Electromagnetic radiation/Visible light

'''Electromagnetic radiation: 8. Visible light'''

This is of course the most familiar form of electromagnetic radiation. It covers wavelengths of around 400-700 nm (frequencies of 430-750 THz); different people have different limits. The range of wavelengths that a person can detect has is nothing to do with colour blindness, which refers to the ability to perceive differences in frequency as colour rather than ability to detect the radiation.

The longest wavelengths correspond to red, then orange, yellow, green, blue, indigo and violet are shorter and shorter. There is no sharp division into different colours; they merge gradually into each other as the wavelength increases.

This is a relatively narrow band, covering a factor of no more than two in wavelength or frequency. However, it is comparatively well defined.

It has been reported that some people who have had cataracts removed can see shorter wavelengths than other people, because they get replacement lenses in their eyes that are more transparent to these wavelengths than natural lenses. A well-documented case is the astronomer Walter Scott Houston.