Why does light reflect?

August 26, 2004 11:55 AM

Why does light reflect? We see objects by the light reflected off of them. Can anyone explain on a molecular/atomic/sub-atomic level what the mechanism is that makes a light wave "bounce" off of an object?
posted by jsonic to Science & Nature (9 answers total)

Here's a few good descriptions.

I couldn't find any good diagrams, which would be quite useful for this one.
Richard Feynman is good at explaining it, either in six easy pieces or QED.
posted by milovoo at 1:40 PM on August 26, 2004

Basically: light is an electromagnetic wave, which is really just a rapidly time-varying electric & magnetic field. When these electric & magnetic fields interact with the electrons in a material, the electrons begin to move around. Moving electrons (or, more accurately, accelerating electrons) give off electromagnetic waves of their own, in all directions. These scattered EM waves are what's reflected to your eyes.

That's the one-hundred-words-or-less version. Let me know if you have more questions.
posted by Johnny Assay at 2:08 PM on August 26, 2004

How Stuff Works has got a good explanation as well: How Light Works. You can think of light as waves or particles, as explained in ways of thinking about light, and when light hits an object, it can be absorbed, reflected, scattered (which is also called diffuse reflection), or refracted.

The basic idea is that the light interacts with the electrons of the atoms. Depending on the energy (colour) of the light, the photons (light particles) are absorbed by the electrons of the atoms the material consists of, and the excited atom then emits a new photon.

The law of reflection can be explained better with the wave model of light. Imagine that a light wave hits a surface. The electromagnetic wave wiggles the charged particles (electrons and nuclei), but on the surface they can only be wiggled in a certain direction which is determined by the direction the wave came from and the direction of the surface. Accelerated charged particles emit an electromagnetic wave, so a new wave is created. The direction of the wave is determined by the direction in which the particles wiggled. A more elaborate answer in scientific terms can be found here: Reflection - from Eric Weisstein's World of Physics.
posted by amf at 2:18 PM on August 26, 2004

Thanks everyone. If I understand correctly, it is not the incoming light that is 'bounced', rather, the energy of incoming light causes electrons in the target material to produce outgoing light due to electron orbital level changes.

One thing I'm unclear on is how exactly light (or electromagnetic radiation in general) actually propagates, especially through a vacuum. The articles define photons as packets of energy. I can conceptualize light as a 'tangible' particle moving through a vacuum, but I'm confused about how energy alone can exist outside of a medium.
posted by jsonic at 4:18 PM on August 26, 2004

"I'm confused about how energy alone can exist outside of a medium."

Think of light as, more or less, a radio wave, and you'll be most of the way there. Radio -- like light -- propagates just fine in a vacuum with no medium. These aren't raw electrons that need a current path, or sound waves that are comprised of macrophysical vibration, they are electromagnetism.

Just as a magnet doesn't need to physically connect to something ferrous to attract it, light needs no medium.

Just as radar (radio) waves bounce off an object to make it detectable, so does light.
posted by majick at 8:26 PM on August 26, 2004

Light isn't more or less like a radio wave; light is exactly like a radio wave. It is the exact same phenomenon--electromagnetic radiation. The only difference is the frequency.

The particle-wave duality of light will help explain this. You'd have no problem thinking of a particle travelling through a vacuum, would you? Light also acts like a particle, and that particle is called a photon. With some phenomena, like interference, it's easier to conceptualize it as a wave.
posted by AstroGuy at 9:51 PM on August 26, 2004

It's also good to remember that EM radiation, including visible light, doesn't just bounce off stuff all that cleanly. Most of the solid stuff in the room around you absorbs certain parts of the spectrum but repels others, in a more or less distinctive or unique signature, what we would call color.

I'm confused about how energy alone can exist outside of a medium.

You're not the only one. Plenty of folks will offer you explanations, but no one understands just what light is, and how it manages to behave like a tangible particle *and/or* as a media-borne wave in different situations. Get used to wrapping your head around counter intuitive concepts if you want to keep venturing down this particular rabbit hole.
posted by scarabic at 11:32 PM on August 26, 2004

Oh - and it's also good to remember that the sub-atomic particles that make up atoms take up very little space. Most of each atom is empty space. Most of all matter is empty space: small particles moving through a vaccuum - no medium.
posted by scarabic at 11:34 PM on August 26, 2004

Most of all matter is empty space: small particles moving through a vaccuum - no medium.

Actually, I'm pretty certain that light travelling through a block of glass actually does notice quite a few glass atoms sitting around the place. Visible light wavelengths are in the hundreds of nanometer range, while atomic radii and separation in a solid are in the tenth of a nanometer range.

I was taught that light propagation through a dense medium like glass involves a lot of interaction with the atoms of the medium. As the wavefront travels through the medium, it causes each successive layer of atoms to absorb energy and radiate light.

This densely-packed sheet of radiating atoms produces a complicated interference pattern, with the net result that interference in the forward/backward direction is constructive, while interference in the sideways direction averages out to zero.

So why doesn't light travel backwards as well as forwards? Well, the atoms 'behind' the current position of the wavefront are still radiating, and their contribution cancels out the backwards-going part of the light. In front of the current position of the wavefront, the atoms haven't started radiating yet, so the radiation is free to travel forwards.

Incidentally, this is why light travels more slowly in a block of glass than it does in a vacuum. Even though each individual photon can only exist at a speed c and no slower, the absorption and re-radiation process takes time, thus slowing down the apparent wavefront.

The reason you get reflection at an air/glass boundary is that when the wavefront reaches the start of the glass, there are no radiating glass atoms behind it -- just air. So the first layer of glass is free to radiate in a backwards direction as well as a forwards direction. It's this very thin layer, about half a wavelength deep, that is responsible for all the reflection.
posted by chrismear at 2:51 AM on August 27, 2004