A person wants a real image of his own, 3 times enlarged. Where should he stand infront of a concave mirror of radius of curvature 30 cm?

Options

A. 30 cm
B. 20 cm
C. 10 cm
D. 90 cm

Answer

B

Explanation

As radius of curvature (R ) is negative for a concave mirror , so
`R = - 30 cm `
` :. ` The focal length of the mirror is
`f = R/2 = (-30 cm )/2 = - 15 cm `
As the person wants his real enlarged image , so magnification , `m = v/u = - 3 or v = 3u `
Accordng to mirror formula
` 1/v + 1/u = 1/f `
` :. 1/(3u) +1/u 1/(-15 cm )`
`(1+3)/(3u_ = 1/(-15 cm ) or 4/(3u) = 1/(-15 cm )`
` u = (-60 cm )/3 = - 20 cm `
- ve sign shows that the person should stand infront of the mirror