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.
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.
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
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
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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Update: June 2012