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# Optics of magnifying glass HW question. Watch

1. Is the image formed when using a magnifying glass real or virtual? For an observer with a near point of 25cm from the eye, what is the greatest magnification that can be obtained with a magnifying glass of focal length 10cm and where should the object be placed to achieve this?

I have been using this picture;

This is how magnifying glasses work, right? The object is placed within the focal length of the convex lens and so the image appears to be coming from behind the focal point and this image is greater is size, so it is a virtual image.

However I do no understand how to use the parameters given. Exactly where would I be placing my eye in this picture? At the focal point on the right side? If it has a near point of 25cm then this means that the minimum the distance from the eye to the virtual image is 25cm, right? The distance could be greater than that.

That's where I'm facing a problem. If the distance from eye to virtual image must be a minimum of 25cm, then surely one could just move the object closer and closer to the focal point and increase the magnification, if the eye stays in the same spot?
2. (Original post by Sasukekun)
Is the image formed when using a magnifying glass real or virtual? For an observer with a near point of 25cm from the eye, what is the greatest magnification that can be obtained with a magnifying glass of focal length 10cm and where should the object be placed to achieve this?

I have been using this picture;

This is how magnifying glasses work, right? The object is placed within the focal length of the convex lens and so the image appears to be coming from behind the focal point and this image is greater is size, so it is a virtual image.

However I do no understand how to use the parameters given. Exactly where would I be placing my eye in this picture? At the focal point on the right side? If it has a near point of 25cm then this means that the minimum the distance from the eye to the virtual image is 25cm, right? The distance could be greater than that.

That's where I'm facing a problem. If the distance from eye to virtual image must be a minimum of 25cm, then surely one could just move the object closer and closer to the focal point and increase the magnification, if the eye stays in the same spot?
there are 2 ways (that I know of) to get magnification with a hand lens.
1. The way you're thinking of, which produces a slightly magnified but distant image - sherlock holmes movie style.

2. Jewelers loupe style where the eye is very close to the lens, less than f and the object is also close to the lens.
There's a ray diagram of a loupe on wiki, but I can't do links on the phone.
I think the question about magnification only makes sense for the jewellers loupe case.
3. (Original post by Joinedup)
there are 2 ways (that I know of) to get magnification with a hand lens.
1. The way you're thinking of, which produces a slightly magnified but distant image - sherlock holmes movie style.

2. Jewelers loupe style where the eye is very close to the lens, less than f and the object is also close to the lens.
There's a ray diagram of a loupe on wiki, but I can't do links on the phone.
I think the question about magnification only makes sense for the jewellers loupe case.
Thanks, the second case works if the eye is placed directly on the convex lens and so the distance of the virtual image from the eye is exactly , the distance of the virtual image as in the lens formula.

The negative of v that is.

Placing v = -25cm gives me the correct answer. However I don't understand how there's a maximum magnification, as the object is placed closer and closer to the focal point the virtual image moves further back and becomes larger and larger giving a greater magnification. The near-eye point just placed a minimum distance of the distance from the virtual image to the eye, not a maximum.
4. The way a loupe works is by allowing you to focus on an object that's closer to your eye than the unaided near point. Producing a 'large' virtual image at near infinity isn't really helpful, you want to be thinking about the angles of the rays coming into the iris at the lens assisted near point compared to the unassisted near point.

Have a go with an actual loupe if you can, don't try to touch it on your cornea though.

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