MadSci Network: Physics |
The question is pretty complex because of the multiple elements, but I'll try to talk to the underlying physical principle -- then answer the individual elements. A laser beam is just that -- a bunch of photons of similar (but probably not identical wavelength) moving together along roughly the same line. If you direct them toward a mirror, part (but not all) of the beam will be reflected. The reflected beam will be reduced in power because no mirror is perfectly reflecting. The best-optimized reflectors aren't metallic, but are stacks of optically clear dielectric materials (like magnesium fluoride or silicon dioxide or hafnium dioxide or aluminum oxide) which are specially made up so that each layer provides partial reflection and that each partial reflection adds with all the others exactly in phase so that the result is exceptionally good reflection -- it can be 99.9%. If you take a laser beam and put it onto a target with another laser beam, it's pretty likely that you won't see anything very remarkable. If the beams are of significantly different wavelength or aren't made by very very very very very stable lasers, the beams don't really interact with each other in a meaningful way. If the two beams are very close in wavelength, you may observe interference as the parts of the beams which are in phase with each other add to produce a bright area and the parts that are out of phase subtract to produce a dark area. Depending on whether the beams are expanding or contracting or going in exactly the same direction, the area of interference may exhibit vertical or horizontal or circular or elliptical dark and light stripes... or other complex shapes depending on the details of the beam characteristics. Unfortunately, laser beams don't carry image information on themselves unless they have somehow been modulated -- either in position across the beam or in time: they're just pure beams of light. If you arrange to modulate them, you can recover the information, but that's generally pretty complex stuff and gets either into optical data transmission (usually high speed fiber optics) or holography -- both of which are beyond the scope of my answer here. Image formation, on the other hand, occurs on a daily basis any time that light -- from a laser, or the sun, or a light -- falls on an object and is scattered by the object. The scattered light is collected by a lens and focused -- not to a point, but to a series of points depending on the angle of every point on the object from the center of the lens. So now, back to your questions: 1. What is the maximum times a beam splitter can split a single laser beam? A beam can be split an infinite number of times, but it just keeps getting weaker and weaker and, if the mirrors aren't perfectly flat, the beams start to look progressively more and more mottled in appearance. 2. If two laser beams intersect will they provide constructive interference and will a point of light appear? In this case, the waves will interfere if they come from a common source, but the exact form of the beams determines whether and where on the beam the interference is constructive and/or destructive. I should also add here that for visible interference to occur, it's necessary to have the beams closely matched. This can be easily done by splitting a single beam into two beams, but it's also necessary in most cases to make sure the pathlengths followed by the beams before they hit a target are very nearly identical. It turns out that lasers "burp" every now and then (in the trade, they are said to take mode hops which change the wavelength or phase), so that they won't even interfere with themselves unless they are interacting with a piece of the beam that was created at exactly the same time. Generally, the interference area will be the full beam diameter, not a spot, but there are some pathological conditions that might produce a spot -- but never an image in the usual sense unless the beam is sent through a transparency or modulated or some such thing. 3. I told (him or her?) that in order for the light to be visible it would need to be reflected to the eye from something, like a screen???? Yep, in general you're exactly right, for the reasons I indicated above. Thanks for the question!
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