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But,what is the reason that they are parallel ?phinds said:They are not mathematically parallel but for PRACTICAL purposes, yes they are.
Ok!i will wait for your answer!Drakkith said:The reason that the Sun's rays are nearly parallel to each other has nothing to do with its size. It is solely the result of how far away the Sun is. Unfortunately I don't have a good diagram available and don't have the time to make one at the moment. I'll try to get back to this tomorrow or something.
If they were parallel, the sun would be a single point in the sky, not a disc:navneet9431 said:Are the Sun's rays reaching us always **PARALLEL rays**?
navneet9431 said:
Are the Sun's rays reaching us always **PARALLEL rays**?
The Geometry (Basic School stuff) is what counts here - along with a sense of what's practical. The value of the angle between two radial lines is given by the ratio of the separation where they arrive and their length. Comparing the width of even the widest practical conventional telescope with the distance from the Sun gives a ratio that's as near as dammit to Zero so rays from a particular point on the Sun can be regarded as pretty near parallel. Looking at it the other way round; the Sun's image subtends an angle of about 0.5° on Earth, which means that rays from the Sun are up to about half a degree out of parallel. That's near enough parallel for many practical purposes. When you need more accuracy from a measurement (for instance in Astral Navigation) of the Sun's position in the sky then you have to take this half degree into account. In a Solar Oven, the half degree doesn't matter but when you are taking a photo of the Sun (be careful if you ever try this), that half degree is enough to show the Sun as a small disc and not a single point.navneet9431 said:Are the Sun's rays reaching us always **PARALLEL rays**
Strictly speaking, that is absolutely not true. As glappkaeft correctly pointed out, it is due to the relationship between size and distance, with the size being trivial relative to the distance. If the size were substantial relative to the distance, they would not appear parallel. Think about it, @Drakkith. What would be situation be if the sun were the same distance from the Earth (93 million miles), but 50 million miles in diameter? Would the size then be irrelevant?Drakkith said:The reason that the Sun's rays are nearly parallel to each other has nothing to do with its size. It is solely the result of how far away the Sun is.
navneet9431 said:My book says that the Sun's rays reaching us are always parallel.
See this image <snip>
Are the Sun's rays reaching us always **PARALLEL rays**?
A google search revealed http://web.mit.edu/2.009/www/experiments/deathray/10_ArchimedesResult.html How about that?Andy Resnick said:The fallacy that sunlight is collimated leads to the erroneous claim of Archimedes 'death ray' being practical.
phinds said:Strictly speaking, that is absolutely not true. As glappkaeft correctly pointed out, it is due to the relationship between size and distance, with the size being trivial relative to the distance. If the size were substantial relative to the distance, they would not appear parallel. Think about it, @Drakkith. What would be situation be if the sun were the same distance from the Earth (93 million miles), but 50 million miles in diameter? Would the size then be irrelevant?
Andy Resnick said:No, because the sun is an extended object, subtending 32 arcminutes. The fallacy that sunlight is collimated leads to the erroneous claim of Archimedes 'death ray' being practical.
There is no way that an image of the Sun in any normal optics can be regarded as a "point". The focussed image is in the Image Plane and not at a point.Drakkith said:the reason the image of the Sun is formed at the focal point
sophiecentaur said:There is no way that an image of the Sun in any normal optics can be regarded as a "point". The focussed image is in the Image Plane and not at a point.
Drakkith said:Because of this, the image of the Sun will be formed at the mirror's focal point, again, as the book says (focal plane actually, but I'll use focal point to keep things simple).
sophiecentaur said:All those diagrams are 'correct', just not all of them are relevant to the question. You can select any rays you choose, depending what you want to know.
Drakkith said:Hold on, I think there's some confusion about what "parallel" or "collimated" rays mean in this context. A collimated bundle of rays are parallel to each other, as I'm sure most people understand. However, when we speak of collimated rays, we usually mean a collimated bundle of rays, and this bundle is, as far as I know, always emitted from the same point on the object. I've never read anything talking about a bundle of collimated rays where the rays are emitted from different points on the object except perhaps starlight. However the OP's book is talking about the light from the Sun, not starlight.
<snip>
sophiecentaur said:A google search revealed http://web.mit.edu/2.009/www/experiments/deathray/10_ArchimedesResult.html How about that?
The image size must be what counts because that will dictate the energy flux at the focus for a given aperture. I think the area of the image will be proportional to the f2 and you can only compensate for that with a proportionally big aperture area. Isn't this where the f number comes into photography? If the projected distance is small enough then you can make burning happen. There are some massive solar ovens, after all and a ship's hull with a layer of pitch on it would be pretty vulnerable.Andy Resnick said:Indeed- how about that? It's silliness, is what it is. Until someone points out the flaw in Slyusarev's clear argument, I maintain that the death ray and related 'technologies' like solar concentrators are no different than claiming second law violations.
http://www.dtic.mil/dtic/tr/fulltext/u2/281847.pdf
parshyaa said:These are the only rays which are coming in the direction of our planet, rest of them are not in our frame, since sun is too far and our planet is so small in comparison to sun , thus only rays coming from the middle part reaches Earth and they are almost parallel as you can frame from the rough diagram
Thats due to atmospheric refractions, disturbance in their pathA.T. said:
Nope. Seen from orbit the sun still has an angular size of ~0.5°.parshyaa said:Thats due to atmospheric refractions, disturbance in their path
If that were true then we would see a fuzzy ball and not an object with sharp, well defined edges. Also, all stars would look the same as a fuzzy Sun but fainter.parshyaa said:Thats due to atmospheric refractions, disturbance in their path
All rays from one location on the surface can be treated as parallel with no error. But rays from different locations are definitely not parallel. A half degree of difference in arrival angle is very significant. I reckon a high performance racing engine with bores with half a degree of taper would not last long. The poor rings would be knackered pretty soon. Half a degree is like a Barn Door!Shane Kennedy said:Those rays are not parallel.
Agreed! I was shocked this thread made it to page 2!ZapperZ said:Wow!
This was a question that was asked at such an elementary level, but it has taken off in a way too-complicated direction.