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 Subject: Mirrors and light rays Category: Science > Physics Asked by: brian999-ga List Price: \$10.00 Posted: 12 May 2006 11:46 PDT Expires: 11 Jun 2006 11:46 PDT Question ID: 728174
 ```As far as I understand, the film inside a photo camera is positioned in such a way that every light ray that hits some spot X on the film comes from a single source on the object being photographed. Here's what I mean: http://static.howstuffworks.com/gif/camera-diagram3.gif The various light rays from the top of the pencil, no matter where or at what angle they hit the lens, will be focused to one point on the film. If so, then I have two questions: 1. What about interference effects between the rays of light? Since they all hit the same point on the film, and come from the same source, it seems they will be coherent and should produce interferene at that point. Yet we don't see that when taking photographs. 2. What about when the source of the rays is not an object (pencil, etc.) but a mirror? In that case, even if the rays come from the same point on the mirror, they are still of different colors (frequencies) since they are reflections of light rays entering the mirror. And yet, all these various frequencies from the same point in the mirror will hit one single spot on the film. Shouldn't this produce a mix of colors on the resulting photograph, instead of a picture of reflection that we expect? (The following image shows the situation I have in mind: http://i2.tinypic.com/wsql2r.jpg The green and blue rays are reflected off a mirror, at the same point, and then enter the lens and are focused onto a single point A - and this will happen for every ray coming from the same point on the mirror. As a result, any photodector/film/etc. on point A should detect a mix of all colors (from rays coming from all angles), yet in reality we only see one color from one ray).```
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 Subject: Re: Mirrors and light rays From: rracecarr-ga on 12 May 2006 13:19 PDT
 ```Great questions. 1) You do see interference. More specifically, a point source on the object produces not a point in the image, but an Airy disk. The bigger the lens, the smaller the resulting Airy disk. So a pinhole camera cannot make as sharp an image as a lens can. The interference pattern you get from light that passes through a single aperature is called a diffraction pattern, and when the source is far away, it's called Fraunhofer diffraction. A second, theoretical point, ignoring diffraction, is that the optical distance from a point on the object to its corresponding point on the image is identical regardless of the path. This is a consequence of the priciple of least time: in going from point A to point B, a light ray will always take the path that minimizes the travel time. A lens makes all possible paths from A through the lens to B take the same time to complete, so rays take all the paths. The result is that rays that left the source point in phase with each other will still be in phase upon reaching the image point, and will interfere only constructively. 2) If a camera is close to a mirror and adjusted to bring the mirror into focus (that is, so that the edges of the mirror or writing on the mirror appear sharp), then the image in the mirror will be blurry. The effective distance to the source of the light rays that form the image in the mirror is greater than the distance to the mirror. So yes, you do get a 'mix of colors' in the photograph, and the result is a blurry image.```
 Subject: Re: Mirrors and light rays From: redfoxjumps-ga on 12 May 2006 20:06 PDT
 ```In xray lithography when they need The best possible image to produce the smaller and smaller more detailed image to make microprocessors, etc. They are now jumping thru many hoops just to get a blury image to make it work. X-rays can be thought of as light with the frequency turned way up.```
 Subject: Re: Mirrors and light rays From: brian999-ga on 13 May 2006 18:26 PDT
 ```rracecarr-ga wrote: >A second, theoretical point, ignoring diffraction, is that the optical >distance from a point on the object to its corresponding point on the >image is identical regardless of the path. This is a consequence of >the priciple of least time: in going from point A to point B, a light >ray will always take the path that minimizes the travel time. A lens >makes all possible paths from A through the lens to B take the same >time to complete, so rays take all the paths. How can we prove that the lens makes all possible paths from A to B take the same time (and hence have the same optical distance)? Is there a proof for this available online, in some text, or elsewhere? >2) If a camera is close to a mirror and adjusted to bring the mirror >into focus (that is, so that the edges of the mirror or writing on the >mirror appear sharp), then the image in the mirror will be blurry. >The effective distance to the source of the light rays that form the >image in the mirror is greater than the distance to the mirror. But it's not just the effective distance that's different - the frequencies of the various light rays that come from a single point in the mirror are different, because they originally came from different sources. My question is how come we are, in fact, able to take photographs (or see with our eyes) images reflected in a mirror, no matter how we adjust the focus?```
 Subject: Re: Mirrors and light rays From: rracecarr-ga on 15 May 2006 11:57 PDT
