We said that color depends on wavelength. For example, for an object emitting 650 nm light in air, we see red. But this is true only in air.

We said that color depends on wavelength. For

example, for an object emitting 650 nm light in

air, we see red. But this is true only in air. If we

observe this same object when under water, it

still looks red. But the wavelength in water λn

is 650 nm/1.33 = 489 nm. Light with wavelength

489 nm would appear blue in air. Can you

explain why the light appears red rather than

blue when observed under water