$\\_{w}$ - 1)  = (1.5 -1) 15 / 1.5/1.33 -1

f= .5*15/ 1.128 -1 = 4.5 / .128 = 35.15 cm

difference in focal length = 5.15cm

Explanation:$\\_{w}$