POGIL-ish: Light Waves (ACADEMIC)

READ THIS A vibrating guitar string generates sound waves. These sound waves are disturbances that can transport energy without transporting matter. Each molecule of air bumps into it’s neighboring particle to pass the energy at great speeds without the individual particles moving far from their original location. This can be seen at any sporting event where the fans stand in place but their energy “wave” passes around the stadium. Light, sound, earthquake and even ocean waves share the same basic properties relating energy, frequency, and wavelength. Since stars are both very hot and very distant, much of our understanding of the universe relies on the analysis of light waves that those distant stars send us.

MODEL 1: Wave Properties ( )- - - - - ->

1. What is the high point of the wave called? The low point? What is that point where the wave is exactly halfway between the high and the low?

2. A complete wave is one full cycle that starts and ends at equilibrium, and includes one crest and one trough. Highlight a complete wave on the diagram. What is the name of the distance a complete wave travels?

3. This value is represented with what symbol?

4. What is wave amplitude? How does it relate to the crest and trough?

5. What is the direction of travel for the wave? How does that compare to the direction of the moving particles that are disturbed by the passing wave?

(power)

READ THIS The amount of energy a wave has is related to its wavelength and frequency. Standing knee deep in the ocean, each wave that passes a swimmer moves both the water particles, and the energy of the wave. How often the waves pass reflects their frequency. The base unit for frequency is Hertz (Hz) or 1 wave/second. Radio stations are identified by the frequency they transmit their signal on – locally, NPR transmits their broadcast at 88.5 waves per second. FM radio is broadcast in megahertz (MHz), 1 x 106 Hz.

MODEL 2: Wave Frequency

6. Frequency (f) is the number of wave cycles over time. What is the frequency of wave D?

7. Determine the frequency of the other waves in Model 2.

I----------- 1 second ------------I

8. Summarize the relationship between wavelength and frequency.

9. Claim: despite having the same frequency, waves 1 and 2 are not the same. Justify.

10. If waves 1 and 2 were ocean waves, infer which would be smaller.

If they were light, which would be brighter?

If they were sound, which would be quieter?

11. Each wave is a packet of energy. Which wave has the greatest energy?

12. Describe the relationship between wavelength and energy.

A

B

C

D

READ THIS: Even though all waves are generated by a vibration and have the same basic relationships between energy, wavelength and frequency there are some differences between different types of waves. Sound waves and primary earthquake waves are both move by compressing the matter (or medium) they travel

through. They pass energy along by collisions that alternately squeeze and stretch the medium. The density of the medium thus will affect the speed at which the wave will travel.

MODEL 3 – Longitudinal Compression Waves (Sound and Earthquake P-waves)

13. Label the compressions as the wave crests and the stretched areas with less particles as the wave troughs. Which of the two waves would sound louder?

14. How do the frequencies (or tones) of the two waves compare?

15. What direction do the particles and waves move in a longitudinal wave? Infer why it is given the name longitudinal.

16. Given that sound is a longitudinal compression wave, why doesn’t sound travel in space?

READ THIS: Light is created by a vibration of an electric charge that generate a magnetic field. The vibrating magnetic field in turn generates an electric charge thus requiring no matter to move itself along. Light waves are electromagnetic (EM) waves that come in various wavelengths and frequencies, and are organized as the electromagnetic (EM) spectrum. Only a narrow band of the EM spectrum is detectable by the human eye and thus called visible light. Other animals are able to detect wavelengths outside the visible; bees can see ultraviolet wavelengths, and snakes have a sense organ that detects infrared waves given off by warm-blooded prey. However, we regularly interact with non-visible forms of light: to cook in a microwave, receive cell phone signals, and to watch television.

Direction of particle compression

MODEL 4 – The EM light spectrum is a range of waves

12. Model 4 displays the range of light types found in the electromagnetic spectrum. Which of these has:

a. The longest wavelength? B. The lowest frequency? C. The greatest energy?

13. List the differences between a radio EM wave and a visible light EM wave.

READ THIS Wave speed is the product of wavelength and frequency. Symbols are used for each variable in the relationship: c for speed, l for wavelength, and f for frequency. The units are also specific to the relationship: frequency (f ) is in hertz (Hz), wavelength (l) in meters (m) and speed (c) is in meters per second (m/sec). The speed of light (c) is 300,000,000 m/sec and is constant for all types of EM light when traveling through space.

14. Write the relationship between speed, wavelength and frequency as a sentence formula and again using symbols.

15. How fast is visible light? How fast is ultraviolet light?

16. If you stretched a wave so it had a longer wavelength, what would happen to its frequency?

17. As the wave is stretched it would become a different type of EM wave. What is the longest it could be? READ THIS: As light waves travel through different medium materials of different densities it’s speed changes. You see this in the visible wavelengths when a straight pencil appears bent if placed in a glass of water. The light slows as it enters the more dense water, and then speeds up again when it re-enters the air. This bending (refracting) of light is evident when a glass prism separates white light into its component colors. Other light waves behave in the same manner, and these differences can also be detected, albeit not visually.

MODEL 5: White light

18. Color in the rainbow and wavelength data in Model 5. Which color has the shortest wavelength?

19. Which color has the lowest energy?

20. Which color is bent or refracted the most? Relate this to frequency.

21. The colors of the rainbow make up the visible portion of light. What type of light is just beyond the red?

22. What light type is beyond violet? Look back to Model 4 to confirm this relationship.

620-780 nm585-620 nm570-585 nm490-570 nm440-490 nm420-440 nm 400-420 nm

Ultraviolet (UV)

Infrared (IR)

Glass prism

Air

23. What happened to white light as it moved through the glass prism? Why?

THINK FURTHER

24. Stars and other light-producing objects emit some, but not all types of EM light. Suggest a way this characteristic can be used to identify different types of objects in space.