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Propagation of light

Light and all electromagnetic radiation propagate in a vacuum at a speed of approximately 300,000 kilometers per second. When light passes through a medium (such as air or glass), its speed is lower than in a vacuum. The ratio of these two speeds is called the refractive index of the medium. The speed of any wave is equal to the product of its wavelength and its frequency v.

V = vλ (1.1)

Here, frequency (v) is the number of waves passing a fixed point per second. For example, violet light with a wavelength of 400 nanometers has a frequency in a vacuum of 7.5 x 10' Hz; red light with a wavelength of 750 nanometers has a frequency in a vacuum of 4 x 10' Hz. When a light wave passes from one medium into another, the frequency does not change, but any change in velocity causes a proportional change in wavelength, since V1λ should be constant according to Equation 1.1. When referring to the wavelength of light without specifying the medium, it is usually referring to the wavelength of light in air. The wavelength of light in air is only slightly shorter than that in a vacuum because air's refractive index is close to 1.