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GCSE waves-frequency and speed question-urgent

Hi,
How come when describing refraction, it is described as light bending due to a change in medium, causing a change in speed, but not frequency; and then when describing the speed of light in the electromagnetic spectrum the speed is the same but the different types of light have different frequencies?

Could someone give a GCSE explanation, please?

Thanks a lot :-)
Original post by wallflower#1
Hi,
How come when describing refraction, it is described as light bending due to a change in medium, causing a change in speed, but not frequency; and then when describing the speed of light in the electromagnetic spectrum the speed is the same but the different types of light have different frequencies?

Could someone give a GCSE explanation, please?

Thanks a lot :-)


Frequency relates to the energy of the wave, but not its speed. Speed depends on what the light wave is travelling through. [not a physicist]


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Reply 2
Avatar for wallflower#1
wallflower#1
OP
Original post by William Turtle
Frequency relates to the energy of the wave, but not its speed. Speed depends on what the light wave is travelling through. [not a physicist]


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Thanks anyways :wink:
Original post by wallflower#1
Hi,
How come when describing refraction, it is described as light bending due to a change in medium, causing a change in speed, but not frequency; and then when describing the speed of light in the electromagnetic spectrum the speed is the same but the different types of light have different frequencies?

Could someone give a GCSE explanation, please?

Thanks a lot :-)


Speed through a medium:

Light is an electromagnetic wave that carries energy. Light must always travel at the same speed through a vacuum (300 million metres per second).

When light interacts with matter (atoms) individual atoms will absorb the energy, and if the atoms are of a specific type (for instance, water is hydrogen and oxygen), they will both absorb some energy but re-emit most of it.

That re-emitted light is then absorbed by other atoms nearby which also absorb some but re-emit most of the energy.

In this way, the light travels through the water but critically, all those absorption and re-emissions means the light is scattered so the light no longer takes a direct path through the water. All the while, the light is travelling at the same 300 million metres per second through the intervening space between the atoms.

Think of it like an athlete in a race having to negotiate obstacles. While overcoming the obstacles, the athlete spends time doing it and having to travel further in the process, but between obstacles the athlete can run flat out. The overall result is, it takes a longer time to get from one end of the course to the other. i.e. the distance covered by the light whilst it is bouncing back and forth in the water has increased. So the time taken to get to the other side must also increase so it takes longer to achieve.

So really, light does not slow down as it moves through, but the interaction with atoms causes a delay which, when all added up, makes light 'appear' to have slowed down as it passed through the water.

Refraction:

The light approached the surface of the water as a wave-front. (rather like a line of soldiers marching on parade).

If the wave front hits the surface of the water at an angle, then not all the wave front will interact with the water atoms at the same time. The light that entered the water first gets delayed as it moves through the water. However, the light that is still to hit the water is s travelling at it's original velocity.

The combination of absorption and re-emission of light already interacting with the water atoms, together with the different time at which all the wave front gets to the water, causes the wave front to deviate from its original path. This is why the light refracts.

Frequency and energy

The amount of energy the light has is purely governed by it's frequency. The higher the frequency, the more energy it has. Which is why ultra-violet (higher frequency) light is more harmful to human tissue than say infra-red (lower frequency).

Critically though, light is constrained to travel at the same speed through a vacuum - as stated before, 300 million metres per second. It's only when it interacts with atoms that it is delayed by the process of energy absorption and then re-emission. Which makes light appear to slow down.



Ask if you need further clarification. :smile:
(edited 9 years ago)
Reply 4
Avatar for wallflower#1
wallflower#1
OP
Original post by uberteknik
Speed through a medium:

Light is an electromagnetic wave that carries energy. Light must always travel at the same speed through a vacuum (300 million metres per second).

When light interacts with matter (atoms) individual atoms will absorb the energy, and if the atoms are of a specific type (for instance, water is hydrogen and oxygen), they will both absorb some energy but re-emit most of it.

That re-emitted light is then absorbed by other atoms nearby which also absorb some but re-emit most of the energy.

In this way, the light travels through the water but critically, all those absorption and re-emissions means the light is scattered so the light no longer takes a direct path through the water. All the while, the light is travelling at the same 300 million metres per second through the intervening space between the atoms.

Think of it like an athlete in a race having to negotiate obstacles. While overcoming the obstacles, the athlete spends time doing it and having to travel further in the process, but between obstacles the athlete can run flat out. The overall result is, it takes a longer time to get from one end of the course to the other. i.e. the distance covered by the light whilst it is bouncing back and forth in the water has increased. So the time taken to get to the other side must also increase so it takes longer to achieve.

So really, light does not slow down as it moves through, but the interaction with atoms causes a delay which, when all added up, makes light 'appear' to have slowed down as it passed through the water.

Refraction:

The light approached the surface of the water as a wave-front. (rather like a line of soldiers marching on parade).

If the wave front hits the surface of the water at an angle, then not all the wave front will interact with the water atoms at the same time. The light that entered the water first gets delayed as it moves through the water. However, the light that is still to hit the water is s travelling at it's original velocity.

The combination of absorption and re-emission of light already interacting with the water atoms, together with the different time at which all the wave front gets to the water, causes the wave front to deviate from its original path. This is why the light refracts.

Frequency and energy

The amount of energy the light has is purely governed by it's frequency. The higher the frequency, the more energy it has. Which is why ultra-violet (higher frequency) light is more harmful to human tissue than say infra-red (lower frequency).

Critically though, light is constrained to travel at the same speed through a vacuum - as stated before, 300 million metres per second. It's only when it interacts with atoms that it is delayed by the process of energy absorption and then re-emission. Which makes light appear to slow down.



Ask if you need further clarification. :smile:

Thanks so much...this is very helpful =)

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