SCIENCE EXPERIMENTS ON FILE Revised Edition 6.25-1 Roy G. Biv Charles W. McLaughlin Topic Relationship between wavelength and frequency of light Time 1 hour! Safety Please click on the safety icon to view the safety precautions. Materials roll of adding-machine tape red, blue, and green colored pencils yardstick or ruler cardboard about 8 in. square scissors six books stopwatch Procedure You will need an assistant for part of the procedure. 1. Draw a vertical line about 6 in. from the beginning of the adding-machine tape, and label it start (figure 1). With the yardstick, mark a point 60 in. from the starting point. Draw a vertical line and label it end. Do not cut the marked portion of the tape off the roll. Figure 1 start Top line is blue, marked every 2 in.; middle line is green, marked every 3 in.; bottom line is red, marked every 6 in.
6.25-2 SCIENCE EXPERIMENTS ON FILE Revised Edition 2. Using colored pencils, draw three evenly spaced horizontal lines along the tape from Start to End. Make the top line blue, the middle line green, and the bottom line red to represent three different colors in the spectrum of light. 3. Divide the red line every 6 in. with clearly visible vertical marks in red pencil. Divide the green line every 3 in. and the blue line every 2 in. The marks at different intervals on the three lines represent the different wavelengths of the different colors of light. (The difference between wavelengths of actual blue, green, and red lightwaves is proportionately smaller than in your model, but their relationship is the same: red has the longest wavelength, green is shorter, and blue shorter still. 4. Wind the marked tape back onto the spool, and insert a pencil through the middle of the spool. 5. Cut a rectangular opening in the middle of the cardboard about 3 in. high and 1 2 in. wide, as shown in figure 2. Figure 2 cardboard 6. Set the cardboard on the table, supporting it with books (figure 3). Feed the end of the adding machine tape through the narrow space between the cardboard and the back pile of books until start appears in the middle of the opening in the cardboard. Figure 3 pencil cardboard tape start
SCIENCE EXPERIMENTS ON FILE Revised Edition 6.25-3 7. Divide a sheet of paper into three vertical columns, and label them red, green, and blue. 8. Sit in a chair at the table in front of the cardboard door and tape apparatus. Have your assistant call out start and begin timing as he or she slowly pulls the tape along. Each time you see a wavelength mark put a dash in the box of that color on a data sheet. When the vertical end line appears in the door, tell your assistant to stop timing. 9. Repeat the procedure several times, until you get consistent results. 10. On the data table, record the average number of wavelengths observed for each color and the average time of the trials. 11. Frequency is defined as the number of wavelengths passing a given point per second; determine the frequency for each of your colored lightwaves and record the results in the frequency column. DATA TABLE Color Wavelength Avg. no. of Time Frequency (in.) wavelengths observed Red 6 Green 3 Blue 2 12. Compare the wavelengths and frequencies of the three waves. Do you notice any pattern in their relationship? What is the mathematical relationship of the red wavelength to the green? To the blue? What is the relationship of the red frequency to the green frequency? To the blue? Which wave has the highest frequency? The longest wavelength? Which has the shortest wavelength? The lowest frequency? 13. From your answers to the questions above, can you deduce a constant relationship between wavelength and frequency in waves that travel at the same velocity like the ones represented on the paper strip? 14. Using what you now know about the relationship among time, distance, and velocity, what is the velocity of the waves? 15. Multiply the wavelength of the red wave by its frequency. Do this for the blue and green waves also. What do you notice about the results? 16. Write a constant formula for the velocity of waves in terms of wavelength and frequency instead of distance and time. What s Going On If you observed all the wavelengths on the strip, you would have seen 10 red, 20 green, and 30 blue. Time will vary. The frequency of each color will be the number of wavelengths observed divided by the time in seconds. The red wavelength is 2 times the green wavelength and 3 times the blue wavelength. The frequencies of the waves
6.25-4 SCIENCE EXPERIMENTS ON FILE Revised Edition will vary depending on the value you are using for time. However, the relationships among them will always be the same: the red frequency is one-half the green frequency and one-third the blue frequency. Red has the lowest frequency and longest wavelength; blue has the highest frequency and shortest wavelength; and green is in between. In waves with the same velocity, there is an inverse relationship between wavelength and frequency; that is, the longer the wavelength the lower the frequency, or the shorter the wavelength the higher the frequency. Velocity = distance/time. So, the velocity of the waves = 60 in./time (in seconds) for all the waves. Divide by the number of seconds to put your results in inches per second. The products of all three multiplications will be the same. The values used for frequency will vary depending on the value being used for time. However, regardless of these variations, in all three cases multiplying the wavelength by the frequency will produce the same answer. The results will also equal the velocity of the waves that you calculated above: Velocity = wavelength 2 frequency Connections The white light from the sun is actually composed of the colors red, orange, yellow, green, blue, indigo, and violet, which become visible when sunlight passes through a prism. The colors always appear in this order; a good way to remember the order is to note that the first letters of the colors spell out the name ROY G. BIV. We can think of light as traveling in waves with properties of wavelength and frequency. Wavelength is the distance between identical locations on adjacent waves (figure 4). wavelength Figure 4 Frequency is the number of complete waves (or wavelengths) that pass a given point each second. All light travels at the same velocity, but each color has a different wavelength and frequency. It is their different wavelengths that cause the different colors of light to separate and become visible when passing through a prism. Red light has the longest wavelength. The wavelengths of the other colors decrease in order, with violet light being the shortest. In this project you constructed a simplified model of different lightwaves to identify a constant relationship between wavelength and frequency.
