What decides the colour of light?

[Apoorv3012]

Consider a beam of light passing through a slab of some refractive index.
We know that the speed and wavelength of the light changes, but its frequency remains the same.
Since the wavelength of the light changes, does its colour change, or does it remain the same as its frequency remains the same.

Does the wavelength really change after refraction?

 

[DaveC426913]

Does the wavelength really change after refraction?

Yes.

 

[Apoorv3012]

Yes.

Thanks Dave ,I read some further on other threads and understood that happens because the light's speed change per medium and since it's frequency remains constant, the wavelength must change.

 

[evan-e-cent]

All the above discussion is very interesting with different perspectives from people coming from different backgrounds, and with different areas of expertise.

I was about to comment on the fact that the basic chemical structure of the light sensing molecule Retinal is the same in all the different color sensing cones but that it may be influenced by hydrogen bonding, weaker Van der Waal's forces, and other electromagnetic effects of the surrounding protein structures when Yggdrasil gave us a much more eloquent description and demonstrated a beautiful example.

Is it true that the speed of light varies with wavelength? I assume this also applies to the speed of light in a vacuum?

I had not appreciated that, but this Wikipedia article uses it to explain how a prism separates colors. Here's the quote.

For electromagnetic waves the speed in a medium is governed by its refractive index according to

where c is the speed of light in vacuum and n(λ0) is the refractive index of the medium at wavelength λ0, where the latter is measured in vacuum rather than in the medium. The corresponding wavelength in the medium is

When wavelengths of electromagnetic radiation are quoted, the wavelength in vacuum usually is intended unless the wavelength is specifically identified as the wavelength in some other medium. In acoustics, where a medium is essential for the waves to exist, the wavelength value is given for a specified medium.

The variation in speed of light with vacuum wavelength is known as dispersion, and is also responsible for the familiar phenomenon in which light is separated into component colors by a prism. Separation occurs when the refractive index inside the prism varies with wavelength, so different wavelengths propagate at different speeds inside the prism, causing them to refract at different angles. The mathematical relationship that describes how the speed of light within a medium varies with wavelength is known as a dispersion relation.

 

[evan-e-cent]

I asked "Has anyone wondered why water is perfectly clear". No one took my bait. Well I will give you my answer anyway.

Vision evolved in water, in species that lived in the sea, or otherwise had eyes that contain water. Any wavelength of light that is absorbed by water would not penetrate the eye and therefore would be invisible. So vision evolved to avoid these wavelengths. Consequently water MUST be transparent as a requirement of the evolutionary process.

 

[rootone]

That kinda makes sense when thinking about UV and beyond, but Infrared and below goes through water quite well, apart from a quite narrow band in the microwwave region, or maybe I'm wrong about that.

 

[collinsmark]

I asked "Has anyone wondered why water is perfectly clear". No one took my bait. Well I will give you my answer anyway.

Vision evolved in water, in species that lived in the sea, or otherwise had eyes that contain water. Any wavelength of light that is absorbed by water would not penetrate the eye and therefore would be invisible. So vision evolved to avoid these wavelengths. Consequently water MUST be transparent as a requirement of the evolutionary process.

I never thought about it like that. Nice insight! Eyes and their photoreceptors wouldn't be so useful if their peak sensitives were at parts of the spectrum readily absorbed by water or even the eyes' own fluids.

That goes along with another nice thing that the peak power spectral density of sunlight falls within the visible range.

 

[sophiecentaur]

I asked "Has anyone wondered why water is perfectly clear". No one took my bait. Well I will give you my answer anyway.

I guess that one reason for our visible range of wavelengths is primarily based on the spectrum of the Sun's light and having a response that covers the black body spectral peak. The fact that water has an appropriate trough in its absorption spectrum is just 'good luck' on our part - mainly on account of the ability of plants to photosynthesise under a significant layer of water. It is another Goldilocks situation, if you like.
As you say, vision developed under water and the photochemistry of vision evolved around this window, just as with photosynthesis. But the absorption of water shows a sharp slope in our visual region (see this wiki link). Even at a depth of a few metres, the red is severely attenuated and colours become bluer and bluer. Our colour vision was evolved in an air environment, I think. This wiki article says that many fish have UV sensitivity in their colour vision- which would make sense, as there's so little red to be seen down there.

