Why is a blue pencil blue? White light, which contains all of the colors of the visible spectrum, arrives at (or is incident on) the pencil. Some of the photons that white light is composed of are absorbed by the paint on the surface of the pencil. Other photons are reflected from the surface of the pencil, and eventually reach your eye. The photons that are reflected are the blue photons, or the blue waves. Everything else is absorbed. One side of the pencil is light because a lot of photons are reflecting off that side, so a lot are reaching your eye. Another side is darker because there are fewer photons reaching that side, so fewer reflect from the side and reach your eye. ABSENCE of light is interpreted by your brain as darkness. So the black lead of the pencil is black because?

Yes, all of the rays incident on the lead are absorbed, and none are reflected back, so the absence of light rays coming from the direction of the lead is interpreted as BLACK.

Absorption of a light ray simply means that the energy in the photon is changed into another kind of energy.

• Photosynthesis occurs because chlorophyll uses the energy from the sun's rays to cause a chemical reaction.  Photochromatic lenses change color because when they are exposed to UV rays some special molecules undergo a chemical change that causes them to absorb visible light.
• The silicone in the little plates at the top of a solar powered calculator changes the light that it absorbs into electrical energy.
• The blacktop in the street gets very hot in the summer sun because the tar converts the light that falls on it into heat.

When you look in a mirror, all of the light in its various colors and textures and intensities are reflected from the surface of the mirror.

When you look at the moon rising over an ocean or a lake or a pond or a pool or a puddle . . . oh, well, anyway, it is the rays that leave the sun, reflect from the surface of the moon, reach earth and reflect from the surface of the water that cause you to see the trail of moonlight as if it originated at the surface of the water instead of millions of miles away in the sun.

I know, I know, you got the picture. (Was that a pun?) (I know my artwork is primitive.  I'm and Optician.)

We think of mirrors as reflecting surfaces, but we do not think about the carpet as a reflecting surface. Why?

The surface of the mirror is polished very smooth. If you were to inspect this surface under a very powerful microscope you would probably see irregularities, but the irregularities would be much smaller that the wavelength of visible light. As a result, the waves of light that are incident on the surface reflect back travelling in the same relationship to each other that they had before they hit the surface. This is specular reflection, or regular reflection. In order for an X-ray reflecting surface to have specular reflection for X-rays it has to have irregularities polished even smaller than for visible light reflection. In order for a radio wave reflecting surface to have specular reflection for radio waves, the irregularities only have to be 'much smaller' than a meter.

The irregularities on the surface of a carpet are certainly much smaller that a meter, but they are gigantic in comparison to a light wave. So the reflection of light from the carpet is diffuse reflection, and our brains interpret it as color and texture rather than reflection.

Are you getting the idea that everything that you see is the result of reflection, either specular or diffuse?

Well, anything that you interpret as being opaque is precisely that. It is dark or light depending on the intensity of the light reflected from it, and it is a color depending on what wavelengths are reflected from the surface rather than absorbed or transmitted.

An opaque shinny object is shinny because, along with reflecting light back, it is relatively smooth. A matte-finish object is not shinny because . . . you get the idea?

By the way, if only some of the blue and yellow light is absorbed by a material, and some of the blue and yellow are reflected diffusely along with all of the red, then you get a light red or a pink, because there is white light (made up of all colors) plus extra red light reflecting diffusely from the surface.

   Now we get to refraction. This is what happens to the light that goes through the material. We discussed this briefly in the history lecture. Look again at what happens to the waves of light as they travel from one material or medium to another. If the wave is slowed down and it is incident on the surface at an angle, it changes direction toward being closer to parallel to the surface. If it speeds up it changes direction away from being parallel to the surface.

Imagine a car travelling on a nice smooth dry hard surface. Minimum friction. Maximum speed for the amount of energy expended in driving the car.

Now imagine the car coming to an area of sand. If the car is facing the sand ‘head on’, or both wheels enter the sand at the same time, both wheels slow at the same time and the car slows down but does not turn. More friction, so less speed for the same amount of energy. Because the car entered the sand head-on, it does not change direction. Why, if you are riding a bike, and you see a patch of sand that you cannot avoid, do you steer so that you will enter the sand perpendicular to it? So it will not throw you off by turning the wheel of the bike.

In drawing (b) one front wheel enters the sand before the other front wheel does, so the car is pulled off course - it changes direction toward the normal to the surface. The normal is the imaginary line drawn perpendicular to the surface at the point where the car enters the sand.

When the car reaches the other side of the sand trap, if one wheel speeds up before the other one does, the car will again change direction, this time away from the normal, since the wheel that exited the slow material first is the wheel that is away from the normal.
 
 
 
 

OK, now what I recommend that you do is go to the worksheet for this week and fill out the first part. Please enter the answers in your own words. Thinking through the concepts, and then writing them down on paper, is part of the process of learning it. So print the worksheet and write it out. When the lesson is complete submit the work sheet to your instructor.