two parallel plane mirror s M1 and M2 have a length of 2m each and are 10 mm apart.A Ray of light is incident on one end of the mirror M2 at the angle of 30 degree the number of reflection light undergoes before reaching the other end is

The property of reflection in a mirror deviates the incident ray through an angle equal to the very angle of incidence. So for the two parallel mirrors, existing near vicinity of the incident ray falling at one end, could cause such deviation of the reflected ray through multiple reflections until reaching out at the other end. The angle of incident being 30° and their separation being 10mm or 1cm=>trigonometric application to the ray moved by reflections to deviate from the normal by 1×tan30°=0.577cm in every such reflections to reach the other end. Assuming the no of reflections to be N,the total deviation of the incident ray from the point of incidence is 0.577N=length of mirror ie 2m or 200cm.                            So N=(200/0.577=346. 62) as 0.62×0.577=0.35<0.577 .So the total number of reflections is 346.Ans.. N=(200/0.577=346.