Focal Length & Concave Lens: Multiple Images?

In summary, using a concave lens will result in a virtual image that can only be seen by the eye. It is not possible to obtain multiple sharp images with a concave lens, and the sharpness of the image depends on the eye's ability to focus. It is possible to get two images with a concave mirror, one real and one virtual, by adjusting the position of the object. The story of finding a concave mirror at a garage sale ended with the mirror being made into a table and still being in use. However, caution should be taken if the mirror is silvered as it can cause fires if left in direct sunlight.
  • #1
nfsmwbe
2
0
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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  • #2
nfsmwbe said:
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!
nfsmwbe said:
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.
 
  • #3
what will happen if I use convex lens...

thanks in advance:)
 
  • #4
nfsmwbe said:
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.
 
  • #5
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.
 
  • #6
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!)
 
  • #7
sophiecentaur said:
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.
 
  • #8
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.
 

1. What is focal length?

Focal length is the distance between the center of a lens and its focal point. It is measured in millimeters and determines how strongly a lens converges or diverges light rays.

2. How does focal length affect the image produced by a lens?

The focal length of a lens affects the magnification and field of view of the image produced. A shorter focal length will produce a wider field of view and a smaller image, while a longer focal length will produce a narrower field of view and a larger image.

3. What is a concave lens?

A concave lens is a lens that is thinner in the middle and thicker at the edges. It causes light rays to diverge and is commonly used to correct nearsightedness.

4. How does a concave lens create multiple images?

A concave lens creates multiple images by causing light rays to diverge. This means that each point on an object will have its own focal point, resulting in multiple images being formed.

5. How can the number of images formed by a concave lens be calculated?

The number of images formed by a concave lens can be calculated using the formula N = (360°/α) - 1, where N is the number of images and α is the angular aperture of the lens. This formula applies when the object is placed within the angular aperture of the lens.

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