How does the Earth's atmosphere affect the perception of color in space?

[Kerrie]

When astronauts travel to outer space, the moon, etc, can their eyes still perceive color?

 

[enigma]

Yes. If you watch the videos of the Apollo landings, the astronauts were surprised that underneath the dust was orange rock on one of the missions.

 

[Kerrie]

thanks enigma...i thought our ability to see color had to do with the Earth's atomosphere...

 

[kleinma]

No i think it is more that Earth's atmosphere changes the color of things in space when you observe them from Earth though...

 

[Gonzolo]

Don't forget that any light that goes into our eye goes through eye fluid before hitting our retina.

Atmosphere is transparent to visible wavelength (almost coincidently...). It absorbs many other wavelengths though, beneath purple and beyond red.

 

[kleinma]

doesn't visible light bend through our atmosphere though? like the effect you get on the moon during an eclipse?

 

[chroot]

Light only changes direction when it goes from one medium to another (this is Snell's Law). Light does not bend within our atmosphere, but it does bend when moving from vacuum to air, or from air to vacuum.

- Warren

 

[kleinma]

Light only changes direction when it goes from one medium to another (this is Snell's Law). Light does not bend within our atmosphere, but it does bend when moving from vacuum to air, or from air to vacuum.

- Warren

right but any light you observe past our atmosphere out in space is doing this move you talk about

 

[chroot]

Of course, Snell's law predicts there will be no change in direction at all when the light hits the interface perpendicularly.

When you observe an object near the zenith (the point directly overhead) there is no bending of the light involved.

When you observe an object near the horizon, the light is most certainly bent in going from vacuum to air. The amount of refraction is actually dependent on wavelength. Red light is refracted more than blue light. A simple telescope will show this effect on any bright object illuminated by sunlight, like the Moon or Venus.

- Warren

 

[tony873004]

They put those color charts on spacecraft so they can color calibrate the camera in its new surroundings. I'm guessing that that's because colors look slightly different in Mars' atmosphere than in Earth's atmosphere.

Scuba divers sometimes take flashlights on dives in bright daylight, not to brighten what they're looking at, but to restore the color. Water robs light of its color, the deeper you go.

So maybe a lot of moisture in the atmosphere makes a difference too.

 

[kleinma]

Of course, Snell's law predicts there will be no change in direction at all when the light hits the interface perpendicularly.

When you observe an object near the zenith (the point directly overhead) there is no bending of the light involved.

When you observe an object near the horizon, the light is most certainly bent in going from vacuum to air. The amount of refraction is actually dependent on wavelength. Red light is refracted more than blue light. A simple telescope will show this effect on any bright object illuminated by sunlight, like the Moon or Venus.

- Warren

im probably wrong, but wouldn't this light bending not depend so much on where I am physically looking in the sky (directly up, or at the horizon) but where I physically am on the Earth (NY versus one of the poles). Or is it more to do with the curvature of the Earth on the horizon? I guess it does make sense though since the sun or moon looks huge and color distorted when it is near the horizon versus when it is high in the sky

 

[chroot]

im probably wrong, but wouldn't this light bending not depend so much on where I am physically looking in the sky (directly up, or at the horizon) but where I physically am on the Earth (NY versus one of the poles).

No, it only depends on the direction you look in the sky.

I guess it does make sense though since the sun or moon looks huge and color distorted when it is near the horizon versus when it is high in the sky

Actually, neither the color or apparent size changes have anything to do with refraction.

The change in color is largely due to the fact that the atmosphere is better at scattering blue light than red light (which is the reason the sky is blue, incidentally). When you're looking near the horizon, you're looking through a lot more atmosphere than you are when looking near the zenith. More atmosphere means more selective color absorption which means more change in color.

The Sun and Moon are not actually any larger when near the horizon -- it's just an illusion. This can be demonstrated by looking through a paper-towel tube.

The only way that I know to directly see the refraction of the atmosphere is to look at a bright object like the Moon or Venus when it is low on the horizon with a telescope used at moderately high magnification. You'll see overlapping blue and red images of the object.

- Warren

 

[Nereid]

They put those color charts on spacecraft so they can color calibrate the camera in its new surroundings. I'm guessing that that's because colors look slightly different in Mars' atmosphere than in Earth's atmosphere.

Scuba divers sometimes take flashlights on dives in bright daylight, not to brighten what they're looking at, but to restore the color. Water robs light of its color, the deeper you go.

So maybe a lot of moisture in the atmosphere makes a difference too.

There are several reasons for the colour charts, calibration of cameras is certainly important.

Colour perception seems like it should be simple, but as it has to do with the human brain, it's anything but. A well calibrated spectrophotometer will tell you unambiguously how much light of each wavelength is being received by the detector, but translating that into how the brain perceives the same light isn't at all straight-forward.

A simple example is the different sensitivities of the rods and cones ... as light gets fainter, we see that it loses its colour, well before we say that it's all black (no light at all); of course, the colour is still there, just that the cones (which the brain uses for the 'colour' signals) aren't as sensitive as the rods.

 

[Gonzolo]

Light only changes direction when it goes from one medium to another (this is Snell's Law). Light does not bend within our atmosphere, but it does bend when moving from vacuum to air, or from air to vacuum.

- Warren

Actually, since the atmosphere has many overlapping layers of varying composition, its index of refraction should be a continuous function of altitude - a graded index of refraction. So strictly speeking light should slightly bend throughout the atmosphere. Whether it is ever useful to take this into account, I am not sure.

 

[chroot]

Actually, since the atmosphere has many overlapping layers of varying composition, its index of refraction should be a continuous function of altitude - a graded index of refraction. So strictly speeking light should slightly bend throughout the atmosphere. Whether it is ever useful to take this into account, I am not sure.

Well, that's true -- I was thinking "through the atmosphere" meant "parallel to the ground." It doesn't necessarily mean that, though, you're right.

- Warren

 

[pervect]

Actually, since the atmosphere has many overlapping layers of varying composition, its index of refraction should be a continuous function of altitude - a graded index of refraction. So strictly speeking light should slightly bend throughout the atmosphere. Whether it is ever useful to take this into account, I am not sure.

This effect (variation of density of the atmosphere with height) has to be taken into account when surveying. Because the density and thus the refreactive index of the atmosphere depends strongly on temperature, it's necessary to model the temperature profile of the atomsophere as well as the gravitational gradient to determine the density / refractive index as a function of height. Under the right temperature conditions light can bend more than the Earth curves - this is what causes mirages.

I couldn't find the first URL I had in mind on this topic, but

http://mintaka.sdsu.edu/GF/explain/atmos_refr/bending.html

is pretty good. The URL I had in mind originally discussed the adiabatic atmosphere in more detail, which is what the atmosphere would look like if it were in theromdynamic equilibrium (generally a good overall aproximation on Earth. Unfortunately I couldn't find this URL.