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• Light travels as a transverse wave and can be reflected by surfaces and objects.
• Smooth, shiny surfaces produce specular reflections, and rough surfaces produce diffuse reflections.
• Ray diagrams are used to show the path of light rays which are reflected and how images are formed in a mirror.

When light hits a mirror, it reflects from the surface of the mirror.

The angle at which the light is reflected depends on the angle it hits the mirror, and a ray diagram can be used to show the path of the light rays.

The diagram below shows the path of a light ray being reflected from a plane mirror.

• The reflective surface of the mirror is represented by a vertical line, with hatching ( \ \ \ \ \ ) to show the reverse (non-reflective) side of the mirror.
• The ray of light travelling towards the mirror is called the incident ray.
• The ray of light which is travelling away from the mirror is called the reflected ray.
• The dashed line is called the normal, and is drawn at 90° to the surface of the mirror. All angles are measured from the normal.
• The angle of incidence, i, is the angle between the normal and incident ray.
• The angle of reflection, r, is the angle between the normal and reflected ray.

The angle the ray is reflected is always the same as the angle the light hits the mirror, with both angles being measured from the normal.

This is summarised by the law of reflection, which states: “the angle of reflection is equal to the angle of incidence”, or simply “i=r”.

This law works for any angle. For example:

If parallel rays of light hit the surface of a plane mirror, they are all reflected in the same direction. This is known as specular reflection. Because all the light reflects in a predictable way, images can be formed by specular reflections.

If light meets a rough surface, the rays of light reflect in different directions.

Each ray obeys the law of reflection but, because the different parts of the rough surface point in different directions, the light is not all reflected in one direction. Instead, the light is reflected in all directions – this is called diffuse reflection or scattering.

This is why you can see a clear image of yourself in a shiny flat mirror, but not in a dull rough wall.

When light reflects from a plane mirror, it produces a clear image. This image:

• appears to be behind the mirror
• is the right way up
• is ‘laterally inverted’ (letters and words look as if they have been written backwards)

A ray diagram can be used to show how an image in a mirror forms:

Notice that the ‘real’ rays, the ones leaving the object and the mirror, are shown as solid lines.

The ‘virtual’ rays, the ones that appear to come from the image behind the mirror, are shown as dashed lines ( - - - - - - ). Remember that each incident ray will obey the law of reflection.

To draw a ray diagram showing reflection:

1. Draw two rays from the object to the mirror. Each ray should obey the law of reflection.
2. Trace the rays back using dashed lines, until they meet.
3. The image will appear to be at the position these dashed lines meet.