# Spectral colours

If we shine a prism with white light we get the light spectrum.I am interested what will happen if we block all the formed spectral components but the yellow light and let the yellow pass through another prism.Will this yellow light divide and split into red and green light again?If yes why light didn't decide on first prism to form just green and red?

• no, it will not. Prism just deflects different wave-lengths at a different angle,- so called dispersion. And btw color is parameter produced just in brain, which is affected by many conditions, like brightness, weather conditions and even state of human itself. Only wave-length is meaningful physical parameter of light – Agnius Vasiliauskas Oct 1 at 17:27
• @AgniusVasiliauskas wavelength is too primitive a parameter: few natural light sources have a well-defined wavelength. It's the spectral power distribution which matters and is general enough to speak of any possible color. – Ruslan Oct 1 at 18:30
• @Ruslan, if wavelength is not defined for some source then wavelength bandwidth may be defined instead. Many parameters may be useful, except "color" - that is what i wanted to say – Agnius Vasiliauskas Oct 2 at 6:01
• @AgniusVasiliauskas I wonder how useful the bandwidth would be for solar disk at sunset... – Ruslan Oct 2 at 6:56
• @Ruslan I never said that bandwidth is always useful, nor that it is useful for solar disk - that's just your interpretation. Wave-length bandwidth is most useful for Lasers, because even most coherent laser will not emit 1 wave-length photons, but a packet of them with different wave-lengths in some bandwidth, with power distribution peak at some wave-length, usually in Gaussian form. And btw, you change topic. All I wanted to say that COLOR is not very meaningful term for measurements, thus it is not worth to concentrate on it – Agnius Vasiliauskas Oct 2 at 7:05

No, the yellow light will not split into red and green light. When you block all of the spectrum except yellow, only the 580nm(yellow) wavelength remains. This cannot further split and become 700nm(red) and 530nm(green) light.

The association that red + green = yellow is just an act of the brain. Our eyes have 3 types of cone cells which 'light up' in different intensities in response to a wave in the visible spectrum hitting the eye. When RED and GREEN are simultaneously shown, the cones fire in the same intensity as it would have in response to YELLOW light.

You can verify this- load a yellow screen on your computer. Then take a magnifying lens and look at the screen. You will see that there are no YELLOW LEDs in the display. It is just the RED and GREEN LEDs firing simultaneously.

• To readers: this possibility to "fool" the brain into seeing yellow when there's actually no yellow component in the mixture is called metamerism. – Ruslan Oct 1 at 18:28

If we shine a prism with white light we get the light spectrum.

Let's remember what happens in detail. Polychromatic light is a stream of photons with different wavelengths. At the boundary between two media - in your example between air and glass - these photons are deflected (in the case that the beam hits the glass between zero degrees and the total reflection angle). The deflection angle is different for each wavelength. High-energy photons are deflected less than low-energy photons.

... if we block all the formed spectral components but the yellow light and let the yellow pass through another prism. Will this yellow light divide and split into red and green light again?

Since the duality of wave and particle behavior of light has been established, it is not wrong to use both properties for one's thoughts. The photon is an indivisible unit from its emission (from the subatomic particles, often electrons) to its absorption. From a superficial point of view, the photons from your beam pass through air and glass undisturbed. At second glance we should agree that air and glass are not 100% transparent.

The photons from your beam interact with the air molecules and the glass molecules. In any interaction with absorption and re-emission in the air or glass, the emitted photon has less energy than the previous one. (And the air and the glass are heated.) As a result, a general redshift occurs. If you perform your experiment with yellow in series, you will get a redshifted result.