Question

The light bulb in your room is not very bright. You want to use a concave mirror to direct its illumination into a tight, parallel beam of light for reading. The radius of curvature of the mirror is 30 centimeters. At what distance from the mirror should you place the light bulb to produce this parallel light beam?

Answer 1

Radius of curvature of a spherical mirror = 2 × Focal length
and the ray of light is parallel to the principle axis if the incident Passes through focus

therefore, focal length =30/2=15cm

The mirror should be placed at a distance of 15cm from the light bulb to produce this parallel light beam.