NSC 101 Laboratory for 3/31/98

Overview:

This week's lab will consist of one experiment and a collection of demonstrations to help you understand color and human perception of color. First, you will use a diffraction grating to determine at what wavelengths you perceive various colors.

Related to our new topic of the nucleus, there will also be a station with a multi-channel analyzer where you will be able to see how the various types of radioactive decay particles penetrate different barriers.

Wavelength and Color:

In class, you were given a brief description of the human retina and how three different cone cell receptors give rise to color perception. The cones respond to different ranges in wavelength of light. These wavelengths correspond roughly to the colors red, green, and blue. But the response of the cones varies from one person to the next. You will calculate the wavelengths of the various colors for your eye and compare them with the values for everyone else.

A lamp will be placed just behind a meter stick at the 50 cm mark. A diffraction grating is place about a meter from the lamp. Looking through the diffraction grating, you will see the first order image of the lamp on either side. The image will be spread out into the colors from violet to red. When viewing the spectrum, record the positions where:

1. you can just detect violet and where you can just detect red.
2. where the color changes from red to yellow,
3. where the color changes from yellow to green and
4. where the color changes from green to blue.

For some people, there will not be a clear dividing line between some of the colors. For others, the distinction will be clear. Each student will calculate the wavelength corresponding to these positions and the results will be tabulated and discussed at the blackboard. The formula for calculating the wavelength is:

where

• d = the spacing between the lines of the diffraction grating
• y = average distance from the lamp to the image position
• D = perpendicular distance from the diffraction grating to the lamp

  The diagram below shows the setup.

  

  Color Addition and Subtraction

  Visit the following stations:

  1. There will be four color filters resting on the overhead projector. Overlap them in all possible combinations and record what you see. Can you figure out why you see what you do?

  2. Look around the room through each of the filters. Note the effect it has on the various colors of the objects in the room. Do the filters add or subtract color?

  3. Stare at the green heart with the yellow border for about 30 seconds. Now stare at the blank sheet of white paper. What do you see?

  4. Find the blind spot on the retina of your eye. With one eye covered, keep your gaze fixed on the apple. You should be able to see the orange in your peripheral vision. (Do not look directly at the orange!) NOw move foward and backward until the orange disappears. The image of the orange is now located at the blind spot of your eye. This where the optic nerve connects to the retina to carry the image information to the brain.

  Radioactivity

  Your instructor will demonstate how to start, stop, and clear the multichannel analyzer. You will use it to detect the beta and gamma radiation coming from the radioactive sample provided. (Don't worry, they are too weak to cause any harm!)

  Note the count rate of radiation hitting the detector. Now place various thickness of the paraffin and lead shields between the samples and the detector. Note how the count rate changes. What effect does thickness have? Is there a difference between the lead and paraffin? Is there a difference between the gamma and beta radiation? Is this consistent with what you were taught about the relative dangers of these two types of radiation?

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