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Waves

Waves. EM spectrum. As wavelength increases frequency decreases. Can we interconvert between the two? Units: wavelength= distance (meters, nanometers ect ….) Units frequency= Hz (aka 1/sec) What does c (speed of light) have for units?. Problem 1- Conceptual questions.

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Waves

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  1. Waves

  2. EM spectrum • As wavelength increases frequency decreases. • Can we interconvert between the two? • Units: wavelength= distance (meters, nanometers ect….) • Units frequency= Hz (aka 1/sec) • What does c (speed of light) have for units?

  3. Problem 1- Conceptual questions A) Red light falls around 700 nm green light falls around 500 nm. Which color is highest in frequency? Which color is highest in energy ?

  4. Problem 1- Conceptual questions B) Given the electromagnetic spectrum, which is more damaging to the skin, UV or microwaves? Why ?

  5. Problem 1- Conceptual questions C) UV light is subdivided into classifications UVAI (longest wave), UVAII, UVB and UVC light (shortest). Most UVC light does not reach the surface of earth. Why?  Originally it was thought that only UVA rays caused sun cancer. Given the information about UVC rays not reaching the surface, why might this have been previous common wisdom ? Why isn’t it true?

  6. Problem 2-Freqency to wavelength and wavelength to frequency A) I’ll do this one: The wavelength of radiation when you get an xray is 10.0nm. Find the frequency of the xrays. (3s.f.) . • B) You try this one: The frequency of red light from a traffic signal is centered at 4.11x1014Hz. What is the wavelength of this radiation.

  7. Problem 3- Frequency/Wavelength to energy A) I’ll do this one: The frequency of radiation when you get an xray is 3.0x1016 Hz. Find the energy of the xrays. (2s.f.) B) You do this one: The wavelength of red light from a traffic signal is centered at 730 nm. What is the energy of this radiation . (2 s. f.)

  8. Planck’s Quantum Theory • When solids are heated they emit radiation. • Planck discovered molecules can only emit light at specialized energies or “quanta” • A “quantum” is the smallest quantity of energy and is equal to: • h=6.626x10-34J*s

  9. Problem 4- Introduction to Blackbody Radiation A)As the voltage is increased, what happens to the light bulb ? B) How does Wein’s Law relate to incandescent lightbulbs of different brands ?  Intensity increased and color changes from a red to yellow to white.

  10. Challenge Problem • A) ***A lamp rated at 40 W (1W= 1J/s) emits violet light of wavelength 470 nm. How many photons of blue light can the lamp generate in 2.0s ? (Note on difficulty: I know that 3 stars is much lower than a typical challenge problem, that’s because this is really only a challenge problem given that we haven’t really discussed these sorts of problems yet, you will definitely want to be able to do this sort of problem before the exam).

  11. Challenge Problem • A)Critical Thinking Question****(same reasoning on difficulty as all critical thinking questions): Black lights emit long wave UVA rays along with a very small amount of visible light (seen as purple to our eyes). Does this mean we can develop skin and eye cancer from black lights ? Why ?

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