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Appendix C C-73<br />

vertically downward to the point at the center of a coin lying at the bottom of a<br />

cup of water. The solid red rays emerge from this point and enter the eye after<br />

bending at the water’s surface. The ray emerging from the center of the coin to<br />

the center of the eye is called the chief ray. On the right side of the figure, one<br />

ray emerging from the center of the coin is shown refracted at the surface of the<br />

water and entering the pupil of the eye. The dashed line extends the part of this<br />

ray in air back through the water intersecting the chief ray. We could perform<br />

this same construction for any other non-chief ray in Figure D. Our goal is to<br />

show that all the dashed extensions would intersect the chief ray at approximately<br />

the same point.<br />

In Figure E we lay out the geometry for a typical ray in great detail.<br />

Air: nª=1<br />

P<br />

˙ª<br />

1<br />

y<br />

O<br />

yª<br />

A<br />

˙<br />

I B<br />

4<br />

Water: n=<br />

3<br />

Figure D<br />

Figure E<br />

Exercise 1<br />

(a) Show that a ray emerging from the coin making an angle f with the vertical<br />

intersects the air-water boundary with angle of incidence f. Then show<br />

that angle BPI is equal to the angle of refraction f.<br />

(b) Show that the length of line segment IB is sin f, which by Snell’s law is<br />

n sin f, then show that the length of PB is 21 n 2 sin 2 f. Caution: Pay<br />

attention to the direction of the rays in Figure E.<br />

(c) Show that the length of line segment OI is y tan f.<br />

(d) Show that triangles IOA and IBP are similar.<br />

y tan f21 n 2 sin 2 f<br />

(e) Use parts (a) through (d) to show that y¿ <br />

and<br />

n sin f<br />

simplify to show<br />

y¿ 21 n2 sin 2 f<br />

cos f<br />

1<br />

n y<br />

The last formula allows us to compute the point at which the center of the<br />

coin would appear to be located if we just consider the chief ray and a particular<br />

ray emerging from the center point at an angle f to the vertical. To complete

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