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:
(1)
where
is the index of refraction of the first medium
is the index of refraction of the second medium
is the angle of incidence (the angle between the incident ray and the normal to the boundary)
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):
Using this definition, we can rewrite eq.(1) as
Where is the speed of light in the 2nd medium and
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:
This means that
And therefore,
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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