The process involved in reflection?

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Main Question or Discussion Point

when we have light reflecting off a material, what's going on there? is it something analogous to a gas particle rebounding off a container wall due to Coulomb repulsion? If so, what is it that exerts a force to reverse the photon's momentum perpendicular to the surface it's reflecting off, given that a photon carries no charge of any variety?

or I was thinking that if the above description is incorrect, maybe the photon is absorbed and an electron in an atom gets excited to a higher state, and then relaxes and emits the photon back out very rapidly?
But if that is what happens, then how does the photon get emitted in the right direction (ie. angle of incidence = angle of reflection)?
 

Answers and Replies

  • #2
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Not a good analogy.

Reflection happens when the ratio of electric and magnetic fields is not the same in two different materials.

When the reflected field is included with the incident and transmitted fields, the boundary conditions are satisfied.
 
  • #3
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are you referring to the way that if R = reflected power / incident power, and T = transmitted power / incident power, then R + T = 1 due to conservation of energy?

that much makes sense, but it's not really saying anything about the mechanism by which it occurs...?
 
  • #4
morrobay
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Not a good analogy.

Reflection happens when the ratio of electric and magnetic fields is not the same in two different materials.

When the reflected field is included with the incident and transmitted fields, the boundary conditions are satisfied.
Could you expand on this with a detailed classical explanation ? thank you
 
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  • #5
Meir Achuz
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The detailed classical explanation is in all EM physics books that include optics.
All QM does is that the amplitude of the wave is quantized.
 
  • #6
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I + R = T

It can be found in most textbooks but often not in the very simple terms I will do it in.

With any EM wave, there are two fields, electric and magnetic. (In reality this explanation works for all waves; it's just that the variables are different. For acoustic waves it's the presure and velocity of a differential gas element etc.). Also in the following I use MKS or "engineering" units.

When any EM wave moves through a linear isotropic medium, the amplitudes of the electric and magnetic fields satisfy a simple relation that looks a lit like Ohm's law. Engineers don't call it a resistance, they use the word Impedance which is almost the same thing.

|E| (Volts/meter) = 376.7 (Ohms) * |H| (Amperes/meter)

Now in a dielectric medium like glass, one of the Maxwell equations tells you that the electric field of a charge is smaller than it would be in free space.

In free space, 8.85x10^-12 * Div.E = q
In glass, it's 20.45x10^-12 * Div.E = q.

So in glass, the electric field of a wave going through is 2.3 times smaller than when that same wave is going through the air. But the magnetic field of the wave in glass and in air is the same because glass only affects the electric and not the magnetic fields.

When the wave enters the glass from the air, the magnetic field doesn't notice. But the electric field goes through an abrupt change because the following equation are true.

In the glass E = 870.3 * H
In the air, E = 376.7 * H

Ok, no big deal except- Maxwell's equations demand that across any material boundary like air/glass, the tangential components of the fields must be continuous. In other words, there are different ratios of electric and magnetic fields just inside and just outside the interface when you consider the incoming wave from air and the wave in the glass just inside the surface.

So what happens? A reflected wave happens because without it, there would be discontinuous fields at the boundary; not permitted by Maxwell's equations.

The reflected wave in the air side has an electric field value which is the difference of the incident and transmitted fields. That means in the air right at the surface, the electric field becomes smaller than it was with only the incident field. But then how can it still be E = 377 * H? Well, we'll put in a smaller magnetic field too. This smaller electric and magnetic field which is required to be added to the air side to lessen the field to match the glass fields; this is called a reflected field.

In plain English, the electric and magnetic fields have a natural ratio in each material. When waves reach a material boundary, there is a new ratio which must be established inside the new material. Because the fields are equal right at the boundary, the new field ratio on the air side is "unnatural". Nature reacts by launching a third wave that restores the balance between E and H in both materials and on both sides of the interface.
 
  • #7
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ah right, I'd never heard that before. I'm asking because I've just done a semester's worth of the optical properties of soids, and this was never addressed at all.
Still though, what you said seems to make sense to me but it's not really a mechanism is it?
Doesn't that just say that the maths only checks out because reflection does indeed exist, rather than providing a mental picture such as, say, two electrons exhanging a photon to repel each other and make each other change direction?

Am I asking too much of this, is this the limit of what is understood about reflection, or have I overlooked something you said?
 
  • #8
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I _think_ you can conceptualize reflection from a _conducting_ surface like this: You can't have electric fields inside a conductor, since charges rush around to create buildups of charge that cancel out any electric field very quickly. So light, being an oscillation of the electric and magnetic fields, can't propagate very far in a conductor. When light impinges on a conducting surface, the electric field makes the charges on the surface rush around in a way that cancels out any wave that would have propagated further into the conductor. But the oscillations of these charges also set up EM waves that propagate out away from the conductor, forming the reflected wave.
 
  • #9
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ah right, I'd never heard that before. I'm asking because I've just done a semester's worth of the optical properties of soids, and this was never addressed at all.
Still though, what you said seems to make sense to me but it's not really a mechanism is it?
Doesn't that just say that the maths only checks out because reflection does indeed exist, rather than providing a mental picture such as, say, two electrons exhanging a photon to repel each other and make each other change direction?

Am I asking too much of this, is this the limit of what is understood about reflection, or have I overlooked something you said?
The_Duck has provided a reasonable picture of physical mechanisms for conducting reflection.

The same sort of thing happens through the volume of a dielectric and right up to the air interface. The dipoles of the material oscillate due to the wave going through the material from the the air side. The oscillating dipoles form a volume current described by the polarization vector P. The P vector radiates fields just like any other oscillating charge. The P vector at the air interface ends up radiating a small wave back into the air side which becomes the reflected field.

Let me know if you need a third way to visualize this.
 
  • #10
morrobay
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Regarding what wavelengths are reflected from an opaque non conducting surface
and what wavelengths are absorbed:
Would that explanation be classical, related to the specific resonance frequencies of the atoms and molecules in the particular material, that determines selective absorption or selective reflection in a particular wavelength.
Or would that explanation be with QM , related to specific energy level transitions, by those
same particles.
 
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  • #11
morrobay
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Regarding what wavelengths are reflected from an opaque non conducting surface
and what wavelengths are absorbed:
Would that explanation be classical, related to the specific resonance frequencies of the atoms and molecules in the particular material, that determines selective absorption or selective reflection in a particular wavelength.
Or would that explanation be with QM , related to specific energy level transitions, by those
same particles.
Hello Antiphon.
That was a good explanation of the reflective waves in post # 9
But what determines the wavelengths of those waves ?
 

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