What is the Difference between Reflection and Refraction?

Waves transfer energy without transferring matter, they can be both longitudinal (for example sound waves) or transverse (for example electromagnetic waves and water waves). When a wave encounters a boundary between two different substances, the speed and/or direction of the wave will change as it passes from one medium to the next. Most often one of three things will occur at a material boundary:

1. The wave will be absorbed, that is it will be taken in by the material, and undergo an energy transfer, usually to the thermal energy store;
2. The wave will be transmitted, that is it will be allowed to pass through the second material. This usually results in the wave being refracted;
3. The wave will be reflected, that is the wave ‘bounces off’ the boundary and changes direction, but does not pass through the second medium.

Depending on both the nature of the wave and the nature of the boundary, different amounts of these processes will happen. This 3-minute read aims to provide you with the quick revision you need on the differences between reflection and refraction, including the use of ray diagrams to show each process.

Reflection

When reflection occurs, there are two ways that waves (particularly light) can be reflected:

• diffuse reflection (the surface is rough so waves are reflected irregularly) which leads to a blurry image, or no image at all;
• specular reflection (the surface is smooth so waves are reflected regularly) which leads to a regular image, visible ‘within’ the surface.

Although all waves can be reflected, we tend to focus on light waves as these are easier to model and investigate in the classroom.

‍The Law of Reflection

One of the key experiments undertaken in physics within this topic is to use ray box and single slit, with a plane mirror, to investigate the law of reflection. A pencil is used to mark out the path of the ray of light and a protractor is used to measure the angles. A ray diagram is drawn as part of the investigation, and to show the results:

Refraction

If a wave is transmitted through a material, then it will undergo refraction. This is when the speed of the wave changes as it travels into a material with a different density. A wave will speed up as it enters a less dense material, and slow down as it enters a more dense material. Unless the wave is travelling perpendicular (at right angles) to the boundary (i.e. along the normal), the change in speed will also cause the wave to change direction. The easiest way to show this is using another ray diagram:

The table below should help you remember key differences between reflection and refraction often asked in examinations.

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