# Magnifying lens

#### pm1

##### Full Member
Can someone please explain this problem for me: 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.

Thanks!

#### milski

##### 1K member
5+ Year Member
How much the light is "bend" when it goes through the lens depends on the ration of the refraction indices of the two media. The water has a higher refraction index than air, so the ration between that and the glass refraction index will be closer to one and the lens will bend the rays less. Less bending - less magnification.

#### pm1

##### Full Member
How much the light is "bend" when it goes through the lens depends on the ration of the refraction indices of the two media. The water has a higher refraction index than air, so the ration between that and the glass refraction index will be closer to one and the lens will bend the rays less. Less bending - less magnification.

okay, and why less bending means less magnification?

#### pm1

##### Full Member

thanks! but I'm still a bit confused.. you said

So relatively speaking, the light rays will not bend drastically and so the focal point will move farther away from the lens in water.

the focal point moves? Is the focal point here referring to where the image is going to be formed?

sorry.. I get confused with optics/snells law

#### chiddler

##### Full Member
10+ Year Member
I was trying to prove it mathematically but i'm stuck and could use help. the 1 in that equation is the n value of air. if we change it to water, it becomes 1.33 and makes that value smaller and therefore the focal length larger.

here are two test values:

One, with air, focal length is 0.4

Two, with water, focal length increases to 1.17.

if object is within 1 f distance originally (for a magnifying lens to work), and then you increase focal length, distance of image becomes larger which means larger magnification.

?

#### pm1

##### Full Member
I was trying to prove it mathematically but i'm stuck and could use help. the 1 in that equation is the n value of air. if we change it to water, it becomes 1.33 and makes that value smaller and therefore the focal length larger.

here are two test values:

One, with air, focal length is 0.4

Two, with water, focal length increases to 1.17.

if object is within 1 f distance originally (for a magnifying lens to work), and then you increase focal length, distance of image becomes larger which means larger magnification.

?

so to find the focal length we need the lens makers equation? and the lens makers equation states that focal length is dependent on n? Should I memorize this equation though?
Saint jude, did you make your conclusions based on this equation or it was a conceptual thing?

#### milski

##### 1K member
5+ Year Member
One way to define focal length is the distance from the lens where parallel rays would cross. Less bending means that the rays will cross further away and the focal length will increase.

The magnification is f/(f-d)=1+d/(f-d). When f increases, the magnification will decrease.

#### SaintJude

##### Full Member
so to find the focal length we need the lens makers equation? and the lens makers equation states that focal length is dependent on n? Should I memorize this equation though?
Saint jude, did you make your conclusions based on this equation or it was a conceptual thing?

conceptual.

#### pm1

##### Full Member
One way to define focal length is the distance from the lens where parallel rays would cross. Less bending means that the rays will cross further away and the focal length will increase.

The magnification is f/(f-d)=1+d/(f-d). When f increases, the magnification will decrease.

OK!! got it! Thank you!

#### chiddler

##### Full Member
10+ Year Member
nevermind. figured.

Last edited:

#### milski

##### 1K member
5+ Year Member
There's also the "no bending - no magnification, so increased bending will be increased magnification" route. Not as safe as math but still quite viable. This thread is more than 9 years old.

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