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Why does reflection occur on transparent materials? For example, air/glass or air/water (two different interfaces)

Mostly it's refraction that's occurring, but I also see some reflection. Why is that?

Kelly Choi
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    Good question. Check out https://en.wikipedia.org/wiki/Fresnel_equations The simple GIF anim at the top of that article is quite good. – PM 2Ring Jan 20 '19 at 13:43
  • @PM2Ring That GIF is a little scant on the "why". A physical demonstration might help clarify. (The video talks about sound waves, and the physical mockup [and explanation] doesn't exactly match the details of either sound or light, but it's a approximate analogy to help get a grip on the basic concepts). – R.M. Jan 21 '19 at 04:24
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    Possible duplicate of Why does a window become a mirror at night? –  Jan 21 '19 at 08:28
  • Wikipedia's "Theory" section on anti-reflective coatings answers this and also has a really interesting description of methods used to reduce reflections, it was a fascinating read for me just now. https://en.wikipedia.org/wiki/Anti-reflective_coating#Theory – Keavon Jan 21 '19 at 11:15
  • You should think of all materials as both reflecting and refracting. It's worth noting that refraction also happens at the boundary of "non-transparent" materials, you just don't notice it as such -- because the refracted light is mostly absorbed by the material itself, or scatters within the material and re-emerges on the same side of the material boundary as the viewer. For any non-metal materials, whatever part of their appearance doesn't look like a sharp specular reflection is probably refracted, scattered one or more times within the material, then refracted back out to your eye. – Larry Gritz Jan 23 '19 at 19:09

4 Answers4

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It is a property of waves that when they meet an interface, where the medium through which they travel changes, some of the wave is reflected from the interface and some of the wave is transmitted through the interface.

In the case of light it is a change in the refractive index (related to the speed of light in the medium) which produces a reflected and a transmitted wave.

All other things being equal a greater amount of reflection occurs at an air/glass interface than at an air/water interface because the difference in refractive index at the air(n=1)/glass(n=1.5) interface is greater than for the air(n=1)/water(n=1.33) interface.

Note that a light wave traveling in a medium of higher refractive index than the refractive index of the medium on the other side of the interface can be totally internally reflected if the angle of incidence exceeds the critical angle.

In terms of what happens on the molecular scale the incident light waves produces currents (oscillating charges) in the medium and these oscillating charges re-radiate the electromagnetic waves in all directions which add together to form a transmitted wave and a reflected wave.
Richard Feynman has produced a novel way of determining the fraction of light which is reflected in Volume 1 Section 33 of Feynman Lectures in Physics..

Farcher
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As a general answer, waves reflect at discontinuities in the velocity of propagation. This is also true for a pulse on a string, for example at a knot where ropes of different densities are joined together. Or at a mass that is fixed on the string.

  • Is this even an answer to the question? OP asks WHY this happens, and you answer that it happens? – Zaibis Jan 21 '19 at 10:27
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    @Zaibis "Why" questions are rarely answered meaningfully by physics. I think this answer very concisely points at propagation velocity discontinuities as the underlying phenomenon. It could provide a little more background (Fresnel equations), but I still believe this is a valid and helpful answer. – dasdingonesin Jan 21 '19 at 13:54
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    @Zaibis There was already the answer by Farcher who described a mechanism for light. I just wanted to point out that similar things happen in other contexts. I considered some background about continuity conditions but found it difficult to write something clear and short and general. –  Jan 21 '19 at 19:23
  • @dasdingonesin: Thanks for your information, I was just wondering, cause it looked for me like this wouldn't give OP any new meaningful knowledge. But not being a physic, my impression doesn't mean much :P – Zaibis Jan 22 '19 at 04:33
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Reflection usually happens because of a change in refractive index. Reflection occurs because the EM field of the incoming radiation causes oscillations. Electrons in the material receiving light jiggle as it hits them, making them each secondary emitters of radiation and they re-emit light (effectively seems like bouncing off). Glass and water both have different refractive indices than air, so when there's an interface between air and water or between air and glass, you get a reflection. This is why you see reflections off glass or off the water, or other materials that are normally transparent. This emitted radiation goes both into the substrate and back into the original medium, with the net sum of all of the secondary emissions forming the reflected and refracted rays, respectively.

exp ikx
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  • "Glass and water both have different refractive indices than air, so when there's an interface between air and water or between air and glass, you get a reflection." That's more an observation than an explanation, isn't it? The thing about emitted radiation gets to the core but does not explain what's special about the transition phase between two substances as opposed to a plane within a homogenous transparent substance (the question may rather be "why are some substances transparent if light in principle propagates in all directions?"). – Peter - Reinstate Monica Jan 21 '19 at 11:18
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Why does reflection occur on transparent materials.

I assume this is because you may be supposing that all the light incident on a transparent is let through. This is not completely correct. Transparent materials aren't fully transparent; they let most of the light through (and this is refracted); and the light which isn't let though, is reflected.

Mozibur Ullah
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