To fully understand the propagation of electromagnetic waves, you will need to study the next section on electromagnetism. However, it is enough to use what we know about other waves to understand the wave nature of light. 

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Electromagnetic spectrum

Light is part of the electromagnetic spectrum. Electromagnetic waves are disturbances in electric and magnetic fields. They are not mechanical waves so require no medium.


When light strikes the boundary between two media, part of it reflects.


While part of the light reflects, the remainder will refract into the medium as it changes speed and direction. The angle of refraction can be calculated by Snell's law, where n is the refractive index (ratio of the speed of light to its speed in the material).

\({\sin i \over \sin r}={n_2 \over n_1}\)

NB: n1 is often equal to 1, as light usually enters a medium from air (where its speed is equal to that in a vacuum).

Total internal reflection

Total internal reflection occurs when light travels into a less optically dense medium at an angle of incidence that exceeds the critical angle - all of it reflects.

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The refractive index of glass depends on the wavelength (colour) of light. It is largest for violet light, but a useful reminder can also be "blue bends best" (rather than red!).

White light disperses into all the colours of the visible light spectrum when it passes through a prism.


Two coherent sources of light (equal frequencies) will interfere where they overlap. This causes the formation of bright and dark patches called fringes.


Light spreads out when it passes through a slit. The angle of spreading depends on the width of the slit:

\(d\sin \theta=\lambda\)

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