Thread: Why we can see light and not heat?

Why we can see light and we can not see heat? Why can't heat be reflected?

Heat is infrared radiation of the electromagnetic spectrum. It is reflected, its just that our eyes are not made to detect IR (infra red) radiation due to our evolution. If you wear IR goggles you can see IR in a form we recognise-light). Heat is reflected, here:

Imagine you are wearing a white top and I am wearing a black top. You will notice if we sit in the 45 degree celcius sun for an hour that your white top is cool, yet my black top is roasting me. The white top reflects heat as much as a mirror reflects visible light (more or less). Yet my black top absorbs it in the same way it does light.

But you would still get hot. Don't you think that the heat that your receive is mostly infrared, and the difference is only there due to the difference in the amount of visual spectrum light absorption?

There's a misconception here. Heat is not infrared. It's just that things in the temperature range we're used to are warm enough to emit infrared. (See blackbody radiation for more on this, but basically all objects emit light. Warmer objects emit more light and at higher frequencies. This is why hot metal glows red, then yellow, etc.) Infrared is a type of light, just like visible light. All forms of light can be absorbed, radiated, reflected, refracted, etc, and all forms of light will make things warmer when they're absorbed. (Light and heat are both forms of energy, and conservation of energy says this must be so.) So the white top hat relfects more light than the black top hat, in total, so it gains less energy and is cooler in the end.

The eye evolved to be sensitive to the visible spectrum because that is the region that passes through the earth's atmosphere with the highest intensity. It has to be a fairly narrow band, because different wavelengths refract differently and so cannot all be in focus at once.

Oh, my bad.

BTW, some birds and insects can see further past the violet end of the spectrum than we can; that is, they can see ultraviolet light as well as human-visible light. Snakes can detect infrared radiation, but I'm not sure it's high enough resolution to call it sight.

how do you define sight? i've never come across that way of thinking before?

you can see signs of heat on a hot day, just get a long stretch of road service with an object at the end and stare at the onject. You should be able to see 'heat haze' which is the particles of the air rising causing distortions

Originally Posted by jacketate

how do you define sight? i've never come across that way of thinking before?

Well, I'd probably define it as the ability to detect a particular wavelength with some spatial resolution. That's pretty vague, but I'm no expert, so I can't give a better definition. Plus, it'd probably make more sense to define it in on a continuum anyway.