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View Full Version : If a rainbow is a solar spectra why can't we see Fraunhofer lines?



Messier Tidy Upper
2009-Sep-02, 02:29 AM
If, as I understand, a rainbow is effectively a natural solar spectroscope then why can't we see the Fraunhofer (dark) lines in it? Are they there but too small at that scale - if we magnifyed a rainbow would we perhaps be able to detect them or not?

Can we use a rainbow as a crude solar spectroscope at all?

stu
2009-Sep-02, 05:34 AM
I would guess that it's because the lines are too small to be in focus and resolvable in a rainbow caused by sunlight filtering through small spherical water droplets in the atmosphere. But, I'm not certain on this so wait until at least one more person replies. ;)

Perikles
2009-Sep-02, 08:54 AM
I suppose in an ideal environment where all the drops are exactly spherical, the lines would be there, but too fine to see. I guess in practice the water drops are never exactly all the same shape, so what we see is the superposition of millions of spectra which are not exactly superimposed, but blurred to an extent that eliminates the lines.

Glom
2009-Sep-02, 12:20 PM
So here's a microgravity experiment. Try generating a rainbow using a splash of water drops in orbit. The drops should be closer to spherical then and maybe the quality would be better. Sort of high definition rainbow. Maybe then we'll be able to see the lines.

ngc3314
2009-Sep-02, 12:28 PM
The problem is that the droplets don't refract the light in completely monochromatic fashion. The usual diagram of the light paths in a droplet (even a perfectly spherical one) shows the most probable angles, but there is a spread (so each location in the rainbow includes light from a significant spread of wavwlengths, smearing spectral features out). My own tests in the classroom with a prism in front of a window show that you need very good spectral purity to see the Fraunhofer lines. This is in addition to the spread from the fact that the Sun is not a point source, which is significant compared to the angular width of the rainbow.

(Related note - some friends viewing the February 1979 total solar eclipse could make it only to a site with thin cirrus. They reported a momentarily amazing display of ice halos and arcs with astonishingly pure colors just as the third-contact diamond ring was seen).

Hornblower
2009-Sep-02, 07:19 PM
The Sun's angular diameter, which is large in proportion to the amount of dispersion, would wash out the lines even if there were no other complications.

Glom
2009-Sep-02, 09:54 PM
Okay, screw the microgravity. Instead, get a good quality prism or diffraction grating and a very stopped down aperture and... oh wait, that's how we normally detect fraunhoffer lines.

mike alexander
2009-Sep-02, 10:35 PM
Okay, screw the microgravity. Instead, get a good quality prism or diffraction grating and a very stopped down aperture and... oh wait, that's how we normally detect fraunhoffer lines.

Yup, that's it. My son built a spectroscope for a science project a few years ago. To test it we aimed it out the window on a cloudy day. Perfect lines. It's the slit that matters.

Amber Robot
2009-Sep-03, 04:03 PM
The Sun's angular diameter, which is large in proportion to the amount of dispersion, would wash out the lines even if there were no other complications.

Here's your answer. Large source, low dispersion.

Messier Tidy Upper
2009-Sep-05, 01:53 PM
Thanks everyone. Pity but I guess I have my answer. Cheers.