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Light

Light and all electromagnetic radiation travel in a vacuum in a straight line at a speed of about 300,000 kilometers per second. When light passes through a medium (such as air or glass), its propagation speed is lower than in a vacuum. The ratio of these two speeds is called the refractive index of the medium.

The propagation speed of any type of wave is equal to the product of wavelength and frequency

V=vλ

The 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 of 7.5x104 Hz in a vacuum; while red light with a wavelength of 750 nanometers has a frequency of 4x10' Hz in a vacuum. When a light wave passes from one medium to another, the frequency does not change, but any change in speed will cause a proportional change in the wavelength, because according to formula 1.1, Vλ should be a constant. When the wavelength of light is mentioned without specifying the medium, it is usually the wavelength of light in air. The wavelength of light in air is only slightly shorter than that in a vacuum because the refractive index of air is close to 1.

At the interface between two media with different refractive indices, incident light is split into two parts, one of which is reflected back into the first medium and the other is refracted into the second medium. The laws that determine the direction of reflected and refracted light can be derived from the wave theory of light.

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