Question

Define focus and principal focus of a lens. Derive mirror formula for a concave mirror when image formed is (i) real (ii) virtual. Also give the sign convention used. [5 MARKS]

Answer 1

Each point: 1 Mark
Problem: 3 Marks

1. In case of a lens, focus is any point on, above or below the principal axis at which light rays intersect or appear to intersect and form image.

2. Principal focus is the point on the principal axis at which a parallel beam of light or light coming from infinity, travelling parallel to the principal axis after refraction through the lens comes to meet or appears to come from.

Problem: 1.5 Mark
For a concave mirror
i) When image formed is real

u is –ve
v is –ve
f is –ve

$\frac{1}{-f}$ = $\frac{1}{-v}$ + $\frac{1}{-u}$

$\frac{1}{f}$ = $\frac{1}{v}$ + $\frac{1}{u}$

ii) When image formed is virtual

u is –ve
v is +ve
f is –ve

$\frac{1}{-f}$ = $\frac{1}{v}$ + $\frac{1}{-u}$

$\frac{1}{f}$ = $\frac{1}{u}$ - $\frac{1}{v}$