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# Waves Help!

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1. Hey! I'm struggling with a question on some work and would hugely appreciate some help! I think have some answers but would appreciate input from others who have confidence in their physics knowledge

A light source emits light of a particular frequency uniformly in all direction. How do each of the following quantities vary with distance from the source?

Intensity - I think I'm right in saying that intensity will decrease as energy is lost through travel.

Amplitude - Same again, decrease in amplitude.

Phase with respect to the source - This I don't quite get, anyone care to venture a guess?

Speed - Same as light always travels at speed of light.

ANY help will be hugely appreciated, thank you!!
2. Anyone? </3
3. (Original post by givemeknowledge)
Hey! I'm struggling with a question on some work and would hugely appreciate some help! I think have some answers but would appreciate input from others who have confidence in their physics knowledge

A light source emits light of a particular frequency uniformly in all direction. How do each of the following quantities vary with distance from the source?

Intensity - I think I'm right in saying that intensity will decrease as energy is lost through travel.

Amplitude - Same again, decrease in amplitude.

Phase with respect to the source - This I don't quite get, anyone care to venture a guess?

Speed - Same as light always travels at speed of light.

ANY help will be hugely appreciated, thank you!!

Since the question stated "a particular frequency " and speed is the same , frequency should be the same.

When you said you don't quite get about the phase part, I assume that you know what is phase if not check out this link.

Phase with respect to the source - Should be the same assuming that the source is not emitting light randomly. It is same as the distance between the source and the point on the wave is constant.

Hope it is simple to understand.
4. (Original post by givemeknowledge)
Intensity - I think I'm right in saying that intensity will decrease as energy is lost through travel.

Amplitude - Same again, decrease in amplitude.

Intensity in this context means power per unit area - if your source was giving out 1 Watt of light that one watt is being spread more thinly the further away you place your unit area from the source - the intensity is in fact proportional to the inverse square of the distance from the source - it's an example of the inverse square. be careful about saying energy is *lost* because you have to explain where it's gone to.

Amplitude is a bit different though because light shows wave and particle like properties, all the photons have the same amplitude, the intensity falls with distance because they travel in straight lines and are spread further apart as they move away from the source.

frequency is tied to wavelength and speed by the formula c=fλ and you've already said correctly that speed and wavelength remain constant... what does this mean for frequency?
5. (Original post by Eimmanuel)

Since the question stated "a particular frequency " and speed is the same , frequency should be the same.

When you said you don't quite get about the phase part, I assume that you know what is phase if not check out this link.

Phase with respect to the source - Should be the same assuming that the source is not emitting light randomly. It is same as the distance between the source and the point on the wave is constant.

Hope it is simple to understand.
Thank you so much for your input!

(Original post by Joinedup)
Intensity in this context means power per unit area - if your source was giving out 1 Watt of light that one watt is being spread more thinly the further away you place your unit area from the source - the intensity is in fact proportional to the inverse square of the distance from the source - it's an example of the inverse square. be careful about saying energy is *lost* because you have to explain where it's gone to.

Amplitude is a bit different though because light shows wave and particle like properties, all the photons have the same amplitude, the intensity falls with distance because they travel in straight lines and are spread further apart as they move away from the source.

frequency is tied to wavelength and speed by the formula c=fλ and you've already said correctly that speed and wavelength remain constant... what does this mean for frequency?
Thank you!

I've put that energy is lost partly through heat and absorption by other particles in the air, does that sound right?
6. (Original post by givemeknowledge)
Thank you so much for your input!

Thank you!

I've put that energy is lost partly through heat and absorption by other particles in the air, does that sound right?
NO that does not sound right.

if you imagine the 1 watt light source is to be placed centrally in cube shaped boxes of various sizes and the intensity (power per unit area) can be measured at the faces.

because of the symmetry of the set up we can say 1/6th the of the light coming from the source hits(is incident upon) the top face, 1/6th hits the bottom face, 1/6th hits the left face, 1/6th hits the right face, 1/6th hits the back face and 1/6th hits the front face... 6 faces of the cube all receive 1/6 th of the light output of the light source. 6/6... all the power coming out of the source has been accounted for.

Now if the side length of the cube is 1m, the area of each face is 1m x 1m= 1m2 - so the average intensity at each face is 1/6th of a watt per 1m2

but if the side length of the cube is 2m the area of each face is 2m x 2m = 4m2 - so the average intensity at each face is 1/4 x 1/6 watts per meter... same amount of power incident on each face..but spread over a larger area because the face is now bigger.

this is where the inverse square law comes from - as the distance r increase the area go up by r2... and area is on the bottom of the fraction
intensity=power/area
so the intensity varies as 1/r2

Same power - but spread across larger surface areas as you increase distance.

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