W9_parallel_resonance.eps

396 Week 12: Lenses and Mirrors

eye

lamp

Figure 155: How the eye sees an object. Light diverging from pointson the surface of the object are
focused onto the retina of the eye, where they form animageof the object that the retina converts
into neural impulses and your brain converts into perception.

Tocompletelyunderstand how your eye can see the object, we have to get halfway through

this week’s work. On the other hand, we can’t understand enough about how mirrors

and lenses work to understand the eye without understanding theeye well enough to

understand how lenses and mirrors work.

Hmmm, a bit of a dilemma. We have tobootstrapjust a bit and draw a few pictures

now that you won’t completely understand later to help you understand what you need

to understand what you need to understand later. Or something like that.

So meditate on the picture above, which shows light diffusely scattered from from a

couple of points on a common object. The light goes inall directionsfromall of the

points on the surface of the object.Some of these rays reach your eye. There the lens of

your eye does its thing, and forms a nice sharpimageof the object cast upon the retina

of the eye. Vision occurs.

s s’

Figure 156: The geometry of forming an image in a plane mirror.

Now consider looking at an object in aplane mirror. Lamps are too hard to draw, so we

consider an arrow, which we will use as a “generic object” in our diagrams.

Rays radiated from the object radiate out in all directions as shownin the figure above.

When they strike the mirror they are reflected with the angle of incidence equal to the

angle of reflection. As we look at the mirror, we see the rays that originated on a single

point on the objectas ifthey were diverging from a single point in space. That point is

theimageof the point on the object. Since every (visible) point on the objectcorresponds