Human eye canít detect all portions of the electromagnetic spectrum or in other words, we canít see all kinds of electromagnetic radiation. Visible light or simply light is the part of the electromagnetic spectrum visible to us. Different sources mention different ranges of wavelengths or frequencies for the visible light but all of them are from about 400 nm to about 700 nm in terms of wavelength or from about 430 THz to about 750 THz in terms of frequency.
The visible spectrum is continuous and not discrete which means the visible spectrum covers all wavelengths inside its boundaries. Each wavelength in visible spectrum has its own color where there is no distinct boundary between colors and change from color of one wavelength to the color of other one is gradual. The visible spectrum doesnít contain all the colors perceivable to the human eye since many perceivable colors such as pink, magenta and brown are mixtures of two or more wavelengths. Since colors associated with only one wavelength and not a mix of them are called spectral or pure colors, the visible spectrum can be considered as a collection of many spectral colors.
Distinguishment of spectral colors with close wavelengths is hard for the human eye and even gets impossible for very close ones. Recognition of six colors: red, orange, yellow, green, blue and violet, in the visible spectrum has global acceptance where each covers a range of wavelengths or frequencies upon which dispute has existed. For example, the red portion of the visible spectrum may be considered to cover wavelengths from 620 nm to 750 nm. From red to violet, frequencies and photon energies increase while wavelengths decrease. Therefore, red light has the lowest frequencies and photon energies with the longest wavelengths while the violet light has the highest frequencies and photon energies with the shortest wavelengths. Other divisions have also existed for the visible spectrum. For example, Isaac Newton divided the visible spectrum into seven colors: red, orange, yellow, green, blue, indigo and violet. However, today, it is believed that Newtonís indigo corresponds to what is today called blue and his blue corresponds to what is today called cyan. Also, it is suggested by some that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet.