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I have a question on white light bending. I know that when white light hits a prism it turns into different colors because white light is simply all the visible light in a rainbow. My question is that why are the colors in the order they are in? In a rainbow, the colors are ordered as red, orange, yellow, etc. Why are the colors ordered that way?

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  • $\begingroup$ The answer will be most appreciated. $\endgroup$ – Ethan Twu Questioner Mar 24 '18 at 18:09
  • $\begingroup$ Ok the answer has nothing to to do with physics. There are 2 reasons: language and your eyes. We will ignore language for now. Your eyes are able to detect light because light falls on your retina and specific cells get activated. Because different light has different frequencies. Hence it excites the cell in different ways. This apparent order does not have a physical reason behind it. But rather it’s how evolution made your eye. $\endgroup$ – physics2000 Mar 24 '18 at 18:14
  • $\begingroup$ There’s also the language thing. We just named the highest frequency light to violet and lowest to red. That’s just the language part. We could have also named the highest frequency light red and lowest violet! $\endgroup$ – physics2000 Mar 24 '18 at 18:21
  • $\begingroup$ These answers assume you know that the refraction angle of light by the prism is dependent on its frequency; and you are asking about colour rather thant why that is. $\endgroup$ – JMLCarter Mar 24 '18 at 18:42
  • $\begingroup$ @JMLCarter that’s the question. He’s just asking why the color order is that way. $\endgroup$ – physics2000 Mar 25 '18 at 3:27
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Let's compare the perception of sound and the perception of light/color. I'm making that comparison to underline that in order to arrive at perception of color we do a lot of processing.

Our perception of pitch is pretty straightforward. When you hear two notes (after each other or together), you can hear which one is pitched higher. The place in the ear where mechanical vibration causes specific neurons to send signals to the part of the brain that processes hearing is called 'cochlea'. Depending on the pitch of the incoming sound a specific part along the length of the cochlea resonates with the sound, triggering corresponding neurons to send signals. It's all very one-on-one.

Light has in common with sound that it is a form of vibration, and it travels in waves. But its frequency is way too high for any biological structure to resonate with it. Instead the light-sensitive cells in the retina of the eye contain molecules that resonate with incoming light. That resonance is somewhat specific, but not as specific as in the case of hearing. Interaction with incoming light causes a change of the light-sensitive molecule, and the work that the retina-cell does to revert that change triggers it to send signals.

The eye has three slightly different light-sensitive molecules such that by comparing response to incoming light we get some sense of the composition of the incoming light.


The color we call 'red' is the color with the longest wavelength.

Prisms and water droplets are not the only things that affect the propagation direction of light. As light travels through the atmosphere it has a small probability of being scattered. The short wavelengths (the colors we perceive as shades of blue) are more prone to being scattered.

In the evening and in the morning the Sun is close to the horizon and the light has to travel through a lot of atmosphere to get to us. In general the composition of the light from the Sun is such that we will perceive that as white light. But with the atmosphere scattering the blue end of the spectrum, we often see the Sun having a red color when the Sun is close to the horizon.

The Sun gives us warmth, so naturally all us humans will associate the color we call 'Red' with warmth. Conversely, we will all associate absence of any hint of red (the blue colors) as comparatively cold.

We perceive Yellow as a particularly bright color, because we perceive incoming light as yellow when it has such a composition that it triggers two of our three light-sensitive molecules in about equal measure. Put differently: we perceive a mix of different colors of light as Yellow when it has the Red portion and The green portion of the spectrum in it, but little blue portion.

To get from light entering the eye to the perception of color by the visual cortex is a long way; it involves stages of processing information. It is remarkable that we experience different colors so vividly different, given that the perception of color is very much a constructed perception.

I suppose that our brain makes the different colors so vividly different by having specific strong associations. For instance, it seems to me that we will naturally associate the spectrum of greens that we perceive with the things around us that are predominantly green. It seems to me that these associations, not all of them conscious associations, allow us to feel a lot of distinction between the colors we perceive.

Your question, 'why are the colors ordered that way'.
One answer to that is that all humans have the same genes for a set of three different light-sensitive molecules, and the responses of the cells containing these respective molecules are processed the same by all humans, so you get for all people pretty much the same pattern of associations between colors and things like warm/cold, scents, flavors, etc. Hence for all people the pattern of associations, from one end of the spectrum to the other, will be the same.

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  • $\begingroup$ It is interesting that a tribe exists for which different (achromatic) vision leads to appreciation of more degrees of gray so that greys got their name. And things like that. But I have forgotten where I read that. $\endgroup$ – Alchimista Mar 24 '18 at 20:42
  • $\begingroup$ @Alchimista That would be the inhabitants of the atol Pingelap, where 10% of the population has a genetic sight disorder, which - among other deficiencies - causes them to have no color vision. One individual, who struggles in full daylight because that is too bright for him, is known to see somewhat better in almost-entirely-dark circumstances. It is speculated that parts of his visual cortex, normally used for color recognition, have been repurposed to aid in processing signals from the rods. $\endgroup$ – Cleonis Mar 24 '18 at 22:00
  • $\begingroup$ Also I remember a group for which there is not green but lots of blue and pale blue recognition/names. I often mention processing here in SE. Many questions "confuse" hf and colour :) $\endgroup$ – Alchimista Mar 24 '18 at 22:17
  • $\begingroup$ Also I remember a group for which there is not green but lots of blue and pale blue recognition/names. I often mention processing here in SE. Many questions "confuse" hf and colour :) $\endgroup$ – Alchimista Mar 24 '18 at 22:20
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Violet bends more than red as refractive index for violet is more than red. Higher the vibration frequency higher it bends.

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