Light waves are often referred to when discussing reflection, refraction and diffraction. However, these behaviours also apply to other waves including sound waves and the other waves in the electromagnetic spectrum, the effects may just not be as great.
Reflection
When a wave travelling through a medium (such as air) hits the surface of another medium (such as a mirror) it is reflected.
The diagram below illustrates a wave being reflected by a surface. Instead of drawing the actual wave a ray is drawn as it is much simpler than drawing the actual wave.
Incident Ray – the ray that hits the surface.
Normal – a line drawn at 90º to the surface of the material reflecting the wave.
Angle of Incidence (i) – the angle between the incident ray and the normal.
Angle of Reflection (r) – the angle between the normal and the reflected ray.
The Law of Reflection
The angle of incidence (i) is equal to the angle of reflection (r).
Refraction
When a wave passes from one medium to another (such as from air to glass) the direction of the wave changes. This change in direction is called refraction.
As with reflection we have an angle of incidence, but instead of an angle of reflection we have and angle of refraction:
Angle of Refraction (r) – the angle between the normal and the refracted ray.
The diagram below shows a wave being diffracted. Most waves will be partially reflected and partially refracted. However, for simplicity only refraction is shown in the diagram.
Refraction occurs because waves travel at different speeds in different mediums and this change in speed is caused by differences in density between the mediums. The angle of refraction is larger or smaller than the angle of incidence depending on how the speed changes:
If medium 2 is denser than medium 1:
- The wave slows down
- The wave bends towards the normal
- The angle of refraction is less than the angle of incidence
If medium 2 is less dense than medium 1:
- The wave speeds up
- The wave bends away from the normal
- The angle of refraction is greater than the angle of incidence
IMPORTANT NOTE: If a wave enters a medium at 90º to the surface (parallel to the normal) it is not refracted.
Diffraction
Diffraction is the spreading out of a wave when it passes through an aperture (gap/hole) or around an object.
The size of the aperture (or object) determines the extent of diffraction, with the most significant diffraction occurring when the aperture is of the same order of magnitude as the wavelength of the wave.
When the aperture is larger than the wavelength most waves pass through and little diffraction occurs and when the aperture is much smaller than the wavelength most waves are blocked.
The diagram below shows the diffraction of a wave:
In the diagram the distance between the the lines is the wavelength of the wave.
IMPORTANT NOTE: Diffraction does not change the wavelength of the wave.
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