C. A could be ruby (speed of light = 170,000 km/s); B could be diamond (speed of light = 120,000 km/s).
Explanation:
Refraction is a phenomenon that occurs when a light rays crosses the boundary between two different mediums.
When this occurs, the light wave changes speed and also direction, according to Snell's law:
[tex]n_1 sin \theta_1 = n_2 sin \theta_2[/tex] (1)
where
[tex]n_1[/tex] is the index of refraction of the first medium
[tex]n_2[/tex] is the index of refraction of the second medium
[tex]\theta_1[/tex] is the angle of incidence (the angle between the incident ray and the normal to the boundary)
[tex]\theta_2[/tex] is the angle of refraction (the angle between the refracted ray and the normal to the boundary)
The index of refraction is the ratio between the speed of light in a vacuum (c) and the speed of light in the medium (v):
[tex]n=\frac{c}{v}[/tex]
Using this definition, we can rewrite eq.(1) as
[tex]\frac{\sin \theta_2}{\sin \theta_1} = \frac{v_2}{v_1}[/tex]
Where [tex]v_2[/tex] is the speed of light in the 2nd medium and [tex]v_1[/tex] the speed of light in the 1st medium.
Now let's analyze the situation represented in the figure: we see that as the light ray enters the 2nd medium, it bends towards the normal, this means that the angle of refraction is smaller than the angle of incidence:
[tex]\theta_2 < \theta_1[/tex]
This means that
[tex]\frac{\sin \theta_2}{\sin \theta_1}<1[/tex]
And therefore,
[tex]\frac{v_2}{v_1}<1[/tex]
So, the speed of light in the second medium is smaller than the speed of light in the first medium: this occurs only in option C), which is therefore the correct choice.
Learn more about refraction:
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