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Q & A: underwater focal lengths

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Most recent answer: 10/22/2007
How is the focal length of a lens affected when it is immersed in water?
- Nimish (age 15)
The focal length of a lens is very much affected when immersed in water.  The reason is that the angle of refraction is dependent on the relative difference  between the indices of refraction of the incoming medium and the medium of the lens.  To be precise, the formula for the focal length is:  1/flens = (nglass - nincident)*(1/R1 - 1/R2),  where R1 and R2 are the radii of curvature of the entrance and exit of the lens. For a lens in air nincident  is close to oneFor water it is 1.33 or so.  For various types of glass it’s often in the neighborhood of 1.5.   You can experience the effect yourself if you try to read some, waterproof, printing under water.  Your eyes’ focal length change dramatically.


(published on 10/22/2007)

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