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Missing Lens Homework due Friday Physics Club SLC on Thursday Test next week Be able to calculate the temperature of very distant stars. Be able to use the Doppler Effect to explain how distant galaxies are moving relative to Earth. Understand how we know if the universe is expanding or not. How do we figure out the temperature of a star? Do you think all stars have the same temperature? How would you try to figure out the temperature of a star? “Thermal radiation, every body does it!” Thermal radiation is any type of electromagnetic radiation from an object that depends on its temperature. Sometimes called Blackbody radiation These photons get their energy from heat not from electrons dropping energy levels so thermal radiation spectrums are the most common type of continuous spectra because of the large variation of photon energies. Note: the wavelength stay in the ultraviolet to infrared range. Law 1 (Stefan-Boltzmann Law): Each square meter of a hotter object’s surface emits more light at all wavelengths. Law 2 (Wein’s Law): Hotter objects emit photons with a higher average energy. What happens when you heat up metal in a furnace. Hot plate demo Let’s ask my close friend Randy McDaniel! http://www.youtube.com/watch?v=e_uO7PU8l8Y When the sword is at a lower temperature it only emits infrared light. When it is heated it starts to emit visible light and ultraviolet light. He came up with an equation that related the peak wavelength with temperature. Using this equation all we need to do is find the peak wavelength of a star and we can calculate its temperature. If people are constantly emitting thermal radiation why don’t we see it? How do night vision goggles work? How can we tell the temperature of a planet without visiting it? Planets do not generate light like a stars do. They are heated by the light they absorb, not by the light they reflect. If the peak frequency of a star is 5.20x10^14Hz find its temperature using Wein’s law: Green-Yellow!! If the peak frequency of a star is 1.2x10^15Hz find its temperature using Wein’s law: If this peak frequency is in the ultraviolet part of the electromagnetic spectrum can we see the star with our own eyes? If the peak frequency of a star is 1.4x10^14Hz find its temperature using Wein’s law: T=1353K If this peak frequency is in the infrared part of the electromagnetic spectrum can we see the star with our own eyes? Is there anything else you think we can learn about the universe from electromagnetic waves?