# why do some things reflect?

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#### annie123

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##### why do some things reflect?
« on: 01/01/2013 00:00:07 »
Heard a programme about mirrors and wondered why some material reflect light and some don't.This wasn't explained on the programme - just accepted that mirrors work because material 'a' reflects more than substance 'b' with no explanation.

#### evan_au

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##### Re: why do some things reflect?
« Reply #1 on: 01/01/2013 01:06:49 »
One cause of reflection is where there is a big change in refractive index between materials.

You can see this clearly when swimming underwater - beyond a critical angle, all light is reflected back underwater, since water has a much higher refractive index than air.
It is also visible when sunlight reflects off the surface of the water, and a small amount of light is reflected back into the air.
These reflection effects are a problem for camera lenses, which are often given an anti-reflective coating having a lower index of refraction.

Both effects occur more strongly in Diamond (carbon) which has a much higher refractive index than water or glass, and light reflects back from the far side, and appears as many rainbows, and also reflects from the near side as flashes of light.

Refractive Index is a measure of the speed of light in a material, and is a function of the material's electronic properties, and the frequency of light that is being used.

Metals are the classic reflectors, as their electrons are very mobile, and react to incoming electromagnetic waves, causing a reflection from the near side.

Semiconductors like pure silicon also reflect visible light from the near side, and look metallic at optical frequencies. However, they are transparent at infra-red frequencies, so would presumably reflect infra red back from the far side, if it exceeds the critical angle...

See http://en.wikipedia.org/wiki/Refractive_index

#### evan_au

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##### Re: why do some things reflect?
« Reply #2 on: 01/01/2013 01:16:54 »
PS: A major problem for X-Ray telescopes is lack of reflections - X-Rays tend to go straight through most materials.

This is solved by building mirrors of materials like gold, and having the X-Rays strike it at a very shallow grazing angle. http://en.wikipedia.org/wiki/X-ray_telescope#Mirrors

#### Pmb

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##### Re: why do some things reflect?
« Reply #3 on: 01/01/2013 16:58:35 »
One cause of reflection is where there is a big change in refractive index between materials.
The refraction index doesn't tell us what the cause is. It merely describes what happens.

What causes it requires an understaning of what happens when photons/EM waves interact with the medium its moving in. Its been too long since I studied/used those things so I forgot the details. But I think an understanding of solid state physics might be neccesary to understand what's going on.

#### annie123

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##### Re: why do some things reflect?
« Reply #4 on: 02/01/2013 02:00:52 »
Thanks. Yes, I can see that it's a function of the way light interacts with the substance etc. evan_au and I went to Wikipedia as you suggest, and I did know about the refractive index before in such simple expts as seeing a pencil in and above water etc. but as Pmb says my question is a bit further on in that i wanted to know why particular things vary. The talk of  speed of electrons etc helps, but without a lot of equations I was looking for some idea as to why these things happen differently in different substances. Perhaps I'm asking too much, like saying why does light travel at a certain speed. Some thing just have to be accepted as  proved/measured.

#### JP

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##### Re: why do some things reflect?
« Reply #5 on: 02/01/2013 02:27:46 »
The index of refraction of a material can be explained in terms of how the electrons within that material wiggle in response to incident light.  This happens since light is an electromagnetic field and electric and magnetic fields can move electrons.  These electrons wave with the same frequency as the incident light, but will lag behind it.  These waving electrons emit light (moving charges emit a field) of the same frequency as the incident light but which is phase shifted which means it lags behind the incident light wave.  This emitted light is what we call reflection and the interference between this emitted light and the incident light gives rise to Snell's law, which specifies how light bends upon transmission through the material.

#### evan_au

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##### Re: why do some things reflect?
« Reply #6 on: 02/01/2013 10:42:11 »
There is a principle in electrical engineering that if you want maximum power transfer from a source to a load, source and load should have the same impedance, otherwise some energy is reflected back towards the source.

Moving away from electromagnetic radiation, reflections occur wherever there is a difference in the impedance of a medium. For example:
• Ocean waves reflect off coastlines and sandbars; in this case the varying water depth represents a change in impedance
• Waves in a rope reflect when they meet a change in impedance, which could be an end fixed to an immovable object, or a change in the thickness/mass of the rope.

