We would be able to see very little or almost nothing without SUPERPOSITION OF LIGHT WAVES. What is SUPERPOSITION?

WAVE SUPERPOSITION IS AN EVENT WHEREBY TWO COLLIDING WAVES PASS THROUGH EACH OTHER WITHOUT CHANGING SHAPE.

Think about it. Two colliding cars cannot pass through each other. Two colliding people -- as you see in a football game -- cannot pass through each other. When you clap your hands, they do not pass through each other. But light can do something that these material "players" cannot do. Light can SUPERPOSE.

In a moment, I'll explain SEEING in terms of SUPERPOSITION.

But, first, I wish to show you an ACTIVITY that DEMONSTRATES THE SUPERPOSABILITY OF ANY FORM OF WAVES.

This is easy to DEMONSTRATE with WATER WAVES. The fact is, you've seen it happen many times in your life, but you weren't paying attention to it.

In the bathtub or in a pool, you can use the hands of your outstretched arms for SIMULTANEOUS CREATION OF TWO COLLIDING WAVES. In a small pool, two of you can do this experiment, at "opposite sides", acting on an agreed signal.

What happens? You'll see THE TWO WAVES PASS THROUGH EACH OTHER IN NO WAY AFFECTED BY THE "COLLISION"!

Think about it. You look out the window and see a tree in the yard. How is this possible?

Light rays from the sun REFLECT (BOUNCE) OFF THE BRANCHES, LEAVES, TRUNK OF THE TREE and PASS THROUGH THE WINDOW PANE TO YOUR EYES. So what?

Well, at the same time, sunlight, or incandescent light of a lamp, is REFLECTING OFF YOU AND THE OBJECTS IN THE ROOM. THEY PASS THROUGH THE PANE WHILE GOING IN THE DIRECTION OPPOSITE TO THE INCOMNG TREE IMAGE. Some of these INCOMING WAVES MUST MEET OUTGOING WAVES. If SUPERPOSITION DIDN'T HAPPEN, THERE WOULD BE NOTHING TO SEE AT THEIR POINT OF CONTACT! That's "what".

If LIGHT WAVES DIDN'T SUPERPOSE -- PASSING UNCHANGED THROUGH EACH OTHER -- WE WOULDN'T SEE WHAT WE SEE! Dig?

When you are older, you may study a little NUMERICAL ALGEBRA, which provides an EXPLANATION OR MODEL for SUPERPOSITION.

LIGHT WAVES CAN BE MODELED SATISFACTORILY BY LINEAR OR FIRST DEGREE ALGEBRAIC EQUATIONS. THESE EQUATIONS -- UNLIKE SECOND DEGREE OR HIGHER DEGREE EQUATIONS -- ADD THE WAY NUMBERS ADD. NONLINEAR EQUATIONS CAN'T ADD THAT WAY. AND THE PHYSICAL MODEL OF SUPERPOSITION REQUIRES THAT KIND OF ADDITION.

IF LIGHT WAVES WERE NOT LINEARLY MODELED, THE SUPERPOSITION WOULD NOT TAKE PLACE.

(Actually, to tell "the whole story", physicists are aware of "tiny" NONLINEAR ASPECTS OF LIGHT'S BEHAVIOR. Some of these NONLINEAR EFFECTS they understand; some they don't. And there may be other NONLINEAR EFFECTS to be discovered later. But not to worry. These NONLINEARITIES can only be detected by powerful and highly technical experiments, using special instruments. So they can be ignored in discussing EVERYDAY SEEING.)

GOOD OBSERVING!