The object is sufficiently far from the mirror that was incident light may be tr
ID: 2206955 • Letter: T
Question
The object is sufficiently far from the mirror that was incident light may be treated as a plane wave. The radius of curvature of the mirror is r = 5 m. The light is reflected from the curved mirror and then from the plane mirror as shown. The paths of two rays are illustrated in the diagram. (i) Calculate the distance d. Hint ? parallel light meets where? If this light is the object for a flat mirror, then where would that image be found? (ii) If d is changed to be exactly 0.75 m, what type of mirror (concave or convex) is required to replace the plane mirror such that the path of the light is identical ? i.e. the two rays still meet at the center of the right mirror. Find the radius of curvature of this new mirror.
Explanation / Answer
Had the plane mirror not been there then the rays would have converged at the focus
1/u+1/v=2/r
since,1/u=0,v=r/2=2.5m
due to symmetry d=v/2=1.25m
if d=0.75m
then u=-(2.5-0.75)m=-1.75m
v=0.75m
using 1/u+1/v=2/r
-1/1.75+1/0.75=2/r
r=2.625 m(concave mirror)