I studied two figures of sunlight reflecting in a raindrop:
In the first image, red is shown above violet, but in the second image, red is below violet. Are both cases possible?
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.Sign up to join this community
Clearly both diagrams cannot be correct. Look closely at the refraction of the rays as they exit. In the left image, it appears as though the rays refract towards the normal while the light is transitioning from a region of high refractive index to a region of low refractive index:
As a general rule, refractive index decreases as wavelength increases (unless you are at a resonance, where it gets a 'bump'). So you expect red to be refracted less than violet -- but both of them have to be refracted away from the normal. A better diagram for showing what an observer sees (recalling that the sun is "very far away", the incoming rays are "parallel"):
Of course I have exaggerated the angles to make clear what is going on. In reality the difference in angle of deflection is quite small - the refractive index between 400 nm and 800 nm drops from about 1.57 to 1.55, and the angles are only different by about 1.7°. I am also ignoring the fact that since the sun is not a point source, you actually get a range of angles for both violet and red light: this smears out the colors of the rainbow a little bit.
The violet ray deviates through a greater angle in both diagrams.
The diagram on the left shows what the observer sees - different colours arriving at the eye from different angles. This is a misleading diagram because the red and violet reaching the eye do not come from the same incident ray of sunlight - they come from different rays.
The diagram on the right correctly shows how the colours in a single ray are dispersed.
Both diagrams imply that the raindrops work exactly like a prism does when it projects a spectrum on the wall, and that is wrong. The error is that sunlight hits the drop across an entire hemisphere, not just one point, and the angles are different at each point.