Question

The focal length of converging lens is measured
for violet, green and red colours. It is respectively
$f_v , f_g, f_r $ We will get

• A. $f_v < f_r $
• B. $f_g> f_r $
• C. $f_v = f_g $
• D. $f_g > f_r$

Answer 1

Answer: (A)

$\frac{1}{f} = (\mu -1) \bigg( \frac{1}{R_1} - \frac{1}{f_2 } \bigg) $
Since the refractive index of violet colour $({\mu}_v) $ is
greater than the refractive index of red colour,$({\mu}_r)$
therefore focal length of violet colour is less than
the focal length of red colour or in other words ,
$f_v < f_r $