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Using lenses to create a beam of normal light?? Answered

If you had, say, light from a torch or whatever, directed it through a metal pipe or fiber optic to get it kind of directional, and then into some arrangement of lenses; what kind of maximum range could you expect before it completely lost focus? And would it still obey the inverse square law, or is that just for expanding spheres of light?


That's pretty cool, interesting to see what just a single LED can do. It's still emitting from a point ish light source though... it'd be great to get this going with a 'extended' light source. ie, one that emits from an area.

get some loupes and glasses and experiment with it i think focusing the light source into a binocular (from the eye side) and focusing the binocular may emit a ray as wanted

A perfect converging lens will produce a perfectly collimated beam only for the case of a point source of light placed exactly at the focus of the lens. If your light source is spread out at all, then you'll get a spreading beam coming out of the lens.

If you put your light into something like an optical fiber, as you suggest, you'll have very high losses, but the light coming out the other end can be an excellent approximation to a point source. So you could get a pretty decent beam with solid fixturing to keep the optical fiber and lens perfectly aligned.

Your comment about "completely lost focus" is misplaced. A parallel (collimated) beam is, by definition, completely unfocused. You've taken a point source at the lens focus and inverted it.

If I was putting the light through a tube, so that coming out the other end the light was spread out and going all over the place, is there a series of lenses I could use to get it back to a point, or a column?

Depending on the tube, the light may or may not go "all over the place." With something like a cardboard tube from wrapping paper, there's a lot of scattering and absorption, so you get a dim output, with a spread of a few tens of degrees. With a fiber-optic cable, much of the light undergoes total internal reflection (to maximize the throughput), so at the other end it has a much larger angular spread.

But if it was a wideish tube, can that be got back under control with lenses?

It still is dependant on, as Kelseymh has said, the material of the tube and what it is coated with inside; and some of it will be lost from absorption, and even atmospheric scattering (in a vacuum, it would not have an atmosphere to deal with, but that is difficult over long distances too).

I was thinking of something with total internal reflection, otherwise not much useful would make it out the other end.

What about if I channelled the light into a long conical concentrator with a very small exit hole. Would that in effect create a point light source? And if the cone is, say, 2cm at the wide end, and 1 or 2 mm at the point, how long would it have to be?

Theoretically, if you were to focus a light source into a perfectly collimated beam, it would continue in a straight line forever at the same brightness, except for bending from gravity and loss of brightness from absorption into the medium through which it travels. Basically, a laser. Practically, you will never be able to focus all of the light emitted, it will not be perfectly collimated, and every lens or optical element it passes through will reduce the brightness. However, it will not fade nearly as rapidly as a light source that emits in all direction. To better understand the inverse square law, think about an expanding sphere of light. The total energy emitted is spread out over the surface of this sphere. If you double the radius of the sphere, the total surface area is quadrupled, because area is a square measurement. Since the area is four times bigger, the energy is spread out over four times the area, and thus appears dimmer over a given area. I hope that last paragraph helped you better understand this property, and didn't just confuse you.

Cheers Cameron. In the real world, with reasonably good but not amazing lenses, what kind of range would be realistic?

. What are you trying to accomplish in the real world? Will the light be used for general illumination? Are you transmitting an image? Sending data? Just doing a what-if experiment?

I was thinking of some kind of signaling device, but really want as tight and bright a spot as possible, over as great a distance as possible.

Yeah, that would be the obvious answer, but I'm interested to see if it can be done with a torch or something similar. Also laser pointers tend to chew through batteries pretty quick, whereas an led will last a hundred hours plus.

Now that we have his attention, direct any questions toward kelsey, he's the smart one here.

Yes, it is, but t requires a heck of a lot of gravity. This has been confirmed during solar eclipses-when the sun is blocked out, we can see stars right next to it that aren't really there. They are actually completely blocked by the sun, but the sun's gravity bends the light rays around so they appear to be in a different position.

See gravitational lensing and general relativity.