# Focal length of lens

by nfsmwbe
Tags: focal, length, lens
 P: 2 is it true when we use concave lens, we can obtain focal length when we get the sharp image.. is it possible to get more than one sharp image when we change the object distance (distance between object and lens)
P: 121
 Quote by nfsmwbe is it true when we use concave lens, we can obtain focal length when we get the sharp image..
Using a concave lens you cannot get a real image, to calculate the focal length. All that forms is a virtual one, which only your eyes can see. So try measuring the dist. b/w the lens and your eye when you get to 'see' a virtual image next time!
 Quote by nfsmwbe is it possible to get more than one sharp image when we change the object distance (distance between object and lens)
As far as my knowledge runs through, you can't (I think so..). I've tried getting those kind of 'double' images, but even when they formed, one was real and another virtual. I couldn't get two sharps at a time on the screen.
 P: 2 what will happen if I use convex lens... thanks in advance:)
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P: 10,076

## Focal length of lens

 Quote by nfsmwbe is it true when we use concave lens, we can obtain focal length when we get the sharp image.. is it possible to get more than one sharp image when we change the object distance (distance between object and lens)
You will only get a virtual image with a concave lens (on its own). This cannot be projected onto a screen but can be seen. The sharpness of the image will depend upon whether your eye can actually focus on it. (Long-sighted people may not be able to see the image in a convex lens as it may be too close for them.)

I just wonder whether you are, in fact, talking about a concave mirror. You can often get two images - one real and one virtual by adjusting the position of the object wrt the lens.
OR.... is it a convex lens that you mean? In that case, you can also get one real (projectable) image or a virtual image, depending on the distances involved.
 P: 4,667 About 40 years ago, I was at a garage sale, and spotted a large circular piece of glass, about 66 cm diameter and 5 cm thick, for which the seller wanted bids. I was the only bidder, so I paid \$3.00 and took it home, and started cleaning it. I quickly realized that one surface was concave, so I set it up so that I could reflect a distant light bulb to a sharp image on a piece of paper. The calculated focal length was 2 meters. Apparently the glass was an unsilvered reflecting telescope mirror.
 PF Patron Sci Advisor P: 10,076 Is there a happy end to this story or is that mirror now at be back of your garage? haha (I have several such possessions!)
P: 4,667
 Quote by sophiecentaur Is there a happy end to this story or is that mirror now at the back of your garage? haha (I have several such possessions!)
I bought a redwood 2 x 4, and made a 3-legged table. I still have it, concave side up, in our sun room.
 PF Patron Sci Advisor P: 10,076 Got your money'sworth then. A word of warning - don't every get it silvered and leave it laying around - you'll burn your house down if the Sun catches it.

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