The Disappearing Beaker - North Carolina School of Science

The Disappearing
The simple mathematics you can use to motivate one
of the best and easiest physics demonstrations ever
Hope Concannon
The North Carolina School of Science and Mathematics
1219 Broad Street
Durham, NC 27705
email: [email protected]
We have all
experienced refraction
when we notice the
apparent bending of an
object when it is dipped
in water. What is really
going on here?
The Law of Refraction
Notice the tie between this equation and the Law of Sines!
Some implications and activities using
the Law of Refraction
Light bends at a boundary between
two materials with different optical
If you want a quick lab,
students can measure the
angle of incidence and
angle of refraction and
verify that the Law of
Refraction describes what
they see.
Total Internal Reflection
Think about the direction light
bends when traveling from
water into air. What happens
when the exiting ray travels
along the surface of the water?
What happens at even steeper
incident angles?
Is there an incident angle 1 where the light will
exit along the boundary surface (2 = 90°)?
sin 1
sin 2
becomes sin 1 =
This has a solution only if light is speeding up at
the boundary (so 1 < 2 and
< 1)!
Optical Fibers
Where did the pipette and test tube go?
A full handout of this talk will be available on
the web after this conference at
Thank you for coming!

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