Perhaps this is a question better suited for Physics SE, but since the H-alpha line is so important in astronomy, I'm posting this here....

I would, naively, assume that wavelengths would be longer, not shorter, when traveling through media (other than the vacuum), but perhaps I am misunderstanding...

Are photons released by hydrogen atoms more energetic (shorter wavelength) when first emitted in a medium of air?

In other words, the light is not shortened (or lengthened) when traveling through a particular medium, like air, but is different to begin with?

Why would light be different in wavelength when first emitted in different media?

Is light sometimes weaker (longer wavelength) when first emitted by atoms or molecules in a non-vacuum?

I am befuddled....


1 Answer 1


This is not due to where the light is emitted, but where it is measured.

When light enters a medium, such as air, it slows down. This is called refraction (and is the reason that prisms etc can bend light). As the frequency doesn't change, the wavelength must get shorter.

The speed of light in air is about 0.03% slower in air than in a vacuum, so if you measure the wavelength in air it is about 0.03% shorter. That corresponds to a change from 656,46 to 656.28.

It is nothing to do with where the photons are emitted. If you generate the H-alpha line in the atmosphere, but measure it in a vacuum, you'll get the longer wavelength.


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