A narrow beam of pure red light goes through a prism…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A narrow beam of pure red light goes through a prism. What do you see on the screen? How does this show that the colours of the spectrum are pure?
Need a reminder? The key ideaShow
White light is a mixture of colours. Glass has a slightly larger refractive index for violet than for red, so violet is slowest and bent most: a prism spreads white light into a spectrum, red to violet.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. One colour, one index. Pure red light has one wavelength, so one refractive index. All of it is bent by the same angle.
Step 2. On the screen. A single red spot, moved toward the base, and no spectrum.
Step 3. What it shows. A prism cannot split a pure colour into more colours, so the colours of the spectrum are pure. Newton showed this.
Answer: one red spot; a pure colour cannot be split further.
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