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# Light

REFLECTION

1. Reflection
Light can change its direction when it bounces off a shiny surface like a mirror. Even if the surface is angled light rays will bounce off at the same angle.

Angled Plane Mirror

• The lst law of reflection states that the angle of incidence equals the angle of reflection.

• The 2nd law of reflection states that the angle of incidence and the angle of reflection both
lie in the same plane.

Reflection from a convex mirror (1st image below) and a concave mirror (2nd image below). For both mirror types the focus is half the centre of curvatures.

REFRACTION

Refraction occurs when light moves from one medium to another. As light changes mediums, it changes both speed and direction. A medium is any substance that light can travel through, e.g. air, glass, diamond, water etc.

Refraction through a convex lens 1st image below. Refraction through concave lens 2nd image below..

A glass prism can change the direction of white light and break it up into its component colours. The individual colours of light slow down at different rates and the colours of the red end of the spectrum are refracted less.

Red
Orange
Yellow
Green
Blue
Indigo
Violet

Critical Angle and Total Internal Reflection
This is the maximum angle of incidence which must produce refraction. The light ray is moving towards a less dense medium from a more dense medium, The critical angle is when the angle of refraction
is equal to 900.

• If the angle of incidence is greater than the angle of refraction, then the light ray will pass through into the other medium.

• If the angle if incidence equals the angle of refraction, then the light ray will refract along the boundary line at an angle of refraction of 90°.

• If the angle of refraction is greater than the critical angle, then the light ray will reflect back into the medium. This is called total internal reflection.

The following diagram illustrates these situations:

An application of total internal reflection is fibre optic cables. Here the rays of light always hit the edge of the plastic at an angle greater than the critical angle. This means the rays of light are directed to travel down the fibre optic cable.