Safety Precautions READ AND COPY BEFORE STARTING ANY EXPERIMENT Experimental science can be dangerous. Events can happen very quickly while you are performing an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious accidents. Be sure to follow additional safety precautions and adult supervision requirements for each experiment. If you are working in a lab or in the field, do not work alone. This book assumes that you will read the safety precautions that follow, as well as those at the start of each experiment you perform, and that you will remember them. These precautions will not always be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always say be careful with hot liquids or don t cut yourself with the knife does not mean that you should be careless when simmering water or stripping an electrical wire. It does mean that when you see a special note to be careful, it is extremely important that you pay attention to it. If you ever have a question about whether a procedure or material is dangerous, stop to find out for sure that it is safe before continuing the experiment. To avoid accidents, always pay close attention to your work, take your time, and practice the general safety procedures listed below. PREPARE Clear all surfaces before beginning work. Read through the whole experiment before you start. Identify hazardous procedures and anticipate dangers. PROTECT YOURSELF Follow all directions step by step; do only one procedure at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure that there is adequate ventilation. Do not horseplay. Wear an apron and goggles. Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose. Keep floor and work space neat, clean, and dry. Clean up spills immediately. Never eat, drink, or smoke in the laboratory or near the work space. Do not taste any substances tested unless expressly permitted to do so by a science teacher in charge. USE EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives and other sharp or pointed instruments with caution; always cut away from yourself and others. Pull plugs, not cords, when inserting and removing electrical plugs. Don t use your mouth to pipette; use a suction bulb. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges. Clean up broken glassware immediately. v
vi Safety SCIENCE EXPERIMENTS ON FILE REVISED EDITION Do not use reflected sunlight to illuminate your microscope. Do not touch metal conductors. Use only low-voltage and low-current materials. Be careful when using stepstools, chairs, and ladders. USING CHEMICALS Never taste or inhale chemicals. Label all bottles and apparatus containing chemicals. Read all labels carefully. Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING INSTRUCTIONS Use goggles, apron, and gloves when boiling liquids. Keep your face away from test tubes and beakers. Never leave heating apparatus unattended. Use safety tongs and heat-resistant mittens. Turn off hot plates, bunsen burners, and gas when you are done. Keep flammable substances away from heat. Have a fire extinguisher on hand. WORKING WITH MICROORGANISMS Assume that all microorganisms are infectious; handle them with care. Sterilize all equipment being used to handle microorganisms. GOING ON FIELD TRIPS Do not go on a field trip by yourself. Tell a responsible adult where you are going, and maintain that route. Know the area and its potential hazards, such as poisonous plants, deep water, and rapids. Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold). Bring along a first-aid kit. Do not drink water or eat plants found in the wild. Use the buddy system; do not experiment outdoors alone. FINISHING UP Thoroughly clean your work area and glassware. Be careful not to return chemicals or contaminated reagents to the wrong containers. Don t dispose of materials in the sink unless instructed to do so. Wash your hands thoroughly. Clean up all residue, and containerize it for proper disposal. Dispose of all chemicals according to local, state, and federal laws. BE SAFETY-CONSCIOUS AT ALL TIMES