Discussion
Red light of wavelength 632 nm in free space is incident at an angle of θ = 39° with respect to the normal on a glass slide of thickness d = 0.78 mm and index of refraction n = 1.52. The wavelength of light in glass will become
Answer: C
The wavelength in glass is about 416 nm.
When light enters a medium of refractive index n, its frequency stays the same but its speed and wavelength are divided by n:
\[ \begin{aligned} \lambda_{glass} &= \frac{\lambda_0}{n} = \frac{632}{1.52} \\ &= 415.8\ \text{nm} \end{aligned} \]
The angle of incidence and the thickness of the slide do not affect the wavelength inside the glass; they are extra data. 632 nm would mean no change, and 961 nm is what you get by multiplying by n instead of dividing.
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