Physics 55 Monday, October 17, What light can tell us, continued 2.Application of thermal emission to greenhouse warming. 3.Doppler shift with application.

Mimas and Saturn’s Rings

The Big Picture of What Produces Light Spectra

Solar Spectrum Is Complex!

Thermal Radiation From Dense Media

Astronomical Spectrum Revisited

Why Stars Have Different Colors: They Have Different Surface Temperatures! M80 globular cluster in ScorpiusOrion asterism

Answer 1: Sun emits most of its energy in visual range of spectrum. Answer 2: Water vapor in atmosphere blocks most light except the visual range. Why are living creatures (plants and animals) tuned to the visual spectrum?

If in the future we discover life on some remote planet and If the remote star is similar to our Sun in temperature and If the planet is the right distance from the Sun for water to be in liquid and gaseous form Then: The creatures of that planet will likely see in the visual range just like life on Earth. We Can Make a Prediction

Quantitative Rules of Thermal Radiation I: Wien’s Law Wavelength of peak intensity in thermal emission gives surface temperature T! Example: Sun’s surface temperature of 6000 K implies max of 480 nm, greenish light. Conversely, measuring the light intensity spectrum I( ) and locating the wavelength of largest intensity gives us the surface temperature of the Sun.

PRS Question: Wavelength of Peak Thermal Emission for People? A person’s healthy body temperature is about 36 o C. The wavelength of peak thermal emission for a human is therefore: 1.~80,000 nm (80 microns) 2.~10,000 nm (10 microns) 3.~1,000 nm (1 micron). See site for more info and examples.

Quantitative Rules of Thermal Radiation II: Stefan-Boltzmann Law of Emitted Power Intensity I of emitted light (energy emitted per unit time per unit area) grows as the 4 th power of surface temperature T, where T is measured in kelvins (absolute temperature). Note: A degree symbol o is not used with the Kelvin scale, one writes 290 K, not 290 o K.

Some Astronomical Vocabulary Luminosity of some object is the total energy emitted per second (power), has units of watts (W). Apparent brightness is the amount of energy being received per second per unit area of measuring device. Apparent brightness is easily measured.

Example of Stefan-Boltzmann: Equilibrium Temperature of Earth See Mathematical Insight 11.1 on page 301 of text.

Greenhouse Gas Effect

The Doppler Shift for Waves

Love That Doppler Shift

Use Spectra To Calibrate Doppler Shift Note: A blue shift does not mean spectral line is itself blue, same for red shift. These terms mean qualitative way that any wavelength changes under motion, even for wavelengths that can not be perceived by humans (microwaves, X-rays).

Doppler Shift Formula Speed v is the radial speed toward or away from observer. This formula is valid only for speeds v less than speed of light, say v < ~0.1c. Speed v is positive if object moving away (change in wavelength  is positive), negative if moving toward.

Doppler Shift Gives Only Radial Speed

Quantitative Doppler Shift Example Rest wavelength of certain hydrogen line is nm. Same wavelength in spectrum of Vega is nm. Conclude that radial speed of Vega is toward us, toward since  is negative, i.e., wavelength has been blue-shifted to a shorter wavelength. The value of the radial speed is then: This is lower bound of Vega’s actual speed since we don’t know Vega’s tangential speed with respect to Earth.