# Light intensity and index of refraction

• Nikitin
In summary: No. All photons that go in will go out (this is exactly the "no absorption" assumption). If their speed gets lower, their density increases, but flow (here: photons per second) stays constant.In summary, the amplitude of the Poynting vector does not stay constant when the wave propagates from one medium to another.

#### Nikitin

Hi. Will the amplitude of Poynting's vector change if the electromagnetic wave goes from one medium to another?
Shouldn't the amplitude remain constant due to conservation of energy? I.e. the photon-density and velocity will change, but their total intensity remains the same.

I could always do the calculations from maxwell's equations, but it seems like too much bother right now if one of you guys already know this (I'm tired).

Thanks

Nikitin said:
Shouldn't the amplitude remain constant due to conservation of energy?
Why? If the wave propagation gets slower, the energy density goes up (because the flow stays constant if we can neglect losses).

But if the wave propagation gets slower, less photons are going thru the surface in question.

Btw, I am talking about linear mediums only!

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Nikitin said:
less photons are going thru the surface in question.
No. Why do you think that?
Nikitin said:
Btw, I am talking about linear mediums only!
Yes I expected that.

mfb said:
No. Why do you think that?
Yes I expected that.

If the velocity of the photons is less, then fewer photons, per second, are passing thru the surface. So you could say that the photon flow gets "denser" if the light enters a medium with higher index of refraction than whence it came.

Sorry, I neglected to add the per second bit

Nikitin said:
If the velocity of the photons is less, then fewer photons, per second, are passing thru the surface.
No. All photons that go in will go out (this is exactly the "no absorption" assumption). If their speed gets lower, their density increases, but flow (here: photons per second) stays constant.

mfb said:
No. All photons that go in will go out (this is exactly the "no absorption" assumption). If their speed gets lower, their density increases, but flow (here: photons per second) stays constant.
Yes this is basically what I was saying with "doesn't the amplitude of Poynting's vector stay constant?".

So the intensity remains unchanged, right?

mfb said:
No. All photons that go in will go out (this is exactly the "no absorption" assumption). If their speed gets lower, their density increases, but flow (here: photons per second) stays constant.

Didn't you imply that the intensity does not remain constant for the light beam, in the beginning?

Power per area stays constant. I don't know how the Poynting vector is defined in a medium.

There is always reflection. Reflected intensity + transmitted intensity = incoming intensity.

mfb said:
Power per area stays constant. I don't know how the Poynting vector is defined in a medium.
Surely if the power per area stays constant, poynting's vector must stay constant?

## 1. What is light intensity?

Light intensity refers to the amount of light energy per unit area that is present in a given space. It is typically measured in units of watts per square meter (W/m²) or lumens (lm).

## 2. How is light intensity related to the index of refraction?

The index of refraction is a measure of how much a material can bend light as it passes through it. Light intensity is affected by the index of refraction because the more a material can bend light, the more light will be concentrated in a smaller area, resulting in a higher intensity.

## 3. What factors can affect light intensity?

Several factors can affect light intensity, including the distance the light travels, the angle at which it enters a material, the properties of the material it travels through, and any obstacles or obstructions in its path.

## 4. How can light intensity be measured?

Light intensity can be measured using a device called a lux meter, which measures the amount of light energy per unit area. Other methods include using a spectrometer to measure the intensity of specific wavelengths of light or using a photometer to measure the intensity of visible light.

## 5. Why is light intensity important?

Light intensity is important because it affects the visibility and brightness of objects, the growth of plants, and the behavior of animals and organisms that rely on light for survival. It is also a crucial factor in various industries, such as photography, lighting design, and optical engineering.