Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Green light from black body radiations...?

Learn more physics!

Most recent answer: 04/16/2015
Why isnt the green light is emitted from the black body radiations...
- Prudhvi Raj Borra (age 16)
Well, in fact,  there is green light.  Black body radiation contains all of the wavelengths.  The reason that it doesn't look green is that the longer wavelength light, like infrared and red, overwhelm the amount of green light.  See:  for a nice discussion.


(published on 12/10/2009)

Follow-Up #1: Color temperature

Is color temperature directly proportional to the distance and intensity of its light source? While the light emitted by the sun shows a wide spectrum of colors from blue to pink, I believe this was made possible by such factors like the Earth's atmosphere and gravity. Does this also apply on small-scale artificial lighting such as LED and fluorescent lighting or is this type of lighting too weak to produce this effect?
- Alvin (age 32)

The fact you are referring to is not a solely atmospheric artefect. However, atmosphere still has an effect, because blue is scattered more. This makes the sky blue by scattered light, but the sun itself will have a slightly redshifted spectrum than it actually is. This becomes more drastic during the sunset and sunrise: blue is scattered, so what you are left in the direct beam is the red fraction.

What determines the original spectrum of a star mainly depends on surface temperature. Some emitters can be considered as black bodies, for which there is a relation called , which estimates the maximum emission wavelength of a source. A black body is totally conceptual: something that absorbs everything shone on it, but thermal radiation based sources can be assumed to be so. Sun is yellowish, corresponding to a surface temperature about 5000K. And the same can be used to approximate the temperature of the tungsten filaments of your incandescent bulbs, because they are "hot" light sources.  There is the Hubble effect that causes higher redshift for distant galaxies, but there is not an appreciable distance or gravity dependence on human scales. Distance basically reduces the intensity of the whole spectrum  by 1/r2.

For other sources, color temperature is still defined, but does not necessarily have something to do with their actual operating temperature. LED is a solid state device, it is a "cold radiation". Green led is not emitting at a wavelength shorter than a red one because of its temperature, but this rather is because of the fine tuned energy gap between transitions. In fact, they operate slightly above the room temperature. LED emission is a quantum phenomenon: certain discrete energy levels and certain transitions between them are allowed. Emission spectrum depends on those energy gaps, which the manufacturer can control by its composition. Fluorescent bulbs do not fall into hot light sources, either. The circuitry basically provides emission of electrons from the filaments along the tube, which ineleastically collide with the discharge gas within the tube. This excites/ionizes the gas, but this state is unstable so it returns to its ground state by emitting high energy photons. As these fall into the UV regime, they are invisible (but harmful) so a conversion is performed by a fluorescent material coating the glass tube. Basically, the absorbed UV photon energy is converted to less energetic visible photons accompanied by heat release. So the spectrum again depends on the emission spectrum of the fluorescent material, which is determined by quantum mechanics. Temperature may still cause changes in the spectrum due to occupancy levels of energy states (and hence transition probabilities), though.


(published on 04/16/2015)

Follow-up on this answer.