Reflection of Light Revision Notes

In these revision notes for Reflection of Light, we cover the following key points:

• What is reflection of waves in general and that of light in particular?
• How many types of reflection are there?
• How do the features of reflecting surfaces affect the reflection of light?
• What happens to light when it falls on objects?
• Why do we use the concept of normal line in reflection?
• What do the Laws of Reflection say?

Reflection of Light Revision Notes

By definition, reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples of waves' reflection include the reflection of sound and water waves and especially of light waves.

Things to consider when dealing with reflection of light:

• If the reflective surface is ideal, light wave does not loose energy. As a result, the reflected wave will have the same features as the original wave (frequency, wavelength, amplitude, speed etc.) except direction.
• The best reflective surfaces are those surfaces that are 100% flat and smooth. This is very difficult to achieve as all surfaces have a number of holes, which distort their smoothness. However, flat surfaces made of glass are very close to be smooth. Therefore, we often consider glass mirrors when discussing about reflection of light.

There are two kinds of reflection based on the properties of reflecting surfaces:

1. Regular reflection: In this kind of reflection, the reflected light beam has the same shape as the original one. Regular reflection occurs when the reflecting surface is very smooth.
2. Diffuse reflection:In this type of reflection, rays forming a regular beam fall on a non-regular (rough) surface. As a result, rays are reflected in different angles (they diffuse). Diffuse reflection is the reason why we see objects around us despite that they may not be exposed to direct sunlight. Indeed, most of rays coming from the Sun are absorbed by objects on which the sunlight falls. As a result, these objects become hot. Only the rays composed by light waves which have the frequency corresponding to the colours of objects are reflected by objects' surface to our eyes. As a result, we are able to see both the shape and colours of objects illuminated by the sunlight.

There are three possible situations a light ray may experience when it falls on objects.

1. Reflection. It occurs when light does not penetrate inside the object but is repelled by the objects surface.
2. Absorption. It occurs when light encounters a rough surface and it is absorbed by the object after making a number of collisions on its surface.
3. Emission. It occurs when the absorbed light wave is too energetic and therefore, the object releases a part of it into the environment.

In general, good absorbers are also good emitters of light. Matt and black surfaces are good absorber (and therefore good emitters) of light while bright and shiny surfaces are bad absorbers (and therefore, good reflectors) of light.

The two laws of reflection are:

1. Incident ray, reflected ray and the normal line lie all at the same plane.
2. The incident angle is equal to the reflection angle.