Q. The wavelength of a certain colour in air is 600 nm. What is the wavelength and speed of this colour in glass of refractive index 1.5?
A) $500\ nm\ and\ 2 \times 10^{10}{cms}^{- 1}$
B) $400\ nm\ and\ 2 \times 10^{8}{ms}^{- 1}$
C) $300\ nm\ and\ 3\ \times 10^{9}{cms}^{- 1}$
D) $700\ nm\ and\ 1.5 \times 10^{9}{ms}^{- 1}$
Explanation
(b) Wavelength of a certain colour in air $\lambda_{air} = 600\ nm.$
Wavelength of a certain colour in glass of refractive index $\mu = 1.5$
$\therefore\lambda_{glass} = \frac{\lambda_{air}}{\mu_{glass}} = \frac{600}{1.5}$
$\lambda_{glass} = 400\ nm$
$Also,\ v_{glass} = \frac{v_{air}}{\mu_{glass}} = \frac{3 \times 10^{8}}{1.5}$
$v_{glass} = \ 2.0\ \times \ 10^{8}\ {ms}^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.