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)

$f_v < f_r$

Answer 2

$\frac{1}{f} = (\mu -1) \bigg( \frac{1}{R_1} - \frac{1}{f_2 } \bigg)$ 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 , $f_v < f_r$