Electromagnetic (Light) Waves Electromagnetic Waves

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1 Physics R Date: Review Questions 1. An ocean wave traveling at 3 m/s has a wavelength of 1.6 meters. a. What is the frequency of the wave? b. What is the period of the wave? Electromagnetic (Light) Waves Electromagnetic Waves Until the 20 th century, reality was everything humans could touch, smell, see, or hear. Since the initial publication of the charted electromagnetic spectrum, humans realized what they could touch, smell, see, and hear is less than one millionth of reality. Electromagnetic Spectrum Visible light, infrared light, x-rays, microwaves, gamma rays, etc. are all part of the electromagnetic spectrum. They all travel with the same in a vacuum. However, they have different (Hz) and (m). Find the spectrum in your reference table. Monochromatic 1

2 Electromagnetic waves all travel at the same speed in a vacuum Practice 2. What type of EM wave is the following? 500,000 hertz 5.0 x 10-9 meter 3.0 x hertz 4.5 x hertz 200 meters 3. What is the frequency of a 632 nanometer EM wave? What color is it? 4. What is the wavelength of a 5 x Hz EM wave? What type of wave is it? Electromagnetic waves are transverse waves. Radio waves travel from New York City to the antenna on your radio in Hicksville. How should you orient the antenna on your radio to get reception, horizontally or vertically? Why? 5. The antenna of a device is (approximately) the wavelength of the incoming signal. If you re purchasing a home phone and want one with a smaller antenna, which should you buy, a 900 Mhz or a 2.4 Ghz phone? Why? 6. How long should you make an antenna to pick up an AM radio signal of 800 khz? Doppler effect Light from a star in distance space is expected to have a frequency of 6.3 x Hz. Instead the star has a slightly lower frequency than what would have been expected. Is the star moving towards or away from Earth? Is this a red shift or blue shift? 2

3 Speed of an electromagnetic waves (light) in a medium When light enters a new medium, the speed changes and the wavelength changes but the frequency remains the same. For all wave phenomena, frequency always remains the same except for the Doppler Effect. 7. What is the speed of light in: a. Corn oil b. Diamond c. Water 8. Light travels at x 10 8 m/s through a material. What is the composition of the material? 9. Light with a frequency of 5.2 x Hz is traveling through water. a. How fast is it traveling? b. What is the wavelength of the light in water? 10. Light with a frequency of 3.84 x Hz is traveling through glycerol. a. How fast is it traveling? b. What is the wavelength of the light in glycerol? Energy of an electromagnetic wave (light) Higher frequency EM waves have higher energy E photon = (found in the Modern Physics part of the reference table) A photon is the smallest particle of light like an atom is the smallest particle of matter. 9. An electromagnetic wave has a frequency of 5.1 x Hz a. What color wave is this? b. What is the energy of the wave? c. What is the speed of this wave in a vacuum? 3

4 10. A photon is traveling with a frequency of 8.6 x Hz a. What type of EM wave is this? b. How much energy does this photon have? c. What is the wavelength of this wave? 11. An electromagnetic wave is traveling with frequency 3 x 10 7 Hz a. What type of EM wave is this? b. How much energy does it have? c. What is the speed of this wave in a vacuum? Phase (True for all waves, not just EM waves) 2 points are in phase if they are on the same part of a wave 2 points are 180 degrees out of phase at opposite parts In Phase Out of Phase Polarization A polarizing filter blocks out all waves except those with particles moving in a certain direction. This only works for transverse waves longitudinal waves can NOT be polarized. Reflection Law of reflection: 12. To the right (using a protractor and ruler) a. Draw the normal b. What is the angle of incidence? c. What is the angle of reflection? d. Draw the reflected ray. 4

5 13. To the right (using a protractor and ruler) An incident ray has an angle of 20 degrees a. Draw the incident ray (and normal) b. Draw the reflected ray Diffraction - Electromagnetic waves diffract just like mechanical waves Air Air There is no change in medium necessary for diffraction 14. A 7x10 14 Hz EM wave in air goes through a hole and diffracts a. What is the speed before it goes through the opening? b. What is the speed after? c. What is the wavelength before the opening? d. What is the wavelength after? e. Sketch four diffracted wave fronts after the opening Note: Larger wavelength and smaller opening -> More diffraction 15. Radio waves and light waves go past tall buildings. Which will diffract more? Why? Interference - Electromagnetic waves interfere just like mechanical waves If you put a paper to observe light along B-D-C, what would the pattern look like? Mechanical versus Electromagnetic Waves Mechanical waves vs Electromagnetic waves Can travel in a vacuum Transverse or longitudinal Energy is related to Requires a medium to travel Tastes good on a pizza 5