 

[Ygggdrasil]

Another reason why vision is confined to the near-IR to near-UV has to do with the fundamental chemistry behind vision. Recall that vision relies upon the absorption of light by a chromophore molecule. The amount of energy that it takes to excite the chromophore is on the order of a few 100 kJ/mol, which corresponds to the energy of photons in the visible range.

If we wanted to engineer an opsin to be sensitive to much lower frequencies, what energy would we want our chromophore to absorb? For example, if you wanted to detect radio frequencies (around 100 MHz), each photon would contain about 0.04 kJ/mol. Therefore, the energy required to excite the chromophore molecule would have to be about 0.04 kJ/mol.

Do you see the problem here? At human body temperature (37^oC, 310K), the amount of thermal energy available is about 2.6 kJ/mol. This means that thermal energy alone will be enough to activate our hypothetical radio-sensitive opsin! The protein would not actually be able to sense radio waves because it would always be on regardless of whether or not radio waves were present.

If you look at calculations like these, you'll see that it is not an accident that animal vision is limited to a small range of the EM spectrum ranging from the near IR to the near UV. At frequencies significantly below the visible region, you get to the point where thermal energy becomes more energetic than the photons and a chromophore would not be able to distinguish thermal energy from the absorption of photons. At frequencies significantly above the visible region, you get into the range of ionizing radiation – photons so energetic that their energies are comparable to the activation energies for breaking chemical bonds. Thus, photons in the far UV and above would destroy components of the cells meant to detect them.

 

[sophiecentaur]

At human body temperature (37oC, 310K), the amount of thermal energy available is about 2.6 kJ/mol. This means that thermal energy alone will be enough to activate our hypothetical radio-sensitive opsin! The protein would not actually be able to sense radio waves because it would always be on regardless of whether or not radio waves were present.

What you would have is effectively a receiver with a really lousy noise figure and you'd have to use a different system for detection. An electronic circuit could do it easily by operating with a narrow enough bandwidth - but that is not a QM based method.
Also, our bodies can cope with existing levels of UV and I should imagine that an alternative biological detection system could work. Again, there are electronic methods for UV imaging. I would suggest that the reason that animals do not use frequencies far outside the human visual range is likely to be that the illumination levels are so much lower. Where it is actually advantageous, some animals can see into IR and UV regions that we can't. We never evolve any characteristic that is not useful to us (or good value, energetically speaking). Lazy development can often correspond to good engineering. (But we would never do it that way because we don't think that way)

 

[collinsmark]

A bit of humor from today's Saturday Morning Breakfast Cereal.

 

[Eltahawy]

HUH? Frequency and wavelength are inverse properties. How can one change and not the other?

For a constant speed of light, but the speed of light changes at different materials

 

[phinds]

For a constant speed of light, but the speed of light changes at different materials

If you are going to respond to a post, be sure you read all of the relevant posts. The issue you are addressing was completely resolved in posts 6, 9, and 10 and you have simply repeated what was said there.

 

[evan-e-cent]

So do we now move onto the mechanism of infrared detection in snakes? Of course IR is still considerably higher energy than the radio waves we were discussing.

I also wonder whether this consideration of energetics has any relevance to the mass fear that radio frequencies could cause problems such as brain cancer in humans. If the energy level is so low it is masked by random thermal energy there probably isn't much chance that it would disrupt covalent bonds such as those in DNA. Ultraviolet light break bonds, and that is how it causes skin cancer. Cells have repair mechanisms to repair DNA damaged by UV light. But that is getting way off topic.

 

[evan-e-cent]

That goes along with another nice thing that the peak power spectral density of sunlight falls within the visible range.

The graph is interesting in that it shows visible light corresponding with the peak produced by the sun. But equally interesting that it does NOT correspond with the light we actually see on the surface of the Earth at sea level!

