Vision

Light is a form of electromagnetic radiation and as such, forms a part of the electromagnetic spectrum. A chart of the electromagnetic spectrum labels types of radiation with their corresponding frequency and/or wavelength. If you look at such a chart you may observe that visible light forms a very small part of the spectrum. This is often remarked upon, but I have never heard any explanation as to why that might be. But this evening as I was out for my daily constitutional, I thought of a reason. Lower frequency radiation is more vague, it behaves more like a wave. It spreads out in all directions from it's source and flows around obstacles in its' path. Light is the lowest frequency radiation that travels in straight lines and does not exhibit this flowing characteristic. This means that visible light will reveal the precise direction of its' source. Radiation at lower frequencies will give you a vague idea of the location of the source. The lower the frequency, the vaguer the location. Infrared radiation, which is just below visible light on frequency spectrum, can be seen by some instruments, but the pictures they produce are a little fuzzy. This could be a limitation of the instruments, but I suspect it is more because of the nature of infrared radiation.

Radiation above that of visible light (ultra violet) and above is too energetic. It takes too much energy to produce and when it impacts living tissue it can cause damage. We survive because there is not too much of it around.

So the fact that we can see is a fluke of nature. Visible light is energetic enough to travel in straight lines and not flow around objects, and it is of low enough energy for it to be produced in enough quantity that being able to detect it is worthwhile.

Note that the frequency times the wavelength is equal to the speed of light, which is very nearly 300,000 kilometers per second, or 3 x 10^5 km/s. The wavelength in the chart below is given in meters. The speed of light in meters per second is 3 x 10^8. You can multiply the values show in the chart below by adding the exponents. Note that this chart uses a logarithmic scale and is only an approximation of the spectrum. For more detailed information, you might want to try Wikipedia.

The Electromagnetic Spectrum