The Nature of Light

Properties of Light Speed of light –Until around 1700, it was still debated whether the speed of light was infinite (traveled instantaneously from point to point) –In 1676, Danish astronomer Olaus Romer used the eclipses of Jupiter’s moons to establish the finite speed of light and make a rough measurement

Nature of Light The exact nature of light was an enigma to scientists for many centuries Light has properties similar to waves –Young’s double-slit experiment

Wave Nature of Light In the case of light, waves are carried by electromagnetic fields Light = Electromagnetic Radiation

Frequency of Light Wavelength & speed of light are related by the frequency This is actually true for any wave

Electromagnetic Spectrum

Nature of Light The exact nature of light was an enigma to scientists for many centuries Light has properties similar to waves –Young’s double-slit experiment Light sometimes behaves like a particle –Photoelectric effect

Photoelectric Effect Light shining on a piece of metal can cause electrons to be ejected from the surface –only happens if the light has a wavelength shorter than some value (depends on the metal) As intensity of light increases, number of electrons increases, but not their energy As the wavelength of light decreases, number of electrons remains the same but their energy increases

Atomic Spectra “Fingerprints” of atoms Caused be transition of electrons between orbits Hydrogen Helium Oxygen Carbon Nitrogen Neon

Line Spectra Can determine the chemical composition of any gas, just by studying the lines in its spectrum Don’t need a sample of the gas, just the light from the gas –planetary atmospheres, stellar atmospheres, interstellar gas clouds, intergalactic gas clouds, accretion disks

Doppler Effect The observed wavelength of any wave depends on the relative motion of the emitter (source) and the receiver –If source & receiver are approaching one another, the wavelength appears shorter (blue shift) –If source & receiver are receding from one another, the wavelength appears longer (red shift)

Doppler Shift The amount that the wavelength is shifted depends on the relative speed of the source and receiver

13 Transit Method

14 Extra-solar planets HR

15 Extra-solar planets Fomalhaut b 15

16 Extra-solar planets

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20 Extra-solar planets

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