How come we can't see Infrared or Ultra Violet light?

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Offline Seany

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Our eyes filter the wavelengths of colours? Too long or short, we can't see it? Why is it filtered then? Because it's harmful to our eyes?
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How come we can't see Infrared or Ultra Violet light?
« Reply #1 on: 03/09/2007 13:11:46 »
The purpose of colour is to discriminate between objects of different types that have the same shape.

For instance, many poisonous plants and animals advertise their poisonous nature by being brightly coloured.  If our eyes were to see all colours without any filtration, then we would not easily be able to discriminate between those organisms that advertise their poisonous nature, and those that do not (I stress the advertisement, because like all advertisers, they sometimes lie).

Colour is also used for personal communication (e.g. when blushing), and if we did not respond differently to different colours, we could not tell when someone is blushing.

Since we have to filter red, green, and blue (OK, the real colours we see do not exactly match these nominal colours, but they are normally used as approximations for what we see), thus it follows that anything outside of this range we will not see at all.



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How come we can't see Infrared or Ultra Violet light?
« Reply #2 on: 03/09/2007 14:26:30 »
It is no accident that the range of sensitivity to electromagnetic radiation for most animals fits the range of wavelengths that filtered / absorbed  least by the atmosphere - we make use of what is most available. The amounts of IR and UV, available at the Earth's surface, vary a lot, so they are probably not worth using except in certain circumstances.
Some specialist animals - pit vipers etc, adapted to be more sensitive to infra red  - to detect the warm bodies of prey.
Some insects 'came to an arrangement' with some plants and the two species use UV to signal the 'flight path' onto flowers for pollination (in Summer, when there is more than average UV around). This is a sort of niche relationship and must have advantages for both organisms.
But the majority use more or less, our visible spectrum.
Distinguishing 'colour' is useful for some animals - so they do it.  For others it's not - so they don't. Three - colour analysis seems to be the best value for money - so that's what we use.


Offline peopleunit

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How come we can't see Infrared or Ultra Violet light?
« Reply #3 on: 04/09/2007 03:32:55 »
Our eyes have two types of photosensitive cells, cones and rods. One type is sensitive to back and white and the other type has three sub-types, sensitive to red, green and blue.

As already mentioned, the pit viper has an organ that is sensitive to infra-red and aids in detecting nocturnal prey. Some insects are able to detect infra red as well - moths, mosquitoes, etc. (up to 100 feet away from what I've read), using external heat sensitive antennae.

Much of the ultra violet light wavelengths are filtered out by our atmosphere. I don't believe our eyes actually filter out IR and UV light, its just that the photo-sensitive cells in our eyes don't respond to those wavelengths.

Our skin however, IS sensitive to infra-red (heat), and responds to UV by releasing pigmentation (tanning) to help filter out the damaging rays. Interestingly enough, plants use the IR and UV wavelengths more than the visible spectrum in their photosynthetic process.