UV Radiation: Why Can't Human Eyes See It?

[gracy]

why it is said that human eyes are unable to see ultraviolet radiation.sun emits ultraviolet radiation ,when we try to look at sun ,we see light coming from it (it is another thing that we can not resist that light longer and eyes shrink and we eventually have to stop doing so)but we do watch light rays, isn't it called ultraviolet radiation?if not,what is difference between that light which i have mentioned and ultraviolet radiation?

 

[phinds]

Visible light is in a different area of the electromagnetic spectrum than is ultraviolet. The sun emits a full spectrum of visible light, plus frequencies above and below. Can you see X-rays? Can you see radiated heat? (not the effects, but the rays themselves).

 

[Bandersnatch]

Have you ever tried to shine sunlight through a prism? Or have watched a rainbow?

Both split solar radiation into its component parts, spacing each part next to another. There is short-wavelength - violet - radiation on the one end, and long wavelength - red - on the other. Those two, and all the colours of the rainbow in-between correspond to energy levels that can interact with, and therefore be detected by, the chemicals making up the structures called "rods" in our eyes.

But there are more bands past violet and past red. This is somewhat less true of actual rainbows, as a multitude of optical effects conspires to muddle the issue, but in principle it's true.
Ultraviolet (literaly "over violet") and infrared ("under red") are the names of the first bands of the rainbow that human eye can no longer detect.

Incidentally, various materials are transparent to various wavelengths and opaque to others. As it happens, UV is blocked by glass, so whenever you see a pair of glasses made of actual glass and not plastic with a "UV filter" sticker, you can be sure that there is no actual filter other than just the regular glass they're made of. Whether it's the same with plastic glasses I don't know, but wouldn't be surprised at all if it were the case.