# Electromagnetic Radiation Electromagnetic radiation - all E-M waves travel at c = 3 x 10 8 m/s. (Slower in water, glass, etc) Speed of light is independent.

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Electromagnetic Radiation Electromagnetic radiation - all E-M waves travel at c = 3 x 10 8 m/s. (Slower in water, glass, etc) Speed of light is independent of the speed of the source or the observer! Electric and magnetic components are perpendicular. Electric field creates magnetic field and vice versa. Electromagnetic radiation - all E-M waves travel at c = 3 x 10 8 m/s. (Slower in water, glass, etc) Speed of light is independent of the speed of the source or the observer! Electric and magnetic components are perpendicular. Electric field creates magnetic field and vice versa.

Wave Equation Wave equation; v = f or c = f f increases, decreases; increases, f decreases. Wave equation; v = f or c = f f increases, decreases; increases, f decreases.

Human eyes can perceive only the narrow range from 7000 Å to 4000Å. Very small fraction of the entire E-M spectrum. Human eyes can perceive only the narrow range from 7000 Å to 4000Å. Very small fraction of the entire E-M spectrum.

Photons Packets of energy; massless. “White light” - All colors. Energy is E = hf; h is Planck’s Constant. Photons of high energy have higher f and shorter Photons of low energy have lower f and longer Packets of energy; massless. “White light” - All colors. Energy is E = hf; h is Planck’s Constant. Photons of high energy have higher f and shorter Photons of low energy have lower f and longer

Sample Problems How much energy in a photon of light with a frequency of 7.5 x 10 14 Hz? Solution: E = hf E = 6.63 x 10 -34 Js x (7.5x 10 14 Hz) E = 5 x 10 -19 J How much energy in a photon of red light with a wavelength of 450 nm? Solution: E = hc/ E = 6.67 x 10 -34 Js x (3 x 10 8 m/s)  450 x 10 -9 m E = 4.4 x 10 -19 J How much energy in a photon of light with a frequency of 7.5 x 10 14 Hz? Solution: E = hf E = 6.63 x 10 -34 Js x (7.5x 10 14 Hz) E = 5 x 10 -19 J How much energy in a photon of red light with a wavelength of 450 nm? Solution: E = hc/ E = 6.67 x 10 -34 Js x (3 x 10 8 m/s)  450 x 10 -9 m E = 4.4 x 10 -19 J

Einstein & Planck

Blackbody Radiation Stars are opaque, luminous balls of gas. Perfect radiators; Planck Curve Hot interior produces a continuum. Cool exterior absorbs ; tell chemical composition. Hot  Blue Medium  White Cool  Red Stars are opaque, luminous balls of gas. Perfect radiators; Planck Curve Hot interior produces a continuum. Cool exterior absorbs ; tell chemical composition. Hot  Blue Medium  White Cool  Red

Temperature Scale K = C + 273 F = (9/5)C + 32 C = 5/9(F - 32) K = C + 273 F = (9/5)C + 32 C = 5/9(F - 32)

Wein’s Law The wavelength at which the spectrum peaks (the color of the star) is a measure of its surface temperature. peak T = 2.9 x 10 7 Å-K or peak T = 2.9 x 10 -3 m-K The wavelength at which the spectrum peaks (the color of the star) is a measure of its surface temperature. peak T = 2.9 x 10 7 Å-K or peak T = 2.9 x 10 -3 m-K

Wein’s Law Sample Problems A star emits light with a peak wavelength of 900 nm. What’s its surface temperature? What color would it appear? What would be the peak wavelength emitted by a star with a surface temp. of 12000 K? A star emits light with a peak wavelength of 900 nm. What’s its surface temperature? What color would it appear? What would be the peak wavelength emitted by a star with a surface temp. of 12000 K?

Infrared Images

Stefan-Boltzmann Law The higher the temperature of a surface, the more energy radiated by each square cm in each second.  =  T 4 How does the energy radiated by a 6000 K star compare to that of a 3000 K star of the same size? Luminosity varies directly with the radius squared of a star. L = 4πR 2  T 4 The higher the temperature of a surface, the more energy radiated by each square cm in each second.  =  T 4 How does the energy radiated by a 6000 K star compare to that of a 3000 K star of the same size? Luminosity varies directly with the radius squared of a star. L = 4πR 2  T 4

The Doppler Effect An apparent change in frequency when either the source or the observer (or both) of a wave is in motion. Examples: Sound, water waves, police radar app f true v recession true f app c An apparent change in frequency when either the source or the observer (or both) of a wave is in motion. Examples: Sound, water waves, police radar app f true v recession true f app c = = 1 +

Doppler Effect Sample A police siren is emitting a 256 hz sound and is approaching you at 34 m/s. Using 340m/s as the speed of sound, what frequency do you hear?

One shift, two shift; red shift, blue shift A beam of blue light normally seen at a wavelength of 400nm is seen at 399 nm. What is the object’s radial speed and direction? Solution: (399 nm/400nm - 1) c = v rec = -750000 m/s (the object is approaching at 750 km/s) Green light that is normally seen at =550 nm is seen at = 560 nm. What is the radial speed of the source? A beam of blue light normally seen at a wavelength of 400nm is seen at 399 nm. What is the object’s radial speed and direction? Solution: (399 nm/400nm - 1) c = v rec = -750000 m/s (the object is approaching at 750 km/s) Green light that is normally seen at =550 nm is seen at = 560 nm. What is the radial speed of the source?

Kirchoff’s Laws 3 Types of spectra 1.Continuous - from blackbody radiator 2.Absorption - passing through a cooler gas 3.Emission - given off by a hot gas 3 Types of spectra 1.Continuous - from blackbody radiator 2.Absorption - passing through a cooler gas 3.Emission - given off by a hot gas

A hot blackbody radiator produces a continuous spectrum. A heated gas will create an emission spectrum showing only the lines of photons corresponding to the energy levels its electrons are allowed to produce. A hot blackbody radiator produces a continuous spectrum. A heated gas will create an emission spectrum showing only the lines of photons corresponding to the energy levels its electrons are allowed to produce.

Emission (bright line) Spectra Bight lines act as “fingerprints” for identifying elements.

Absorption (dark line) Spectra The dark lines absorbed by a cool gas of an element exactly correspond to the bright lines that would be produced by a hot, glowing gas of the same element.

Solar Spectrum The sun’s spectrum, spread out to show all absorption lines. All the elements present on earth are present in the sun’s spectrum. Earth (and us) is made from the same elements that make up the stars and the rest of the universe! Solar Spectrum The sun’s spectrum, spread out to show all absorption lines. All the elements present on earth are present in the sun’s spectrum. Earth (and us) is made from the same elements that make up the stars and the rest of the universe!

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