In figure, AB is the principal axis of the concave mirror. A point object moves on th eline PQ which makes small angle `theta` with the principal axis. Show that image also moves in straight line making same angle ` theta` with principle axis .

Explanation

Let `tantheta=a` then equation of the line PQ is
`y=-ax` (i)
Le y=y-coordinate of image and
`x^(')`=x-coordinate of image. By using the formula of magnification,
`(y^('))/(y)=-(x^('))/(x)`
`rArr y^(')=-(y)/(x)x^(')=-(-a)x^(')=ax^(')`
`rArr y^(')=ax^(')`
This is equation of a straight line, making angle `theta` with the principal axis. `(` Hence, proved. `)`