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Q & A: Objects and their colors

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Most recent answer: 01/23/2016
Q:
Why do objects emit only particular colors?
- Jack Rayen (age 14)
India
A:

So, here there are two different phenomena: some objects emit light, while some objects just have a color for reflected light. We shouldn't confuse them. Objects that emit visible light need energy. They may be just very hot or have some other way of channeling energy to the atoms or molecules that emit the light. Other objects are seen just by the light that reflects off them.

Surfaces that are good at emitting certain frequencies of light are also good at absorbing the same frequencies. So if an object glows say red when energy is supplied to it, that means it is also good at absorbing red light. If you see it just in reflected light, the red will tend to be absorbed and missing from what you see. You'll see the complement of red light, bluish-green cyan.

What makes some frequencies easy to emit or absorb? Optically active particles are often molecules with a particular range of energies for their states. Each "photon" of light emitted or absorbed has the energy that corresponds to the difference between the energies of two states of the particle.  The frequency ("color") of the light is just the photon energy times a constant conversion factor. If the frequency falls into the visible range, you will see it with your eyes. And different frequencies of this light we characterize with a color our brain recognizes. Not all the colors we see correspond to a particular frequency (energy) of the light. Some colors are mixtures of those energies: for example, when you have several types of molecules emitting light or different energy levels of the same molecule.

1. Here's some examples of objects emitting light:

Sun - it's almost white meaning that all the frequencies of the visible spectrum are present. The energy comes from nuclear fusion;
glow in the dark toys - they absorb energy while in the light, store it for some time, and then reemit it, so you can see it glowing in the dark;
fluorescent bulb - the inside of the bulb is covered with a special material, that absorbs ultrviolet light from inside the bulb and re-emits it in the visible range. The ultraviolet light is made by collisions of plasma particles inside the bulb driven by the electrical voltage;
flashlights, powered by energy from batteries;
your cell phone screen, LEDs, fire, and so on.

2. Not all the objects are that interesting though. Most of them don't emit visible light. But they can reflect/absorb the light that is shone onto them. That's why most of the objects we see in a well-lit environment have color. They are made of different materials and have different energy levels of the molecules, again. Though they can't emit anything in the visible range, sometimes they can absorb visible light from the spectrum. For example, a bright red ball absorbs most of the visible colors, but reflects red back to your eyes. That's why under white light you see it as red. You will not see it in the dark because it doesn't emit any light. A black object absorbs almost all the visible light and reflects very little to our eyes. White objects absorb almost nothing and reflect the light back. Usually the light is reflected randomly if the surface is not smooth, and then we see the white color. If it's a mirror-like surface, it reflects the light so perfectly, that you can see the image of everything around that mirror. Also, that's why black objects heat up fast in the sun: they absorb a lot of energy from the light.
NB: objects can also reflect light differently depending on their internal order and molecular/atomic composition. They can vary from being almost perfectly reflective to being completely transparent.

Yulia M.


(published on 01/02/2015)

Follow-Up #1: sensitivity to colors

Q:
Why are our eyes more sensitive to some colours and less to others?Like more to yellow and less to blue?Thanks!
- Shrey Khandelwal (age 15)
India
A:

This has been briefly discussed here before: .

Briefly, our eyes pick up signals via 4 types of cells (rods and 3 types of cones) that absorb different frequencies of light, triggering chemical reactions that start nerve signals. The sensitivity to different frequencies of light depends on how well each type of cell absorbs that frequency and how many of that type of cell we have. It varies from person to person. In particular, some color-blind people are missing one or more cell type.

If you ask about "why" looking farther back than mere anatomy, the approximate typical color sensitivity pattern would have been an easy target for natural selection, once the different types of rod and cone cells arose. I don't know what the particular selective advantages of different sensitivities would have been over most of our evolutionary history.

Mike W.


(published on 01/05/2015)

Follow-Up #2: what are colors?

Q:
I know objects absorb all colors of light except 1. This is the color we see. But does something not absorb 2 or more colors of light? If so how would that look like? Do some objects absorb all colors?
- Kunal (age 13)
Asbury, Iowa, United States of America
A:

It's not really true that any object absorbs "all colors of light except one". Light comes in a whole smooth range of different frequencies, from the lowest ones we can  (they look red) to the highest ones we can see (they look violet). Almost any light that you see is not just one frequency but a whole range of frequencies. (The main exception is laser light, which usually has a very narrow range of frequencies.)

 

(This image from http://www.chemistryland.com/CHM130W/10-ModernAtom/Spectra/VisibleLightSpectrumPlus.jpg  uses wavelengths rather than frequencies in the label.)

 Our brains do interesting tings to give us a sense of one color no matter what sort of combination of frequencies happen to reach them. So if, for example,  you see some light that's coming from one end of that spectrum plus some coming from the other end, your brain doesn't say "I'm seeing a combination of two colors." Instead you sense a single color, just not one of the ones that you could get from a single frequency. I think mixing that violet light and that red light gives you a sense of a sort of pinkish purple.

Things that look different colors (say an orange and a lemon) each partly absorb and partly reflect a broad range of the frequencies of light. The balance between absorption and reflection at different frequencies is different for the orange and the lemon. So the balance of different frequencies yo​u see is different. Your brain makes a different color sensation from these different mixes of frequencies.

Mike W.


(published on 01/23/2016)

Follow-up on this answer.