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Name: Danny
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: 1999 

Will a magnifying glass under water magnify more or less? Please explain why or why not.

Less. A magnifying glass works by refracting light, and the angle through which the light is refracted depends on the amount the index of refraction changes at the interface between the lens and whatever the lens is immersed in. The refractive index of water is greater than that of air, and less than that of glass.

Tim Mooney

Hi Danny,

A magnifying glass works by causing the path of the light passing through it to bend. How much it bends (and thus the focal length) depends on the change in refractive index as the light enters and leaves the magnifying glass. The index of refraction of air is 1.0 and the index of refraction of a typical glass is about 1.4 (a difference of 0.4 for light passing from air to glass). You can find the index of refraction of water and other substances in the CRC Handbook of Chemistry and Physics at your library. It can also be found in many science text books.

Greg Bradburn

P.S. The larger the change in refractive index, the shorter the focal length. The shorter the focal length, the greater the magnifying power of the lens.

Sounds like a homework question to me. Especially with the "why or why not." So, I'll answer the "why or why not" and leave you to answer the question.

The magnification by a magnifying glass depends on the amount that light is bent when it crosses from the surrounding medium (air or water in the case of your problem) into the glass, and then when it goes back into the surrounding medium on the other side. Generally, the more the bending, the greater the magnification. So, the question comes down to how much bending there is in either case (the surrounding medium is water or is air). The important relation here is Snell's law of refraction: (sin i/sin r) = (V1/V2), where i is the angle of incidence, r is the angle of refraction, V1 is the velocity of the wave (light) in the first medium (air or water), and V2 is the velocituy of the wave in the second medium (glass). So, to answer this question qualitatively, you need to know the relative velocities of light in air, water, and glass.

Richard Barrans Jr., Ph.D.

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