6 Refraction (transmission when light enters a new medium) When light (EM waves) enter a different medium, there is a change in which causes a change in or Recall: as light travels through a different medium, its speed changes according to the index of refraction n = Light changes direction (angle) according to Snell s Law 16. A ray of light (f=5.09 x 1014 Hz) is incident on the boundary between air and an unknown material X at an angle of incidence of 55, as shown. The absolute index of refraction of material X is a. Determine the speed of light in material X b. Calculate the angle of refraction of this light ray 17. A monochromatic light ray (f=5.09 x Hz) traveling in air is incident on the surface of a rectangular block of Lucite (n=1.50). a. Measure the angle of incidence for the light ray to the nearest degree b. Calculate the angle of refraction of the light ray when it enters the Lucite block. c. What is the angle of refraction of the light ray as it emerges from the Lucite block back into the air? 6

7 18. A ray of monochromatic light having a frequency of hertz is incident on an interface of air and corn oil at an angle of 35 as shown. The ray is transmitted through parallel layers of corn oil and glycerol and is then reflected from the surface of a plane mirror, located below and parallel to the glycerol layer. The ray then emerges from the corn oil back into the air at point P. a. Calculate the angle of refraction of the light ray as it enters the corn oil from air. b. Explain why the ray does not bend at the corn oil-glycerol interface. c. On the diagram, use a protractor and straightedge to construct the refracted ray representing the light emerging at point P into the air. Dispersion Why do all refraction problems specify the same frequency (f= hertz)? Why is the sky blue? Why is the sun yellow? Why are sunsets red? 7

8 19. A wave generator with a constant frequency produces parallel wave fronts in a tank of water of two different depths. The diagram below represents the wave fronts in the deep water. As the wave fronts travel from the deep water into the shallow water, the speed of the waves decreases. On the diagram at right use a straightedge to draw at least three lines to represent the wave fronts, with appropriate spacing, in the shallow water. 20. Orange light has a frequency of 5.0 x Hz in a vacuum. What is the wavelength of this light? 21. How much time does it take light from the moon to reach Earth? How much time would it take a radio wave to travel from the moon to Earth? 22. What is the energy in Joules of a photon with wavelength 532 nm? 23. A hydrogen atom emits a photon with an energy of 1.63 x J. What type of radiation would this photon be classified? (i.e. infrared, blue light, x-ray, etc.) 24. Radio waves diffract around buildings more than light waves do because compared to light waves, radio waves (move faster) (move slower) (have a higher frequency) (have a longer wavelength) 25. A ray of monochromatic light (f = 5.09 x hertz) in air is incident at an angle of 30 on a boundary with corn oil. What is the angle of refraction? 26. A monochromatic ray of light (f = 5.09 x hertz) traveling in air is incident upon medium A at an angle of 45. If the angle of refraction is 29, what is medium A? (water, fused quartz, Lucite, or flint glass) 27. A ray of light is travelling from water to diamond. If the incident angle is 45, what will the angle of refraction be? 8

9 Review Checklist Determine the part of the EM Spectrum that a particular wave belongs to. Explain the origin of electromagnetic waves and contrast this with the production of mechanical waves. Use equation to determine speed in a medium or index of refraction for a medium. 1. What type of electromagnetic radiation is a wave with a? wavelength of... Frequency of (a) 3.5 x 10 3 meters (e) 3.0 x 10 6 Hz (b) 4.5 x 10-8 meters (f) 2.2 x Hz (c) 2.0 x meters (g) 1.5 x Hz (d) 6.0 meters (h) 5.5 x Hz 2. Determine the speed of a ray of light with a frequency of 5.09 x hertz as it passes through water (index of refraction = 1.33). 3. Determine the absolute index of refraction for a medium in which microwaves travel at a speed of 1.8 x 10 8 meters per second. Explain the Law of Reflection; and identify ray diagrams in which reflection occurs. Explain Snell s Law and identify ray diagrams in which refraction occurs. Use Snell s Law to determine angle of incidence; angle of refraction; or index of refraction. 4. A ray of light moving from air into Lucite enters the Lucite at an angle of 35 relative to a line perpendicular to the Lucite surface. What angle will the light be bent at as it moves through the Lucite? 5. A ray of light is incident on a block of unknown glass at an angle of 45 degrees. The light refracts to an angle of 25.2 degrees. Determine the material of the unknown glass. Answers 1. a. radio/long radio b. x ray c. gamma ray d. radio/tv FM e. radio/am f. ultraviolet g. microwave h. visible/green x 10 8 m/s degrees 5. Flint glass 9