Saved Bookmarks
| 1. |
Define the term 'focal length ofa mirror. With the help of a ray diagram, obtain the relation between its focal length and radius of curvature. |
|
Answer» Solution :Distance between PRINCIPAL focus and pole of a mirror is called its focal length. Let C be the CENTRE of curvature of a spherical mirror. Consider a ray OM travelling parallel to the principal AXIS of mirror and incident at a point M. Obviously CM is normal at M. The reflected ray passes through the focus point. Let MD be the perpendicular drawn from M principal axis of mirror. If angle of incidence `angleOMC =theta,` then `angleCMF= angleMCF=theta` and `angleMFD = 2THETA`. For a mirror of small aperture `theta` = tan `theta` = and `2 theta` `= tan 2 theta = (MD)/(FD) ` `implies (MD)/(FD)= (2MD)/(CD) implies (1)/(FD)=(2)/(CD) implies FD = (CD)/(2)` For small aperture the point D is very close to pole P and so we have `FP = (CP)/(2) or F = (R )/(2) or R = 2f `. 
|
|