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Lecture 11: Light

Lecture 11: Light. The Cosmic Messenger. Basic Properties of Light. light is a form of energy – radiative energy rate of energy output (from Sun, lightbulb, etc.) is measured in Watts (Joule/s). The speed of light.

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Lecture 11: Light

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  1. Lecture 11: Light The Cosmic Messenger

  2. Basic Properties of Light • light is a form of energy – radiative energy • rate of energy output (from Sun, lightbulb, etc.) is measured in Watts (Joule/s)

  3. The speed of light • Galileo tried to measure the speed of light using people with lanterns – failed to detect any finite speed • Romer discovered evidence that light has a finite speed in 1676 based on observations of eclipses of the moons of Jupiter • measured accurately on Earth by Fizeau and Foucault in 1850: c = 3 x 108 m/s

  4. The Fizeau-Foucault experiment

  5. Frequency and Wavelength • all light always travels at a fixed speed (in a vacuum): c = 3 x 108 m/s • frequency (n) and wavelength (l) are related by the formula: l n = c • units of frequency are cycles per second = Hertz = Hz = (1/s)

  6. Frequency and Wavelength: example problem Find the frequency of the first Balmer line of Hydrogen, which has a wavelength of 656.3 nm. remember 1 nm = 10-9 m, so l = 6.563 x 10-7 m • l n = c  n = c/l • = (3 x 108 m/s) / (6.563 x 10-7 m) • = 4.57 x 1014 Hz

  7. Frequency and Energy What is the energy of the photons that produce this Balmer line? Remember the frequency n =4.57 x 1014 Hz E = h n E = (6.626 x 10-34 J s)(4.57 x 1014 Hz) E = 3.02 x 10-19 J

  8. Properties of Light • light can be characterized by two numbers • frequency or wavelength • intensity or brightness (amplitude)

  9. What is Light? • Most waves propagate through some sort of medium (e.g., water waves, sound waves in air, etc.) • if light is a wave, what does it wave in? • the idea of the ‘luminiferous aether’ was invented to answer this question.

  10. The Aether theory and the Michelson-Morley experiment

  11. Electromagnetic radiation

  12. The electromagnetic spectrum

  13. Matter and Light • light interacts with matter in four ways: • emission • absorption • transmission • reflection/scattering

  14. Matter and Light • materials that transmit light are called transparent • materials that absorb light are called opaque • the degree to which a material absorbs light is called its opacity (high opacity  absorbs more light) • objects appear to have different colors because of the way that they transmit or reflect light

  15. Scattering: why the sky is blue

  16. and sunsets are red

  17. Flux = Energy/Area Asphere = 4pR2

  18. Blackbody Radiation radiation from an opaque body follows two laws: • the energy flux is proportional to the temperature of the object to the fourth power (Stephan-Boltzmann law) F = [5.7 x 10-8 W/(m2 K4)] x T4 • the average energy of the light (photons) emitted is higher for higher temperature objects (so the wavelength is shorter; Wien’s Law) lpeak = (2.9 x 10 6 / T [K]) nm

  19. The Sun as a Blackbody • The peak wavelength of the Sun’s light is about 500 nm. What is the surface temperature of the Sun? • we can use Wien’s law: T = (2.9 x 106 nm)/lpeak = (2.9 x 106 )/(500 nm) T = 5800 K

  20. The luminosity of the Sun is 3.90 x 1026 W. Find the temperature of the Sun. • this time we’re going to use the Stephan-Boltzman law: F = [5.7 x 10-8 W/(m2 x K4)] T4 first we need to find the flux at the Sun’s surface. remember flux = energy/area so Fsun = Lsun/(4pR2sun) Rsun = 6.96 x 108 m  F = 6.41 x 107 W m-2 • now we use T = (F/ 5.7 x 10-8 W/(m2 x K4))1/4  T = 5800 K

  21. Other Stars and our Sun • Sirius is the brightest star in the night sky. It appears blue and its peak flux is at 280 nm, in the UV. • is Sirius hotter or cooler than our Sun? What is its temperature? • compare the energy flux at the surface of Sirius with that at the surface of our Sun.

  22. The End

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