The visible spectrum is a very small portion of the electromagnetic spectrum that can be seen by human eyes. This is shown in the figure below, which is not to scale. The exact boundaries of each type of wave are not precisely defined and often different authors have slightly different values. This is not really relevant since this classification is just indicative.
As frequency increases the wavelength becomes smaller and the energy associated to each photon increases. Visible light, infrared and radiation with longer wavelengths are non-ionizing radiations. UV light, X rays, gamma rays and radiations with shorter wavelength are ionizing radiations (each photon carries enough energy to ionize matter). The limit between ionizing and non-ionizing radiations is located somewhere in the UV spectrum.
The visible wavelengths are usually between 400 nm and 700 nm. The energy carried by each visible photon is between 3.1 eV and 1.8 eV respectively and the frequency is in the range between 750 THz and 428 THz.
The sensitivity of the eye to wavelengths beyond this range drops dramatically and they are represented as black in the figure below. The maximum sensitivity is usually between 500 and 550 nm.
The perceived color is dependent on the wavelength and the table below will give a rough idea. Of course the perception of each color varies from one person to another.
| Red | 700...630 nm |
| Orange | 630...600 nm |
| Yellow | 600...570 nm |
| Green | 570...520 nm |
| Cyan | 520...480 nm |
| Blue | 480...430 nm |
| Violet | 430...400 nm |
Wavelengths shorter than 390 nm are part of the UV spectrum and are not visible. Wavelengths longer than 750 nm are part of the IR spectrum and are not visible as well. It's not possible to give a precise boundary between visible and invisible wavelengths since that's very subjective: some people may see light at 720 nm and some others may not.
1 nm = 10-9 m = 0.000'000'001 m
1 THz = 1012 Hz = 1'000'000'000'000 Hz
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