What Is Frequency? What Does a Light Wave Look Like? What Is Wavelength?
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What’ s in a light wave? What does a light wave look like? What is frequency? What is wavelength? What am I going to learn? What are the properties of waves? What is a sine wave? What is the relationship between the speed of a wave, its wavelength, and its frequency? How are waves created? Why do we study waves? Have you ever seen the sun? Heard music? Watched ripples in a pond? Then you were observing waves! All of these waves carry energy in different forms by light, sound and motion. We study wave properties to understand all of the different types of wave energy we see in the natural world. In this lab, you will create lines of sand from a swinging cup and study the properties of the wave trace you create as you slide a sheet of paper under the sand at different speeds. Through quantitative measurements, you will determine a relationship between the wavelength, frequency and speed of a wave. Some useful background! Energy often travels in waves. Waves can take on many shapes. One of the most common wave shapes is called a sine wave, shown below. Sine Wave Light waves are one of many types of waves in nature shaped like sine waves. Their repeating pattern allows us to classify them based on their wavelength. The properties of a wave are shown in the figure below. Besides light, what other types of waves have you seen in nature that are shaped like sine waves? Wavelength1 Direction of Crest Crest Motion P Trough Trough Amplitude The crest of wave is the highest point on the wave, and the trough is the lowest point. Wavelength is the distance measured from one crest to the next crest or from one trough to the next trough. The amplitude of the wave is its height, measured from the center dotted line. The frequency of a wave is the number of crests that pass a fixed point (“P” in the diagram) in a given amount of time as the wave moves. For example, if 3 crests pass point P in one second, the frequency is three cycles per second. Since waves are usually moving, we can also measure their speed. Where have I heard frequency before? You’ve probably heard of frequency on the radio station or the channel on a walkie-talkie. This is the same frequency as described above! The radio waves used in these types of communication also have a frequency. It’s the number of times the radio wave oscillates per second. So 98.1 FM really means 98.1 MHz, or 98.1 million oscillations per second! Pre-lab Activity and Predictions Do this pre-lab activity before you do the lab Steps: 1. With your pen, draw lines repeatedly back and forth starting at the top of a sheet of paper. Make sure your pen moves along the same straight line back and forth. 2. Have your partner pull the paper away from you as shown in the photo. 3. Experiment with the speed your partner moves the paper. What happens to the wavelength of the wave you draw when you pull the paper faster or slower? Experiment with how fast you move your hand back and forth. What happens to the wavelength when you move your pen back and forth with a higher or lower frequency? 4. What relationship did you see between frequency, wavelength, and speed? Pre-lab Activity and Predictions (continued) 5. Consider two waves, A and B, that have different properties. If wave A and wave B have the same wavelength and same speed, how do their frequencies compare (same, A greater, B greater) and why? Justify your answer with a picture. If wave A and wave B have the same wavelength, but A moves faster, how do their frequencies compare (same, A greater, B greater) and why? Justify your answer with a picture. If wave A and wave B have the same speed, but A has a larger wavelength, how do their frequencies compare (same, A greater, B greater) and why? Justify your answer with a picture. Materials sand long roll of paper, paper towels, or plastic cup sheets of paper taped together tape calculator scissors chair or other hanging measuring pen stopwatch string support tape Let’s experiment! Steps: 1. Poke a hole in the bottom of a cup so that sand will flow freely from it. 2. Tie the cup to a low hanging chair and place the long paper below it as shown in the photo. 3. Fill the cup with sand. Plug or tape over the hole until ready to start your experiment. 4. Measure the frequency of the swinging cup by timing its swing for 10 cycles and dividing the time by 10. Record this in the chart below. 5. One person in your group will hold the stopwatch and swing the cup while the other person is in charge of pulling the paper. When you’re ready, pull the cup to one side, open the hole in the cup, and let the cup start swinging. 6. Simultaneously, start the stopwatch as your partner begins to pull the paper at a slow constant speed as shown in the photo. 7. When the paper-puller gets near the end of the paper, stop pulling, and at the same time, stop the stopwatch. 8. Measure the distance from start to end as indicated in the photo, and count the number of wavelengths. Record these numbers and the time on the stopwatch in the table provided. 9. Pour the sand back into your cup and repeat steps 4-8, pulling the paper at slower and faster speeds. 10. Re-hang the cup making the string longer so that it swings at a different frequency. Repeat steps 4-9 for this length and record your data on the second chart provided. 11. Calculate and record the average wavelength and average speed in the columns indicated in the table. Results Length of Frequency for Distance Speed of Time of Pull Number of Wavelength Size String 10 Cycles Pulled Wave Units____ Wavelengths Units____ Units____ Units____ Units____ Units____ Short String Length Short Results Length of Frequency for Distance Speed of Time of Pull Number of Wavelength Size String 10 Cycles Pulled Wave Units____ Wavelengths Units____ Units____ Units____ Units____ Units____ Long String Length Long Let’s Find a Relationship With your data, plot the points of wavelength vs. speed on the chart below. Draw a different line for each frequency/string length you recorded. Label each line with the average frequency from that string height (units) Wavelength ___ Wavelength Speed ___ (units) Play some more! We learned about the shape of light - a sine wave. But waves can come in other shapes such as a triangle wave and a square wave, shown below. Triangle Wave Square Wave Using your set up, try to make waves that look like these, or other types of waves besides sine waves. Draw the shapes you made in the space below and describe how you made them. As a bonus, try these other shapes: 1 What happened? Post-lab questions: 1. Did you find the relationship between speed, wavelength and frequency that you predicted above? If not, how did it differ? What surprised you about this result? 2. What are some sources of error that might have affected your experiment? 3. The speed of light is constant, no matter the wavelength or frequency. Based on your experiment, what is the relationship between wavelength and frequency for light? 4. When we created waves on paper in the pre-lab and then with sand during the lab, we were combining a left-to-right oscillating motion with a perpendicular forward motion. Can you think of a type of wave that oscillates in the same direction as it travels? Try making this kind of wave with the swinging cup. More fun physics! The electromagnetic (EM) spectrum, shown below, describes different types of light. Light is referred to as an EM wave because it is made up of both electric and magnetic waves moving together through space. The EM spectrum is arranged by wavelength––longer wavelengths to the left, shorter wavelengths to the right. Though we are most familiar with visible light, all EM waves in the spectrum are types of light. Did you know X-rays were a type of light? The longest light waves are radio waves. The shortest waves are gamma rays. No matter what the frequency or wavelength, all light travels at 299.8 million m/s. Speed (c) is related to frequency (f) and wavelength (λ) by the equation: c = λ * f Since the speed on the left side of the equation is always constant, that means that as wavelength goes down, the frequency must go up. The shorter the wavelength (as we move to the right on the spectrum) the higher the frequency..