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Old Apr 13th 2014, 06:09 AM   #1
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Lightbulb How can we see things that don't emit photons?

How can we see things that don't emit photons? Photons is what the eye makes signals of to the brain, right? And how come things have different colors? Thanks
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Old Apr 13th 2014, 04:37 PM   #2
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Originally Posted by Anonymous View Post
How can we see things that don't emit photons? Photons is what the eye makes signals of to the brain, right?
Right - if no photons are emitted by an object you can't "see" it. But perhaps you may feel it's gravity (as in a black hole).

Originally Posted by Anonymous View Post
And how come things have different colors? Thanks
Color is function of wavelength of the light. Maybe you have a more specific question?
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Old Apr 14th 2014, 04:38 AM   #3
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But a chair don't emit photons, I can see the chair. Why can I see things that don't emit photons?
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Old Apr 14th 2014, 04:52 AM   #4
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The chair reflects light that impinges on it from other sources, such as the light bulb in a lamp. Turn off the lamp and the room goes dark - you can't see the chair anymore. The way light hits the chair and then is bounced to your eye affects how you perceive the chair - the material covering the chair wil typically absorb certain wavelengths of light and reflect others, so the light that gets to your eye has more power in some colors than others, and you perceive the chair as being a certain color. Also, depending in how the light hits the chair certain parts of the chair are in shadow, which appear darker than others thar are not in shadow. So in summary: the lamp emits photons of essentially all visible wavelengths (white light); some of those photons are absorbed by the chair and some are bounced to your eye, with the result that you see the chair's shape and color.
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Old Apr 14th 2014, 06:06 AM   #5
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Ok. What is wavelenght? Isn't photons emitted as particles? Thanks
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Old Apr 14th 2014, 07:56 AM   #6
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Light can be thought of as both a wave and a particle. Light waves have a characteristic wavelength, which is what distinguishes colors to the human eye. Photons (light particles) also have a characteristic wavelength, consistent with the finding that the energy in a photon is directly related to its wavelength: E = hv = hc/L, where h = Planck's Constant, v = frequency, c is speed of light, and L = wavelength. In classic physics the energy in a light wave would depend only on intensity, not wavelength, but the fact that certain chemical reactions can only occur when the material is subjected to light of a certain wavelength shows that the energy in light is dependent on the wavelength, and leads to the particle theory of light.

Last edited by ChipB; Apr 14th 2014 at 09:43 AM.
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Old Apr 14th 2014, 09:05 AM   #7
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Ok. So wavelength is what distinguishes colors to the human eye. Can you tell more about it? Why is it called wavelength ? Thanks
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Old Apr 14th 2014, 09:40 AM   #8
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Wavelength is an alternative term to Frequency.
The wavelength is the distance between one peak of the wave and the next.

Thus: Wavelength = Speed / Frequency

The speed is constant (for light the speed is "c") so the wavelength and the frequency are inversely proportional (when one goes up the other goes down).

Different chemicals will either absorb or reflect a photon preferentially acording to its wavelength.
This depends on the various electron "orbits" available in the chemical (as outlined in a previous thread).
If the energy difference between two available electron orbits nicely matches the energy available from the photon, then it will be absorbed.

The human eye contains 4 different chemicals (some animals have more, some less).
One is sensitive to quite a wide range of frequencies and is found in the "rod" cells.
These allow you to see in the dark, but don't differentiate colours.
The other 3 chemicals each absorb only within their own particular limited range of frequencies.
These are found in the "cone" cells in the eye, they require quite bright light and allow you to see colour.

When these chemicals absorb a photon, they send a signal to the brain.
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Old Apr 14th 2014, 10:44 AM   #9
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What do you mean by peak of the wave ? Thanks
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Old Apr 14th 2014, 10:55 AM   #10
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Originally Posted by Anonymous View Post
What do you mean by peak of the wave ? Thanks
Maybe this will help:

http://en.wikipedia.org/wiki/Wavelength
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