 ```Fermat's principle of least time is discussed, for example, in the Feynman Lectures on Physics, Volume 1, Chapter 26, sections 3-6. Here is an exerpt (hopefully not a copyright violation) where Feynman starts to discuss lenses: "As another important example of the principle of least time, suppose that we would like to arrange a situation where we have all the light that comes out of one point, P, collected back together at another point, P'. That means, of course, that the light can go in a straight line from P to P'. That is all right. But how can we arrange that not only does it go straight, but also so that the light starting out from P toward Q also ends up at P'? We want to bring all the light back to what we call a focus. How? If the light always takes the path of least time, then certainly it should not want to go over all these other paths. The only way that the light can be perfectly satisfied to take several adjacent paths is to make those times exactly equal! Otherwise, it would select the one of least time. Therefore the problem of making a fousing system is merely to arrange a device so that it takes the same time for the light to go on all the different paths! This is east to do. Supose that we had a pice of glass in which light goes slower than it does in the air. Now consider a ray which goes in air in the path PQP'. That is a longer path the from P directly to P' and no doubt takes a longer time. But if we were to insert a pice of glass of just the right thickness (we shall later figure out how thick), it might exactly compensate the excess time that it would take th elight to go at an angle! In those circumstances we can arrange that the time the light takes to go straight through is the same as the time it takes to go in the path PQP'..." All a flat mirror does is make it appear as though the light bouncing off the mirror comes from behind the mirror. So if you focus your camera for the optical distance to the thing you are photographing (which is the distance from the camera to the mirror plus the distance from the mirror to the object, all the light entering the camera from a given point on the object will end up at the same spot on the film. It does not matter that light from two different locations on the object bounce off the same spot on the mirror. The reflection is specular, and if light from two different points on the object hit the same spot on the mirror, they hit with different angels, and so hit the lens in different locations, and ultimately end up at different spots on the film. That is for a camera focused to take a sharp picture of an image in a mirror. If you focus the camera for just the distance to the mirror (so that the edges of the mirror are in focus) then all light rays coming from the same spot on the mirror will indeed end up at the same spot on the film. That causes blurring of the image. It doesn't completely wipe out the image, because only light from a limited area on the object can bounce of a given spot on the mirror and still enter the camera aperture. So a given point on the film is only 'contaminated' by light from parts of the object close to the 'correct' point on the object.```
 Subject: Re: Mirrors and light rays From: epidavros-ga on 28 May 2006 10:39 PDT
 ```1. There is no reason to suppose the light coming from your object is coherent. Coherence is a property of the source of the light, which in this case is not the object but whatever is illuminating it. In general it is highly unlikely that the light will be coherent. However, any aperture will diffract light. The smaller the aperture, the greater the diffraction it will cause. So a monochromatic point source of light taken through your lens would produce a set of rings known as an Airy Disc on the film. In reality, of course, your object is not a point source, and is not illuminated monchromatically. Hence the effect of diffraction will be to reduce the sharpness of the image, and indeed a larger lens will produce a sharper image for this reason. This is also why telescopes need to be large (it also helps them to collect a lot of light). Some of the best radio telescope results come from linking telescopes on opposite sides of the Earth - this gives the "effect" of a lens the diameter of the Earth, allowing very "sharp" results. 2. Your second points seems a little confused. The diagram cannot possibly represent the setup you describe; trace the incident rays back - they cannot come from the same point on the object. You are persuading yourself that a mirror can separate different wavelengths of light, which it cannot - it will faithfully reflect them to the lens which will then behave in much the same way as in part 1. However, a prism can separate wavelengths of light, and a lens is a lot like a prism. In fact for a single glass lens, the focal length (the point at which the lens focuses parallel rays of light) will be different for different wavelengths of light. As a result, different wavelengths of light coming from the same point on your object will arrive in slightly different places on the film. This leads to coloured fringes around objects in pictures - it is called chromatic abberation, and it is a problem compound lenses attempt to fix. The thing to remember is that colour is not a simple matter of wavelength of light. If you lit your object with white light (equal intensity of each wavelength across the visible spectrum) the point on your object would reflect back a range of wavelengths of different intensity, and would absorb other wavelengths to various degrees. This is what makes up its colour. The visual perception of colour is even more complex - the eye is no good at picking out wavelength of light - it sees colour effectively by comparison with what is around. This is important because light conditions change. For instance, in a room lit by bulbs there is almost no light outside the red end of the spectrum, yet a bit of paper still looks white. But the main issue you need to think about is what is colour?```