Conceptual Physics After Images Lab
Conceptual Physics After Images Lab
To be done in your lab notebook
Pre Lab:
· On a fresh piece of paper in your lab notebook write today’s date and the title of the lab.
· Don’t forget to continue the page numbers in your lab notebook.
· Don’t forget to add this lab and its page number to your Table of Contents.
Procedures:
1. Set up the light box as shown at right, as
if viewed from above.
2. Close the two side mirrors so that there is only one source of white light, as shown in the picture at the right.
3. Create a table in your lab notebook that has two vertical columns and seven horizontal rows. Write “Colored Square” and “After Image” across your first row. In the remaining six rows write: Red, Green, Blue, Cyan, Yellow, Magenta.
4. Place the Red square in front of the white paper with only white light shining on it. Stare at the colored square for 45 or more seconds and then look away at the white portion of the paper that is lit with white light.
5. Make observations in your lab table about what color you see when you look away at the white paper. Have each lab partner try the experiment so that you can agree on a color.
6. Repeat with each colored square and make observations about each in your lab table.
Read the following collection of facts and explanation about After Images:
Color is light, and colored objects absorb and reflect different wavelengths. Color is seen by the human eye because of the two light receptors - rods and cones - located in the retina of the eye. The retina is the back part of the eye that contains the cells that respond to light. These specialized cells are called photoreceptors. Rods and cones vary in sensitivity to different colors and light, and they convey the color of light to our brain.
The rods are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment. Rods are not good for color vision. In a dim room, however, we use mainly our rods, but we are "color blind." Rods are more numerous than cones along the outside of the retina. Next time you want to see a dim star at night, try to look at it with your peripheral vision and use your rod vision to see the dim star. There are about 120 million rods in the human retina.
The cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of color. Cones, however, work only in bright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited for detecting fine details. There are about 6 million cones in the human retina. Some people cannot tell some colors from others - these people are "color blind." Someone who is color blind does not have a particular type of cone in the retina or one type of cone may be weak. In the general population, about 8% of all males are color blind and about 0.5% of all females are color blind.
When our eyes are exposed to a color for a prolonged period, the rods and cones become fatigued. Areas of strong color and high contrast can produce ongoing images when the viewer looks away, these are called After Images. Strong/high combos also result in visual stress with prolonged viewing. The color of the afterimage is the screen color's complementary color. After image colors are involuntary. When you stare at the color red, for example, your red-sensitive cones adapt to the light and lose their sensitivity. When you shift your gaze to a white background, you see white (minus red) where the red-sensitive cells have become adapted. White light minus red light is blue-green light. The after image you see will be in the same shape as the original red image but it will be cyan.
Viewing After Images
There are several set-ups around the room to view after images. Choose any three and stare at them for 30-45 seconds. Copy and fill in the table below. Add color to your after image drawings so that the difference can clearly be seen!
Description of Image / Draw the Image / Draw the After Image1.
2.
3.
Analyzing:
?Underneath your observations, write “Analyzing” and answer in complete sentences. Don’t forget to number your answers!
1. Why do you think you see the colors you do as after images?
2. What does a cone do?
3. What does a rod do?
4. If you have difficulty seeing the after images, what might be the cause?
Applying:
?Underneath your circuit diagrams, write “Applying” and answer these in complete sentences. Don’t forget to number your answers!
7. If you were to stare at the Major League Baseball symbol at right, describe the After Image you would see.
People that are color blind see shades of grey in place of the colors they cannot see. The most common kind of color blindness is being Red-Green color blind. People that are Red-Green color blind can not distinguish between red and green colors.
8. If someone is Red-Green color blind are their rods or cones affected? Which ones?
9. What secondary color would also be difficult for someone that is Red-Green color blind to see? Why?
10. Many animals have excellent night vision such as cats, raccoons and other nocturnal animals. Thinking about how the eye works, which photoreceptors would they have more of than animals that are out more during the day? How do you know?