Focal length of lens in different media

[bdc142]

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Hi all,
I'm confused about what happens to the focal length of a lens if you put it in different media (let's say air in comparison to water). TBR says that it should increase because the angle of refraction decreases, but I can't figure out why.

This is how I approached it:
nmedia x sin incident angle = n lens x sin refracted angle
sin refracted angle = (n media x sin incident angle) / n lens

This would suggest that as n media increases, the refracted angle increases, thereby decreasing focal length?

Thanks

 

[EnginrTheFuture]

Hi all,
I'm confused about what happens to the focal length of a lens if you put it in different media (let's say air in comparison to water). TBR says that it should increase because the angle of refraction decreases, but I can't figure out why.

This is how I approached it:
nmedia x sin incident angle = n lens x sin refracted angle
sin refracted angle = (n media x sin incident angle) / n lens

This would suggest that as n media increases, the refracted angle increases, thereby decreasing focal length?

Thanks

Imagine your media is the same material as your lens 😀 ... sounds like cheating the system but its totally valid. If your lens and the media are the same, you won't even have a focal length because nothing will bend. It goes from having a focal length of f to infinite.

They are assuming the original media is a vacuum. Since n is at is absolute lowest (1) when you are in air, the focal length can only increase.

Note, you are forgetting that the lens has two sides with different curvature. The curvature on the left side is not the same as the right side and thus using snell's law is complicated.

Just think about the right side of the lens though. If the lens is made of glass and the exit material is air, the light will bend, lets say in 30 degrees. If that material is changed from air to something similar to glass, the light will not bend as much upon exiting. Thus the light bends, lets say 1 degree. That extends out the focal length.

It's best to think about lenses in abstract, conceptual ways I think. Looking at formulas is, pun intended, a formula for disaster with lenses 😀

Best of luck

 

[bdc142]

Imagine your media is the same material as your lens 😀 ... sounds like cheating the system but its totally valid. If your lens and the media are the same, you won't even have a focal length because nothing will bend. It goes from having a focal length of f to infinite.

They are assuming the original media is a vacuum. Since n is at is absolute lowest (1) when you are in air, the focal length can only increase.

Note, you are forgetting that the lens has two sides with different curvature. The curvature on the left side is not the same as the right side and thus using snell's law is complicated.

Just think about the right side of the lens though. If the lens is made of glass and the exit material is air, the light will bend, lets say in 30 degrees. If that material is changed from air to something similar to glass, the light will not bend as much upon exiting. Thus the light bends, lets say 1 degree. That extends out the focal length.

It's best to think about lenses in abstract, conceptual ways I think. Looking at formulas is, pun intended, a formula for disaster with lenses 😀

Best of luck

Ok thanks for the help!