Optics/Lens question

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thechairman

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A glass magnifying-lens is submerged in water to view an underwater object. Compared to viewing the object with the magnifying-lens out of water, this will:

A. increase the magnification
B. decrease the magnification
C. not change the magnification
D. the magnifying glass will not work at all under water

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Well when the magnifying glass is under water the index of refraction of water is greater than was in air. This means that as light exists the magnifying glass, there will be an increase in the angle. The converging point of the light will be closer than when it was in the air. Since distance of the image is less than it was before (m = -i/o) you'll find an decrease in magnification.
 
you can also use the lens maker's equation to figure this out. The close the indices of refraction to each other, the less power the magnifying glass will have based on the equation (P = n1/n2 - 1)
 
thanks for all your help

let's say in this case that the lens are made out of glass, which has a refractive index of 1.3, and water has an index of 1.5

from the lensmaker equation, 1/f = (n1/n2 - 1), wouldn't we get a negative value for the focal length? how does that make any sense?
 
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shouldnt u use water and air? not the glass of the lense, which is constant in both examples


also how did u get that derivation of the lensmakers equation, fromwhat i learned its

P = 1/f = (n-1)(1/r1 - 1/r2)



edit: nm, i see ur just looking at it qualitatively and that increasing N means decreasing f
 
shouldnt u use water and air? not the glass of the lense, which is constant in both examples


also how did u get that derivation of the lensmakers equation, fromwhat i learned its

P = 1/f = (n-1)(1/r1 - 1/r2)



edit: nm, i see ur just looking at it qualitatively and that increasing N means decreasing f

yeah, that's how i figured out that question when i was doing it the first time and it turned out to be right so i just went with it :hardy:
 
Edit: Could someone please move this thread to the Q& A forum?

Similar question in Kaplan Q bank.

1.) Still don't understand how ppl figured out this quoted point below & derive that equation from the lens maker equation

"The closer the indices of refraction to each other, the less power the magnifying glass will have based on the equation (P = n1/n2 - 1)"

I see that P = 1/f = (n-1) (blah blah)

I'm interested in a qualitative understanding & don't care so much for a mathematical proof.

2.) Also Kaplan says it will "lose power" does that equal an increase or decrease in magnification? I thought that meant a decrease, but someone said the answer was A.
 
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Alright, this thread will be closed after I post this since it wasn't moved. But since most of the previous answers were kind of mathematical in explanation, I'll offer a more conceptual reason...for closure.

P = 1/f.

Light bends when it enters another medium with a different index of refraction. For a magnifying glass lens (n =1.5) in air (1.003), incoming rays of light will converge to a focal point. Big deal.
Now when a magnifying lens is submerged in water (n=1.33), the ray of light will be characterized by a path with a smaller index of refraction difference ( Δ n now = .2 vs. before Δ n=.5). So relatively speaking, the light rays will not bend drastically and so the focal point will move farther away from the lens in water.
Drawing a pic made this clear. P = 1/f. Bigger f, smaller P. And so the submerged magnifying glass will lose power and the images will reduce in magnification. Answer: B.

I don't know, maybe that didn't help anyone, but for me this makes sense.
 
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