# Light Chapter 4. What is Light? There are two different ways of talking about light: There is the "particle" theory, expressed in part by the word photon.

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Light Chapter 4

What is Light? There are two different ways of talking about light: There is the "particle" theory, expressed in part by the word photon. There is the "wave" theory, expressed by the term light wave. In Year 9 we will focus on light as being understood as an electro-magnetic wave.

The light “wave” Described as alternating waves of electrical and magnetic fields

Light waves and EM spectrum

Speed of light The speed of light depends on what it is travelling through. In a vacuum, the speed of light is 300,000 km/s.

Definitions Light is a electromagnetic wave. It is a form of energy. Refraction is the “bending” of light as it passes into a substance which has a different optical density. Normal: is a line drawn perpendicular (90 o ) to the boundary between two substances where the light ray crosses the boundary.

Refraction The higher the optical density the slower light travels. Due to this when light enters a substance of higher optical density it slows down and bends toward the normal. The reverse is also true, when light enters a substance of lower optical density it bends away from normal.

Refraction

FST = Fast to Slow, Towards Normal If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line. SFA = Slow to Fast, Away From Normal If a ray of light passes across the boundary from a material in which it travels slow into a material in which travels faster, then the light ray will bend away from the normal line.

Reflection Reflection is great. The fact that light bounces off of objects around us allows us to see them. When light hits a smooth object, it bounces off much like billiard balls off the side of a pool table. When light hits a rough object, the same thing happens; each different ray of the incident light is reflected in a different direction.

Reflection

Law of Reflection The angle made by the normal and the incident ray (called the incidence angle) is equal to the angle made by the normal and the reflected ray (the reflection angle)

Total Internal Reflection When light goes through the boundary it bends [red]. If the angle of light reaches the ‘critical’ angle the light [dotted] bends to run along the boundary. When the angle is greater then that of the critical angle the light is no longer refracted but rather reflected back off the boundary

Total Internal Reflection

Individual work Read through the text and complete the question for Unit 4.1 on pages 78/79.

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