Pass light through helium-3 it slows it down?

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Is it true, if you pass light through helium-3 it slows it down? Shouldn't the light speed constant interfere with our perception of it?

By the way, I'm only half sure it's helium-3. But it's definitely helium.
 
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Light in any medium (air, water, glass, helium-3) slows down. The speed of light in vacuum is what is invariant.
 


Thanks, that really helped. :smile:
 

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