Question

A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is:

• A. Blue
• B. Green
• C. Violet
• D. Red

Answer 1

Answer: (D)

Red

Answer 2

Since $\lambda _ { \mathrm { r } } > \lambda _ { \mathrm { g } } > \lambda _ { \mathrm { b } } > \lambda _ { \mathrm { v } } , \mu _ { \mathrm { r } } < \mu _ { \mathrm { g } } < \mu _ { \mathrm { b } } < \mu _ { \mathrm { v } }$

$\left( \because \mu = a + b / \lambda ^ { 2 } \right)$

$\therefore \mathrm { v } _ { \mathrm { r } } > \mathrm { r } _ { \mathrm { g } } > \mathrm { v } _ { \mathrm { b } } > \mathrm { v } _ { \mathrm { v } } \quad ( \because \mathrm { v } = \mathrm { c } / \mu )$

After travelling through the slab, red colour will emerge first because it has largest velocity in slab.