 

[evan-e-cent]

It may be of interest that DNA has absorption peaks in the UV range and if cell nuclei lay over the retina as they do in human eyes they might block UV light. They do block some visible light too. Sharks living underwater have much smarter eyes. They position the supporting cells behind the light sensing components. This would give better low light detection and improve vision on the UV spectrum under water. They and other animals have reflectors behind the retina to return photons that did not get captured on their first pass through the light detection layer.

I suppose human eyes have not needed to make these adaptions as most of us are not nocturnal (except me). In fact there are bigger problems caused by having too much light in daylight. There are cells overlying the retina in human eyes that expand in bright light and "deliberately" block some of the light so that it does not reach the retina where is may cause rapid depletion of the cis-retinal. Like altering the ISO rating of the film in a camera. Of course the eye also adjusts the pupil size in bright light, similar to adjusting the aperture in a camera, but the eye does not have a shutter speed, so instead it adjusts the sensitivity of its detectors. But the neurons and blood vessels also lye on top of the retina in humans and I often wondered why they are not located under the retina to improve night vision when predators may be trying to eat us in the dark!

 

[evan-e-cent]

By the way blocking UV light in the human eye may have another purpose since melanoma of the retina can occur with UV exposure.

A more common problem is the formation of cataracts, especially in the tropics. This occurs when UV light exposure causes the lens of the eye to become opaque, blocking light from reaching the retina and causing blindness. Of course this can be repaired by replacing the crystaline lens with a plastic one. The plastic lenses were very expensive but an old friend of mine in NZ worked out how to make them for a dollar or two in his garage at home and was knighted by the Queen for helping restore sight to thousands of people in the Pacific Islands - Sir Say Avery. A case of scientific knowledge have a very useful application. Keep talking! Apologies for the verbal diarrhea!

 

[sophiecentaur]

melanoma of the retina can occur with UV exposure.

Which is more of a problem with animals that live to a great age, i believe - such as us humans. I wonder if this is relevant in parrots and tortoises, too.

 

[BluberryPi]

Think about the spectrum of electromagnetic radiation.
When the frequency of the wave is increasing, the energy is increasing. It goes from infrared to red, to orange, to yellow, to green, to blue, to indigo to violet. The colors of the rainbow reflect the electromagnetic spectrum; ROY G BIV can help you understand this.
When the light rays from the sun come to earth, they reflect off objects. However, these objects absorb some frequencies of the light from the sun. Then they reflect a certain frequency of light that the didn't absorb. This is the objects color. Then our eyes perceive the light.
So I guess he/she is asking what makes an object reflect only one frequency of light, and what characteristic of that object affects the frequency of light reflected.
Correct me if I'm wrong.

 

[sophiecentaur]

what makes an object reflect only one frequency of light,

Can you think of any object that does that?

 

[bligh]

Wow! Am I the only one who interprets color as a resulting loss of frequency from the process shown below?

Imagine a single photon entering a glass prism. It immediately encounters an electron, proton, or atom, and is absorbed in an instant.
It is almost instantly released in a slightly different angle and missing a tiny amount of frequency. This absorbed energy "heats" up the glass a tiny amount. The photon continues to the next collission. In between collisions it always travels at the SOL.
Totally different view of what I read above.
I think also supported by the so-called Stopping of Light in laboratories by using temperatures close to absolute zero. Abs zero is the normal temp of the fundamental field. The photon travels in the fundamental field. The glass prism is of a much higher density of quantum waves in a localized space.
B

 

[sophiecentaur]

It immediately encounters an electron, proton, or atom, and is absorbed in an instant.
It is almost instantly released in a slightly different angle and missing a tiny amount of frequency.

This is not the accepted description of what happens. Firstly, the frequency does not change and secondly, the interaction is with the bulk of the structure and not with individual particles. If the interaction were with individual particles, the phase of the re-radiated wave would be random and it would not be the same as the rest of the re-radiated waves. The 'ray' would disperse. The physics of 'many particles together' is not the same as that of the Hydrogen atom we learn in our first QM lessons. Have a look at this wiki article.