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Light and Atoms Why study the behavior of light and atoms? – It is only through light that we know anything about the Universe – We can’t experiment on stars and planets – Light tells us about the position and velocity of a star or planet. It can also tell us about the temperature and the composition. An infrared image of a man holding a match. White is the hottest temperature, Blue/Black are the coolest temperatures 1 Light & Atoms • Atoms interact with light by absorbing, emitting and bending light • Atoms leave their unique signature or “fingerprint” in the light they emit or absorb – this can tell us what stars and planets are made of – astronomers see these “fingerprints” in light from objects in distant galaxies. • Atoms in our own atmosphere can blur and absorb light from distant objects 2 The Nature of Light Light is an electromagnetic wave • The speed of light (c) is constant in a vacuum • Light has particle-like properties (photons) and wave-like properties (wavelength and frequency) • Which way you describe light depends on the kind of observation you are making. 3 The Nature of Light • Light has a wavelength and a frequency – wavelength () - distance between wave crests – frequency () - number of wave crests that pass a point in 1 second • When you tune your radio you are actually changing the frequency. • A piano will emit different frequencies of sound based on which key you strike. 4 Wavelength and Frequency The wavelength () and the frequency () are related to the speed (c) by the formula =c If the wavelength increases the frequency must decrease for the speed to stay the same 5 Color and Frequency • The frequency and color of light are related. Red light for example has a lower frequency than blue light. • White light is a mixture of light of many different frequencies. A prism can break light into a rainbow (spectrum) of 6 colors. Electromagnetic Radiation • Visible light is just one form of electromagnetic radiation. • Together they form the electromagnetic spectrum. They differ in their wavelength, frequency and energy. • However specifying the wavelength, frequency or energy uniquely characterizes the form of electromagnetic radiation. 7 The Energy of EM Radiation The energy (E) carried by electromagnetic radiation is related to the frequency (). X-rays have a higher frequency and higher energy than radio waves for example. 8 Wien’s Law • Wien’s law allows astronomers to determine the temperature of a star. • The wavelength at which a star is brightest is related to its temperature • Hotter objects radiate more strongly at shorter wavelengths • Blue has a shorter •Objects can emit radiation at many different wavelengths. wavelength than red, so •The wavelength at which a star is brightest hotter objects look bluer. is related to its temperature. •This is Wien’s Law 9 When can you use Wien’s Law? • • • • Only for objects that emit light not for those that reflect light Light emitted by hot, solid objects obey Wien’s Law Can not use with gases unless they are of a high density The Sun and other stars obey Wien’s Law since the gases they are composed of remain at a high density (at least up to the outermost layers of the star). 10