Question

What is chromatic aberration?

Answer 1

This is a defect with ordinary lenses.White light contains 7 colors and ordinary lens can not focus all colors at same point.

Explanation:

Due to differences in Wave length the 7 colors focus at didfferent points.If you use this lens the stars will appear like rainbow color .
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Answer 2

At an elementary level we often teach that the refraction of light is independent of frequency, but that is obviously false, otherwise prisms would not split white light into colours.

Explanation:

The answer above is entirely correct, I just wanted to point out where I think the misunderstanding comes from. It's a problem in all optical equipment, particularly telescopes.