What decides the colour of light?

[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.