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# Re: bending of object thrown into pool

Carl E. Mungan wrote:

I have a dim memory that Newton once argued from his corpuscular view
that light particles should bend *opposite* to the wave prediction of
Snell's law. If someone remembers why that should be so, I'd be
grateful for a primer on the subject.

According to the PSSC version of Newton's corpuscular model, light
particles bend in the same direction as predicted by Snell's law, the
explanation being that attractive forces in the interface between the
less optically dense medium and the more dense medium cause the normal
component of the velocity of the corpuscles to be greater after they
have passed into the more dense medium, while the tangential component
of velocity (and momentum) remains the same. A little right triangle
trigonometry
for a velocity diagram shows that sin(theta1)/sin(theta2)=v(2)/v(1),
where 1 refers to the medium of the incident corpuscles and 2 to that
of the refracted corpuscles. If medium 1 is vacuum,
sin(theta1)/sin(theta2)=v(2)/c. Thus the index of refraction is
n=v(2)/c, just the reciprocal of the value
according to the wave theory. Although Roemer had estimated the speed of
light in vacuum (outer space) in 1676, the direct measurement of the
speed of light in a medium such as water had to wait until the middle
of the nineteenth century. So there was no way to decide on the basis of
which speed was greater in Newton's day. (I think Carl's assertion is
correct once it is known that light travels slower in the more dense
medium.)

PSSC models corpuscles going from a less dense medium to a more dense
medium with a rolling ball moving on a higher horizontal to a lower
horizontal surface, the surfaces connected by a ramp. (The direction
could be reversed to model a corpuscle going from a more dense to a less
dense medium.)
There are photographs of this in Chapter 14 of the 2nd ed. of _PSSC
Physics_ and Chapter 5 of _College Physics, Physical Science Study
Committee_. This is actually a basis for a PSSC experiment, "The
'Refraction" of Particles" to see if they change direction according to
Snell's law. It is in the PSSC _Laboratory Guide_ at least through the
4th ed. I could not find the corpuscular model of refraction in either
the text or lab guide for the latest (7th) edition from around 1991.

I think the demise of PSSC was a great loss to the teaching of physics.
It brought the idea of models to my attention long before I heard of the
more recent Modeling Instruction -- particularly the contrast of the
models of Newton and Huygens for light.

Hugh Logan