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Q & A: Dark lines in Prism spectrum

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Most recent answer: 01/01/2016
Q:
When we magnify a beam of light that's past through a prism we see black lines. My question is are these assumed as dark light and if so do they travel at the same speed?
- Alan (age 49)
England
A:

A prism is used to separate different wavelengths of light from an incoming light source. White light source (e.g., light from sun) is usually a mixture of different wavelengths. Sometimes, a light source could be just one wavelength (e.g., light from a laser pointer) and thus it will not show you any other colors when put through a prism.

A prism does not magnify the light that is fed to it, it just "directs" the different wavelengths to certain directions so that they are easily noticeable, and the pattern is called a spectrum. When we see dark lines in a spectrum, they correspond to certain wavelengths being missing due to absorption by matter (in the form of atoms/molecules) on their way. So the dark line represents "absence of light" in a spectrum, not any particular wavelength (color) of light.

Mithun


(published on 01/01/2016)

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