The eye is sensitive to electromagnetic radiation.
The electromagnetic spectrum extends from high-energy, short-wavelength light in the form of x-rays, to low-energy, long-wavelength radiation called radio waves. A very small portion of the spectrum is detected by the eye: the visible range. (The concept of wavelength is more fully discussed in Topic 2.)
The eye is sensitive to a wavelength range of about 400 nanometers (nm) to 700 nanometers. This corresponds to a range of color from violet to red. Each wavelength in the visible range corresponds to a color.
As generations of students have learned from the mnemonic ROY G BIV, the colors, from long wavelength to short, are red, orange, yellow, green, blue, indigo, violet.