Waves can be reflected at the boundary between different mediums. When a wave is reflected, it stays in the same medium instead of passing through.

Light reflection occurs when light waves bounce off a surface, changing direction

Diffuse and Specular Reflection

Specular reflection

Reflection from smooth, flat surfaces is known as specular reflection. In this case, light meeting the surface in one direction is reflected in only one direction. In specular reflection, surfaces like mirrors or calm water allow an image to form.

An example of specular reflection is in the picture below.

Specular reflection takes place because the water is smooth and calm, producing a clear image of the trees and mountains.

Diffuse reflection

Diffuse reflection occurs when a surface is rough. Each light ray is still reflected normally, but the roughness causes the reflected light to scatter in many directions. This is why an image does not form when diffuse reflection occurs.

For example, when you look at a rough wall, you do not see your reflection.

Reflection Ray Diagrams

We can use ray diagrams to show how light travels and what happens when it reaches a surface (boundary).

In the diagram above, the grey block is a mirror, which is called a plane mirror because it is a flat surface. The dashes at the bottom show the back side of the mirror.

  • The light ray approaching the mirror on the left is called the incident ray
  • The light ray going away from the mirror on the left is called the reflected ray

The incident ray hits the surface of the mirror and is reflected back as the reflected ray.

In ray diagrams, we draw a vertical line at a 90° angle to the mirror, which is called the normal. We use this line to calculate the angle of incidence and the angle of reflection.

  • The angle of incidence is the angle between the normal line and the incident ray
  • The angle of reflection is between the normal line and the reflection ray

The law of reflection states that:

The angle of incidence = The angle of reflection

So, if the angle of incidence is 40°, the law of reflection states that the angle of reflection is also 40°. They must always be equal.

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