Shine a laser into a pan of shallow water, but on the bottom of the pan place a piece of paper. The result will be an interesting perfect circle of light (Fig. 1). This is caused by internal reflection of the diffusely reflected light. The diameter of the circle depends on both the depth of the water and the index of refraction of the liquid. In this article, I investigate this very simple activity and show examples of how students and teachers can use this phenomenon as a laboratory or demonstration to investigate concepts of the critical angle and total internal reflection.
References
1.
Diffuse reflection is often uniform in intensity in all directions. See https://www.sciencedirect.com/topics/engineering/diffuse-reflection.
2.
It is possible to simply put a lamp underwater, but it complicates geometry because there are now two depths, see
J.
Lincoln
, Visual Physics: “Total internal reflection
,” Phys. Teach.
52
, 227
(2014
).3.
A similar, but not identical, effect in frosted glass is described in
B. R.
Viss
and A. E.
Sikkema
, Apparatus for Teaching Physics: “A demonstration of the critical angle without using total internal reflection
,” Phys. Teach.
43
, 471
–472
(2005
).4.
This geometry is similar to the Snell’s window phenomenon. See
D.
Dibble
, K. Van
Alstyne
, J.
Rohr
, and S.
Ridgway
, “The dolphin in the mirror—A familiar face?
” Phys. Teach.
55
, 8
–12
(2017
).© 2022 Author(s). Published under an exclusive license by American Association of Physics Teachers.
2022
Author(s)
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