# Focal length of lens

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)

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PhysicoRaj
Gold Member
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!
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.

what will happen if I use convex lens...

sophiecentaur
Gold Member
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.

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.

sophiecentaur
Gold Member
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!)

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.

sophiecentaur