Optics 7 Question 15
15. White light is incident on the interface of glass and air as shown in the figure. If green light is just totally internally reflected then the emerging ray in air contains.
(2004, 2M)
(a) yellow, orange, red
(b) violet, indigo, blue
(c) all colours
(d) all colours except green
Show Answer
Solution:
- Critical angle $\theta _C=\sin ^{-1} \frac{1}{\mu}$
Wavelength increases in the sequence of VIBGYOR. According to Cauchy’s formula refractive index $(\mu)$ decreases as the wavelength increases. Hence the refractive index will increase in the sequence of ROYGBIV. The critical angle $\theta _C$ will thus increase in the same order VIBGYOR.
For green light the incidence angle is just equal to the critical angle. For yellow, orange and red the critical angle will be greater than the incidence angle. So, these colours will emerge from the glass air interface.
