Question

Which of the colours of light travels fastest in prism made up of glass?

• A. Red
• B. Violet
• C. Blue
• D. Speed of light in glass is independent of the colour of light

Answer 1

Answer: (A)

We know from Cauchy's expression,
$\mu (\lambda )=a+\frac{b}{{\lambda }^2}+\frac{c}{{\lambda }_4}+\dots$
or ${\lambda }_{\text{red }}>{\lambda }_{\text{blue }}>{\lambda }_{\text{violet }}$
${\mu }_{\text{red }}<{\mu }_{\text{blue }}>{\mu }_{\text{violet }}$ (for glass prism)
So, refractive index of prism for violet colour is more hence from Eq. (i), the velocity of violet colour in medium $2\left(v_2\right)$ will be less than the red colour. Red colour light will travel fastest in glass prism.