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Q & A: From Spectral Bands to Rainbows

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Most recent answer: 10/22/2007
Q:
I have learned that the sun produces visible light because each of its elements has a characteristic emission spectrum. We have, for example, the "D line" emitted by sodium. Yet when I see a rainbow in the sky I do not see a collection of lines. Rather, I see a continuous display of colors ranging from red to violet. How do the emission lines produced by elements become a continuous range of colors?
- Richard Treptow
Chicago State University, Chicago, IL, USA
A:
Richard,
You are right that some of the light produced by the sun (and all stars) comes from emission spectra of specific atoms and/or molecules, but thats not the whole story. It is also true that any object that has a non-zero temperature will emit what is called "black body radiation", which is a very broad and continuous range of wavelengths whose average depends on the temperature of the object. This is what causes the continuous range of colors that makes up sunlight This is also one of the ways we can tell what the temperature a star is...since the range of frequencies of the light emitted depends on temperature, by measuring the spectrum of a star we can figure out how hot it is.

Mats

(published on 10/22/2007)

Follow-Up #1: rainbow bands

Q:
I’m told that there are 9 bands of the rainbow...two of the nine are invisible. Can you tell me more.
- kat johnson (age 56)
nevada city, ca.
A:
I don’t think there’s any special way to divide the rainbow into bands. Thye number of ’bands’ depends on how fine we make the distinctions between different colors. When people say ’two bands are invisible’ I guess they’re lumping all the ultraviolet into one band and all the infrared into the other. Ultraviolet is too high-frequency, short-wavelength to see and infrared is too low-frequency long-wavelength to see.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.