1. Nature of Light PA STEM monthly meeting CCIU February 24, 2015

2. High-school standards HS-PS4-1, 3, and 4 focus on the characteristics of light and evidence for particle versus wave properties PA STEM Science HS-PS4-1: Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media. [Clarification Statement: Examples of data could include electromagnetic radiation traveling in a vacuum and glass, sound waves traveling through air and water, and seismic waves traveling through the Earth.] [Assessment Boundary: Assessment is limited to algebraic relationships and describing those relationships qualitatively.] HS-PS4-3: Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other. [Clarification Statement: Emphasis is on how the experimental evidence supports the claim and how a theory is generally modified in light of new evidence. Examples of a phenomenon could include resonance, interference, diffraction, and photoelectric effect.] [Assessment Boundary: Assessment does not include using quantum theory.]

3. High-school standards HS-PS4-1, 3, and 4 focus on the characteristics of light and evidence for particle versus wave properties (cont.) PA STEM Science HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter. [Clarification Statement: Emphasis is on the idea that photons associated with different frequencies of light have different energies, and the damage to living tissue from electromagnetic radiation depends on the energy of the radiation. Examples of published materials could include trade books, magazines, web resources, videos, and other passages that may reflect bias.] [Assessment Boundary: Assessment is limited to qualitative descriptions.]

4. Electromagnetic radiation (light) travels at a constant speed (c = 2.99792458 x 10 8 m/s) in empty space Galileo (before 1640)—lanterns on mountaintops Ole Rømer (1676)—timing of Jupiter’s moons James Bradley (1728)—aberration of starlight Fizeau (1850)—toothed wheel gives time of travel Foucault (1850)—rotating mirror gives time of travel Michelson (1879)—rotating mirror, longer distance Einstein (1905)—speed of light is a physical law PA STEM Science

5. Since the speed of light is constant for all observers, distances can be calculated for known travel times d = ct Q: If light takes 2.6 seconds to bounce off the moon and return to Earth, how far is the moon from Earth? PA STEM Science

6. Since the speed of light is constant for all observers, distances can be calculated for known travel times d = ct Q: The signal from a radio station to a radio receiver is interfered with by a reflected signal that arrives 6 μs later. How far away is the object that reflected the radio signal? PA STEM Science

7. Recent observations confirm that different types of light travel at exactly the same speed in empty space Light races across the Universe: http://www.sciencedaily.com/releases/2009/10/091028153447.htm PA STEM Science

8. Snell’s Law describes the bending of light as it travels from one medium to another n: index of refractionn space = 1.0 n air = 1.0003 n glass = 1.5 PA STEM Science

9. Angles in Snell’s Law show how much the light is bent from traveling straight in or out of a surface n 1 sin(θ 1 ) = n 2 sin(θ 2 ) Q: Light traveling in air hits the surface of glass at a 45° angle from the normal. What angle does the light have in the glass? PA STEM Science

10. In the 1600’s Snell’s Law prompted scientists to argue over whether light is a wave or a particle PA STEM Science “It’s a particle.” “It’s a wave.”

11. Waves and particles are conceptually very different from each other PA STEM Science Waves: nonlocalized vibrations = v wave T Particles: localized matter or energy d = v particle t

12. Isaac Newton showed how a particle model of light could explain Snell’s Law PA STEM Science Force of material on particles of light is perpendicular to the surface n 1 sin(θ 1 ) = n 2 sin(θ 2 ) n 1 (v ║ /v 1 ) = n 2 (v ║ /v 2 ) v 2 = n 2 c Snell’s Law implies that light travels faster in glass than in empty space!

13. Christiaan Huygens showed how a wave model of light could explain Snell’s Law PA STEM Science Side of wave fronts that hits material first slows down first Wavelengths bunch up in material Snell’s Law implies that light travels slower in glass than in empty space! v 2 = c/n 2 Both models can explain Snell’s Law but imply different speeds in materials

14. In 1801 Thomas Young proved that light is a wave by his famous double-slit experiment PA STEM Science Waves of light from both slits either add to make a bright fringe (constructive interference) or cancel to make a dark fringe (destructive interference)

15. Only waves can destructively interfere PA STEM Science

16. Interference occurs because light from slits travels different distances and overlaps different parts of the each wave PA STEM Science Q: How does the interference pattern depend on wavelength?

17. All easily observed phenomena support the wave model of light—light is an electromagnetic wave PA STEM Science

18. Light in a material travels more slowly and the wavelength is reduced PA STEM Science = v wave T = = v wave /f v wave = f Frequency f stays the same Wave speed is reduced: v wave = c/n material Wavelength is reduced: = space /n material Practice Problem: Red light from a He-Ne laser ( = 632 nm) enters a diamond (n = 2.42). What is the wavelength of the light in the diamond?

19. Despite the dominance of the wave model, some phenomena imply that light is made of particles PA STEM Science  Photoelectric effect (Einstein, 1905) Light is composed of particles called photons The energy in each photon is given by E photon = hf,h = 6.6x10 -34 J/s Each photon can knock a single electron off a surface Compton scattering (Compton, 1922) Each photon carries momentum, given by p = h/

20. Millikan’s famous experiment in 1914 showed that the maximum electron energy obeys E max = hf PA STEM Science Photoelectric effect shows that light simultaneously has particle and wave characteristics

21. Compton’s famous experiment in 1923 showed that photons can knock electrons forward PA STEM Science Photons carry both energy and momentum and collide with matter

22. Various “colors” of the electromagnetic spectrum differ by the amount of energy per photon (E = hf) PA STEM Science Damage to biological tissue is only possible for energetic photons (> visible light) because only one photon is absorbed at a time