Things to remember from Lesson 1 and demonstrations and from the laboratory.
First of all, light is what enables us to see. We see light that enters our eyes either directly from the
source or scattered or reflected to our eyes.
We said that there were three models used to describe light:
1. A ray model: light travels in a straight line or ray.
2. A wave model: light is a traveling electromagnetic wave that moves through space.
3. A particle model: light behaves as tiny bundles of energy called photons. In some ways these photons behave
like particles.
We said that color was a visual sensation. When we make a spectrum with a prism or grating we can relate the
colors in the spectrum to the wavelengths of the light as described in the wave model. On the DVD you saw the
spectrum as three color bars- red, green, and blue. That is an artifact of the television system used. If you
want to see a spectrum for yourself, look at a bright light with the 500 lines/mm diffraction grating in your
lab kit or follow the instructions in Activity 1 of Lab 6.
On the DVD I showed that light travels in a straight line by scattering a laser beam with the spray from an
aerosol can. We could not see the laser light directly as the beam traversed the room,
but we saw the path of the beam as it was scattered into our eyes by the dust. That's because we only
see light if it enters our eyes. We can see light only if there is a direct path from the source to our
eyes or if the light is scattered or reflected to our eyes. You also showed the straight line behavior of
light when you explored the nature of shadows and other effects in Lab. 1.
In carrying out the lab exercise you saw that a point source of light generates a sharp shadow and that
a diffuse or extended source of light produces a soft or blurred shadow.
The region completely obscured from the source is called the umbra. The
lighter shadowed region in which the source is only partially obscured is
called the penumbra. To help remember which is which, note that the word umbra is from the
same Latin root as umbrella which you can use for shade. Then penumbra means not quite umbra,
so it is the partial shade.
How does a pinhole form an image? Can you diagram a simple pinhole camera and
explain how it works?
Questions that you should be able to answer include:
Why is the sky blue?
Why are clouds white?
What are the three models we use to describe light?
What are the three primary colors of light?
Additive colors.
The visible spectrum can be crudely divided into three regions of about 100 nm in width: blue, green, and red. These are the three primary colors of light.
We can mix these primaries additively to get other colors.
red + green = yellow = - blue
red + blue = magenta = - green
green + blue = cyan = - red
The order of the spectrum can be remember from the mnemonic Roy G. Biv, for red, orange, yellow, green, blue, indigo, and violet.
However, when you make a spectrum and examine it, you will find that 6 divisions is often enough. In increasing wavelength from violet to red they are:
Color
Wavlength
Violet
400-450 nm
Blue
450-500 nm
Green
500-570 nm
Yellow
570-590 nm
Orange
590-630 nm
Red
630-700 nm
Remember that purple is not the color from 400-450 nm, rather it is a mixture of red and blue/violet. That is,
purple is a saturated magenta. The television does not see all the colors that the eye can see. You were only
able to see three bands in the spectrum as the TV lost the subtleties that the eye picks up.
Here is a simulation that shows both
color produced both by addition (combining light beams) and by subtraction using filters. Follow this link and click on the
link there to color by addition.
Finally, you saw that the coca-cola bottle and its contents look different when
submerged in the tank of water than when in the open air. Why? (The answer
is found in the ideas of Chapter 4 which we will take up later.)
