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Q.
Which of the colours of light travels fastest in prism made up of glass?
Wave Optics
Solution:
We know from Cauchy's expression,
$\mu(\lambda)=a+\frac{b}{\lambda^{2}}+\frac{c}{\lambda_{4}}+\ldots$
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.