# For all practical purposes can light be bent (without the help of gravity) or just reflected?

For example, if a single beam of light was directed directly at the tangent of a semi circular mirror, would it be considered bending or redirecting many times to form a near circular pattern? When I say bend I mean in a curved trajectory, not at an angle.

• What you refer to as a 'beam of light' is actually composed of many photons in a coherent superposition; you need to be more precise for a physically meaningful answer, or wait and hope someone is nice enough to nuance your question for you then answer it. – Alec Rhea Dec 10 '17 at 4:05
• Certainly one can "bend" light by shining it through a medium that has an gradient in its index of refraction. – Hot Licks Dec 10 '17 at 4:07
• @AlecRhea, the question seems to be about ray optics. – The Photon Dec 10 '17 at 6:15
• @ThePhoton How so? The OP seems to be asking about the nature of bending light non-gravitationally; if we are only capable of answering the question in some very approximate limit like ray optics then so be it, but I don't think a very roughly approximated answer is being requested. – Alec Rhea Dec 10 '17 at 7:09
• @AlecRhea, He asks about a "beam of light", and the level of the question doesn't imply prior knowledge beyond high school physics. If you want to post an answer that jumps straight to quantum mechanics, I won't downvote. But I don't think that's what will teach OP the most. – The Photon Dec 10 '17 at 7:13

## 3 Answers

The phenomenon you're describing, of light being bent, is observed when light passes through a medium with progressively increasing, or decreasing refractive indexes.

You actually observe it when you see a mirage. When it's very hot, the temperature being progressively higher as you approach the ground, the refractive index decreases (because density decreases) and light coming from the sky is bent upwards explaining why you see "water" on the ground when it's very hot. When in fact, what you actually see is light from the sky being bent upwards into your eyes.

Hope my answer helped! :)

This image sums it up pretty well • Thanks for the well-explained answer, but I'm talking specifically about "curving" light with a semi-circular mirror. – Preston H. Dec 10 '17 at 18:23
• Are you talking about light "sliding" on a curved mirror? If that's the case it's seems to me as it's theoretically possible, but not experimentally... – Gornemant Dec 10 '17 at 19:36

a beam of light can be bent through an angle by sending it through a wedge-shaped piece of glass, requiring neither gravity nor reflection. this phenomenon is called refraction and can be studied in detail on wikipedia.

if a single beam of light was directed directly at the tangent of a semi circular mirror, would it be considered bending or redirecting many times to form a near circular pattern?

The problem with this plan is that it's not possible to make an infinitely narrow beam of light. Because of diffraction, the diameter of a beam of light is limited to roughly the wavelength of the light. And if you try to make a beam with a diameter close to this limit, it will diverge very quickly. To make a collimated beam (one that doesn't diverge very much), you have to make the diameter of the beam substantially larger.

So there's no way for all the light to travel exactly tangent to the mirror surface, and the end result will not be the light beam skimming along the surface to exit the other side of the hemisphere. Exactly what will happen depends on the exact shape of the mirror and the beam profile. You normally wouldn't try to understand this problem analytically but just use a computer simulation to approximate the result.

To answer the question in your title,

can light be bent?

If you want to bend light, then it's more easily done with a prism or lens, or possibly gradient index optics, as described in another answer.

• One thought after I wrote this: It's conceivable to get something like what OP suggested if the "semicircular mirror" is a dielectric mirror and the incoming beam approaches at the critical angle for total internal reflection (not tangent to the mirror surface). – The Photon Dec 10 '17 at 18:56
• Ok, then since it is impossible to have a beam that small, could you HYPOTHETICALLY do it? As for a prism or a lens, isn't that just refraction, which is for all intents and purposes is the reverse of reflection? If so, then it's still at an angle, so not a curve per say. – Preston H. Dec 10 '17 at 19:08
• 1. Hypothetically under what circumstances? My comment above suggests one where it might be possible. 2. Refraction isn't the opposite of reflection any more than a bicycle is the opposite of a car. 3. As shown in Gornemant's answer, refraction by a continuously varying-index medium can produce curved beams rather than sharp angles. – The Photon Dec 10 '17 at 19:24