Post by: annie123 on 01/01/2013 00:00:07
Post by: evan_au on 01/01/2013 01:06:49
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
Post by: evan_au on 01/01/2013 01:16:54
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
Post by: Pmb 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.
Post by: annie123 on 02/01/2013 02:00:52
Post by: JP on 02/01/2013 02:27:46
Post by: evan_au on 02/01/2013 10:42:11
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.
Post by: Soul Surfer on 03/01/2013 10:20:43
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.
Post by: evan_au on 04/01/2013 06:13:39
The velocity of a pulse in an "ideal" string is
, 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
, 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
Post by: annie123 on 04/01/2013 19:14:49
Post by: yor_on on 05/01/2013 19:23:55