Q & A: reflection of light by glass

Q:
Why does light sometimes get reflected, and is sometimes allowed through glass? I was playing with my laser, and shining it upon my glass table. There was a spot of light on the point of incidence, there there was a beam of the laser that went through the glass table and fell on the floor which is normal since glass is transparent. But then I also saw a point of light on the roof. I.e., a part of the laser light was reflected off. Why is this? (the angle of incidence was really small so I don't think this is TIR)
- Savyasachi Jha (age 15)
New Delhi, India
A:

This is an old interesting question- why does glass partially reflect light. It was given a good modern answer by Maxwell's theory of electromagnetism. This classical explanation is enough for our purposes, and it applies to any light wave, not just laser light.

So the first thing to consider is that light is an electromagnetic wave. In any standard material, there are charged particles (mainly electrons) that move in response to the electric field of the light. This current creates an extra component of the magnetic field, compared to what would go with the same electric field if the glass weren't there. There are boundary conditions at the surface of the glass, describing the relation between the electrical and magnetic fields inside and outside the glass. If there were just a wave coming in and then continuing in the glass, there would be a jump in the magnetic field at the surface, but no surface current layer or magnetic material to cause the jump. If only some of the wave is transmitted and there's also a reflected wave outside, its magnetic field can add to the field just outside the glass and its electric field subtracts from the electric field there. So long as the ratio of the transmitted to reflected wave strengths is right, the electric and magnetic fields inside and outside the glass match.

If you try to picture light as a stream of particles rather than a continuous wave, the question becomes a bit trickier. I won't try to give the quantum version here, but it's at heart very similar to the classical version. Historically, however, Newton really thought that light was a stream of classical particles, so he did worry about "Why does light sometimes get reflected, and is sometimes allowed through glass?" His amazing answer was that the particles have "fits" of easy and hard refractability. Those fits don't exist, according to any of the common modern interpretations of quantum mechanics.

Mike W.

(published on 12/22/2012)

Follow-Up #1: explaining light reflection by glass

Q:
I thank you so much for taking time out to answer my question. But I'd be lying if I said I had understood any of it. If you could please explain to me like I'm a 15 year old, I'd appreciate that *very* much!
- Savyasachi Jha (age 15)
New Delhi, India
A:

Savyasachi- Thanks for the reminder that we need to keep this clear for our readers, not just for ourselves. Sometimes I forget. So here's a simpler explanation. Please ask for more follow-up if that would help.

First, think of the light as a wave. It's not that it sometimes reflects and sometimes transmits, but rather that it always partly reflects and partly transmits. Picture a water wave going from one medium to another. For example, the water wave could start in a deep region and then transmit to a region where the water is very shallow. You could set that up with a deep pan with some blocks at one end. Start a wave in one part and you'll see that part of it keeps going at the boundary and part bounces back. Very much the same happens with the light wave at the glass/air boundary.

How can you picture what goes on for light at a more microscopic level? The light wave consists of electric and magnetic fields. The electric field alternates direction, just as the water wave alternates going up and down. Electric fields push and pull on charged particles. (That's why a battery drives an electrical current.) There are charged particles (electrons) in the glass, so they wiggle back and forth as the wave hits them. The wiggling electrons send out their own light wave, just the same as wiggling your finger up and down in water sends out a water wave. Some of that wave joins with the other wave going forward and some goes the opposite direction, making the reflected wave. The part that goes forward actually partly cancels the wave that came in (the electric fields point opposite ways) so that the transmitted wave is weaker than the incoming wave. That makes sense, since some of the incoming energy bounced backward in the reflected wave.

Mike W.

(published on 12/25/2012)

Follow-Up #2: basic refraction physics

Q:
That was fabulously explained! But now I have a new question! So, as the electric fields cause the electrons on the glass surface to jiggle, they send out their own light waves. Got it. But then, why do the electrons only send their light waves in the direction of the incident light wave? I mean, the electrons are vibrating in a 3-D environment, so shouldn't there be light waves propagating in all directions around the electron, making a sort of a spherical glow all around the electron? Secondly, does the *reflected* light ray, follow the law of reflection (angle of incidence = angle of reflection)? Because if it does, how do the light waves produced by the jiggling electron know which direction to go, or be reflected in, and don't just go in random directions? Another, question comes to mind. Suppose on a reflective but opaque object, a light beam is incident. Now as the electric and magnetic fields jiggle the electrons on its surface, they produce their own light waves. But what decides, whether the resulting light waves will only be reflected off the surface and not propagated through the medium? And, perhaps, if they are propagated through, how do we know that they are given that the material is opaque?
- Savyasachi Jha (age 15)
New Delhi, India
A:
Great questions.

You're right that any very small piece of the glass is radiating light in all directions. (Strictly speaking, it's in a dipole pattern, but that's a lot more directions than just forward and back.) In fact, if you put a tiny speck of glass in your beam, you will see it in almost any direction. However, with a big flat sheet of glass, the light nearly all goes forward or back. Why?

The reason is that the waves from each little part of the glass add up so that they give net cancellation in all other directions. This wave interference effect is not special to light waves. You can get it with those water waves too. Say that you put one small rock in the water, much smaller than the wavelength. The waves that go out from it will have a circular pattern. However, if you put a long smooth ridge under the water, the waves will mostly be just transmitted and back-reflected. In other directions, the crests and troughs of the circular waves coming from each little part average to about zero.

The answer to your second question is essentially the same. The direction of the reflection is the only backward one where the waves from each part of the surface ad up in phase, i.e. crest and crest or trough and trough, not mixed.

The third question may be a little more complicated. Some opaque objects have molecules that absorb all the light that gets much into them. Others have an internal unevenness that causes the light to scatter around so much that it has almost no chance to make it to the other side. Many have both scattering and absorption.

Mike W.

(published on 12/31/2012)