Term
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Definition
| The bending of the path of a wave (a ray) when it enters a medium of a different density. |
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Definition
| The change in speed when a wave passes from one medium to another of a different density - speed change is the key idea. |
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| What happens to the speed of the light ray when it goes from air to glass? |
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Definition
| It slows down (Light travels fastest in a vacuum). |
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| What happens to the speed of the light ray when it goes from glass to air? |
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Definition
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| Which way does the ray bend when it gets faster? |
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Definition
| Away From the normal ('F' in from to remind you of 'faster') |
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| Which way does the ray bend when it gets slower? |
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Definition
| TowardS the normal (the 'S' to make you think of 'slower') |
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| Sketch a diagram of a ray entering a denser optical medium( eg. going from air to glass) - label it fully. |
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Definition
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| Sketch a diagram of a ray entering a rarer optical medium (eg. going from glass to air - to a less dense medium) - label it fully. |
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| (Gifted and Talented input!!) The speed of light is said to be constant at 3.0 x 108 m/s. So why does 'refraction' tell us that it slows down and speeds up? |
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Definition
| Photons of electromagnetic radiation (of which light is an example) do always travel at the speed of light - 3.0 x 108 m/s - that is why the symbol 'c' was given to that speed - it is constant! However, as it travels though matter it does so in a series of 'stop and go' movements. It interacts with the particles in matter - gets absorbed and then re-emitted and that takes time. The average velocity at which it travels through a medium is therefore always slower than it would travel through a vacuum.
Light travels faster through 'rare' optical media - eg. air - because it does not have to 'waste time' interacting with as many particles in a given space - and fastest it possibly can in a vacuum because there are no particles for it to interact with at all.
Light travels slower through 'dense' optical media such as water or glass because there are more particles to interact with in a given time. |
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Term
| Refraction leads to optical illusions. When we look at something under water (or glass) it does not appear to be as deep as it really is. Why does this happen? |
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Definition
| The brain interprets the rays as having travelled in a straight line and therefore forms the image at the point the refracted rays 'appear to come from' NOT taking into account the fact that they have been bent as they leave the water. In the image below the pencil 'looks broken' because its lower half appears higher up in the water than it really is! [image] |
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| Draw a labelled diagram of 'real and apparent depth' |
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Definition
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| Refraction occurs with all waves. You may be asked which way they bend at a boundary. What is the key thing you have to consider when you look at a boundary the waves cross? |
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Definition
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| What affects the speed of light waves? |
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Definition
| The density of the material - the denser it is the more impeded the path of the light it and the slower it travels. |
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Term
| What affects the speed of sound waves? |
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Definition
| The density of the material - sound travels by mechanical vibrations. These are transferred quicker in a dense material. |
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| What affects the speed of water waves? |
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Definition
| The depth of the water course. Water travels faster in a deep chanel.
How to remember this: Slow in Shallow (therefore the wavelength in shallow water is Small) |
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| What is refraction evidence of? |
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Definition
| Refraction is evidence of the wave nature of light - we can only explain refraction in terms of waves. Therefore light is a wave. |
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| When a light ray is refracted by a glass block what happens to the light? |
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Definition
| It slows on entering the block - bending towards the normal. It then speeds up again by the same amount on leaving - bending back the same way away from the normal. That means it exits parallel to the direction it went in! [image] |
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| Why does white light split into a spectrum when it passes through a prism? |
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Definition
| [image]
White light is made up of a mixture of light photons of different energies. These are refracted to a different degree by the glass. The less energetic ones are not slowed down as much as the high energy ones.Therefore they all change speed by different amounts and all therefore bend by different angles. |
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| Draw a fully labelled diagram of a ray of light refracted by a prism. |
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