I am slightly near sighted. I wear glasses, but pretty much only when I drive. I have noticed that, without my glasses on, visual acuity in the rear view mirror mimics the acuity of a direct view. That is, objects in the mirror get fuzzy as their physical distance from the car increases. Why? The mirror is well within my focal point, and the image is on the mirror. It’s not as if my vision is bouncing off the mirror to the objects. If the mirror is in focus, why aren’t all the reflections?
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It’s kind of hard to answer this without drawing ray diagrams, but I’ll give it a go…
Firstly, the image isn’t ON the mirror. The image you see in the mirror is called a virtual image, and is just as far behind the plane of the mirror as the object is in front of it. Get a big mirror and play with it a while to convince yourself of this.
How does that work? When you look at something, the light from it enters your eye. When you look at something in a mirror, the light from the object bounces off the mirror in such a way that its path is the same as if it came from an identical object behind the mirror. So that is what you see.
(When an image is projected onto a screen, that is called a “real” image and it IS on the screen.)
The short answer is because looking in the mirror is no different than lookng through a window.
I hope this diagram comes out right:
Object You
\ /
\ /
\ /
\ /
/_________ Mirror
/
/
/
/
/
Object
Image
As you can see, the length of the path from the object to the mirror then to you is the same as the distance from you to the image. So your eyes have to focus on the image in order to see it clearly. The problem is that the image is not ON the mirror, but rather, behind it.
Dang, that doesn’t look right, plus matt beat me to it.
The picture is supposed to show the path of light from the object to you via the mirror. The image appears behind the mirror at a distance equal to the from the mirror to the object. The light from the object appears to come from the image, which is behind the mirror.
I can’t draw any diagrams, but I learned from photography books that if you’re taking a picture of something in a mirror, you have to set your focus to account for the distance that the reflected object is away from the lens.
I.e., if you are taking a picture of a flower in a mirror and the flower is three feet away from the mirror, you should focus for six feet away.
I hope I remembered that right. Or else I have a lot of photos out of focus.
As mentioned by Matt, I was also under the impression that only real images could be projected, but compound lenses have me questioning this. A compound lens combines a convex lens with a concave lens. This combination allows a telephoto lens, for example, to have an “effective” focal length as opposed to an “actual” focal length. In other words, the telephoto tube does NOT have to be physically (actual) 200mm in length. The same result is accomplished by combining lenses to gain the same result (i.e.: “effective”).
As I recall from diagrams, as seen from the observer’s eye positioned BEHIND the compound lens, light is first incident upon the convex lens creating a virtual image “projected” so far out IN FRONT of the telephoto lens tube. This serves to add to the total focal length (lens tube + virtual distance to the projected image). Then, the concave lens focuses this image BACK to the eye of the observer.
I guess, the explanation might be that the virtual image I describe here still is not actually projected, but rather a phonomena only observed from behind the compound lens. In other words, I guess you could say this so-called “projected virtual image” I describe still can’t be projected onto a screen, for example.
My facts may be a little blurred…it’s been awhile since I’ve done an optics problem esp. one involving compound lenses.
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I have to say I don’t like Cecil’s answer. Physical distance is not everything. Think about this: what defines the apparent distance? The answer is, it is determined by how parallel the light from the object is. If it is completely parallel, then the object is infinitely far away. If the light is diverging at a large angle, the object appears to be nearby. Near-sighted eyes like mine have no problem with diverging light from a nearby object, but cannot properfly focus light which is more parallel.
Now let’s take a point light source far away, say, a star. The light from it is parallel. Now bounce the light of a flat mirror. What does the reflected light look like? It’s still parallel. So the apparent distance is still infinity.
On the other hand, the light from the TV screen is generated at the screen and diverges from that point. So the apparent distance to the image on TV is the distance to the TV itself, and no more.
I think what Matt meant was that the image projected on the screen is called a real image. Or to be more precice, if there is a point somewhere where you can put a screen or film to obtain a focused image, then the image there is a “real” image. You can look at a virtual image, or use another lens to project it onto a screen/film, but if you try to place a screen where the virtual image appears to be, there’s nothing there.
Something else you may not realize: when light bounces off of an object, it goes off into a bunch of different direction. So for any given point, light from that object isn’t coming just into one point in your eye; some of it is coming into the top of your eye, some to the bottom, some to the left, some to the right, etc. That’s where focusing comes is. Your eyeball manipulates light waves so that all the light coming from a certain point ends up in a certain point. If you’re near-sighted, then that means your eyes can do this “put light from the same point at the same point” trick really well for close points, but not very well for far points.
Okay, so what does this have to do with a mirror? You may think that the mirror is close, so all the light from a certain point on the object is all bouncing off one point on the mirror. For instance, if you’re looking at a Straightdope column in the mirror, there perhaps there is one point on the mirror off of which all the the light from the “S” is bouncing off, and another point where the light from the “t” is bouncing off, etc. This is not the case. Light from every part of the object is bouncing off every point of the mirror. So when your eyes look at the mirror, they don’t see light bouncing off a single point on the mirror. So how do your eyes deal with this? They act as if, instead of bouncing off a point on the mirror, the light is bouncing off some point behind the mirror. This coincides perfectly with how the light comes into your eyes, and so your eyeball can now get to work focusing the light. But if you’re farsighted, then the added distance of the image makes it blurry.
Gotcha, thanks all. The light reflected off the mirror actually travels the full distance from the object, not just from the mirror. That being the case, if distance between me and the the mirror is beyond my focal point and the distance from the mirror to the object is also beyond my focal point, then the object would be more blurry in the mirror than if I looked at the object directly. Assuming, of course that the actual distance between me and the object is within m focal distance. Whew.
Looks like a little home experimentation is in order.
What does your vision consist of? Is it emanating from your eyes? If that is the (relative) way that you believe vision works, then the answer to your question is, yes, your vision is bouncing off the mirrors.
Of course, others have answered the question in terms of rays of light coming from the distant objects, but I like this one just as well.