Question

The circular boundary of the concave mirror subtends a cone of half angle θ at its centre of curvature. The minimum value of θ for which ray incident on this mirror parallel to the principle axis suffers more than one reflection is

Answer 1

Answer:

An incident ray that is parallel to the principal axis will suffer multiple reflections from the mirror only when it is incident on 1 part of the mirror, reflects perpendicularly and goes on to intersect the other part. Thus, this is only possible when angle of incidence is 45

0

Thus, 45

0

+ θ+

2

π

=π

Thus, θ=45

0