Diffraction effects also occur due to a change in the impedance, which produces a change in velocity. This can be seen in Google Maps of ocean waves diffracting around a coastline; geologists model this for earthquake waves moving in different layers of the Earth having different densities.

#### Soul Surfer

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##### Re: why do some things reflect?
« Reply #7 on: 03/01/2013 10:20:43 »
Firstly to get a reflected image of something the surface must be smooth.  That is smoother than a modest fraction (say about one quarter) of the wavelength of the electromagnetic radiation (including visible light) involved in the reflection"  the shorter the wavelength the more perfectly flat the surface must be.

The next requirement is that it must present a big change in impedance from free space to the electromagnetic waves.  Conducting metallic surfaces are the best because they effectively "short circuit" the voltage generated by the waves and the resulting current flow creates a reflected wave.  Alternatively they must have a high dielectric constant and not absorb the light like glass or diamond but in this case only part of the light is reflected say around 10% for glass.
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#### evan_au

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##### Re: why do some things reflect?
« Reply #8 on: 04/01/2013 06:13:39 »
Reflection may be easier to visualise if we consider a transverse wave in a string (much like a plucked guitar string).

The velocity of a pulse in an "ideal" string is $$v=\sqrt{\frac{T}{u}}$$, where T is the Tension, and u is the mass per unit length.
(see http://en.wikipedia.org/wiki/Vibrating_string#Wave)

• If the string is infinitely long, a pulse of energy will continue propagating in a single direction forever, without distortion. The movement of one tiny fragment of the string will pull on the adjacent fragment, causing it to have an equal displacement, causing the pulse to move along the string.
• Pulses heading in opposite directions will pass through each other unaffected.
• If the string is anchored at one end, the string mass is effectively infinite at this point, and the string displacement will be zero. Since energy is force times distance, no energy will propagate beyond the fixed anchor point. The effect on the string is similar to two pulses of equal shape and amplitude but opposite phase passing through each other in opposite directions. You end with total reflection of the incoming pulse, but with opposite phase.
• If the string is able to flap free at one end, the string mass is effectively zero at this point, and the force on it will be zero. Since energy is force times distance, no energy will propagate beyond the free-floating end. The effect on the string is similar to two pulses of equal shape and amplitude and polarity passing through each other in opposite directions. You end with total reflection of the incoming pulse, but with same phase.
• If the pulse strikes a segment of string having greater linear density, it will be deflected less than the amplitude in the lighter string. Some energy will propagate into the heavier string (but not all of it). This results in partial cancellation of the incoming pulse, and a reduced intensity pulse reflected back into the light string (ie partial reflection).
• If the pulse strikes a segment of string having lower linear density, some energy will propagate into the lighter string (but not all of it). This results in partial cancellation of the incoming pulse, and a reduced intensity pulse reflected back into the heavier string (ie partial reflection).

The string analogy is rather physical and also rather analogue, but perhaps easier to imagine than quantum interactions with an electromagnetic field. However, there are some strong similarities: A pulse on a string is similar to a photon in that:
• The photon also carries energy
• The photon has a particular velocity in a given medium $$v=\sqrt{\frac{1}{\text{\epsilon\mu}}}$$, where ε is the electric and μ the magnetic constants of the material.
• Photons can pass through each other
• Photons can interfere with each other anti-phase to cancel or in-phase to add together.
• Photons can have total or partial reflection when they strike a material of different impedance
• Most optical media (like most real strings) are non-ideal in that different frequencies travel at slightly different velocities, so the pulse shape changes slightly over time, an effect known as dispersion. This causes problems for those of us who use it for communications, but gives rise to the beauty of rainbows and the "fire" of diamonds: http://en.wikipedia.org/wiki/Dispersion_relation

#### annie123

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##### Re: why do some things reflect?
« Reply #9 on: 04/01/2013 19:14:49 »
Thanks everyone. Interesting explanations. I certainly learned something.

#### yor_on

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##### Re: why do some things reflect?
« Reply #10 on: 05/01/2013 19:23:55 »
Heh Annie, time to start taking physics. You got a flair for asking the right questions.
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