# lecture8.docx - Lecture 8 How does light tell us the speed...

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Lecture 8How does light tell us the speed of a distant object?o= The Doppler EffectThe Doppler EffectoLight emitted from an object moving towards you will have its wavelengthshortened: BlueshiftoLight emitted from an object moving way form you will have its wavelengthlengthened: redshiftoLight emitted from an object moving perpendicular to your line-of-sight will notchange its wavelengthDoppler shift tells us ONLY about the part of an object’s motion toward or away from usoIf star is moving directly away form us, Doppler shift tell us its full speedoIf star is moving across our line of sight, but not toward or away form us, Dopplershift measures no speed at alloIf star is moving diagonally away form us, Doppler shift measures only the part ofits speed directed away from usThe Doppler Effecto/= (Δλ λλobsλrest)/λrest= v/coThe wavelength of the light shifts to the blue (shorter wavelength) or the red(longer wavelength)oC is speed of lightoBy measuring distance of lines, get velocity of starLooking at spectrum lines…oIf move right, toward right, red-shifted: object moving away from usoIf move left, toward blue, blue-shifted: object moving toward usoThe larger the line jump, the faster the speedMeasuring radial velocityoWe can measure the Doppler shift of emission or absorption lines in the spectrumof an astronomical objectoWe can then calculate the velocity of the object in the direction either towards oraway from Earth (radial velocity)Though question: I measure a line in the lab at 500.7 nm. The same line in a star haswavelength 502.8 nm. What can I say about this star?oIt is moving away from me (wavelength increased)Red shiftedv/c = (502.8 – 500.7)/500.7 = 0.0042Doppler Effect SummaryoMotion toward or away from an observer causes a shift in the observed wavelengthof light:Blueshift (shorter wavelength) -> motion toward youRedshift (longer wavelength) -> motion away from youGreater shiftgreater speed
Chapter 6: Collecting Light with TelescopesBasic lens – convex on two sidesLight from a far-away source comes in paralleloA point-like object (ex: sun) appears as a point in the focal planeoLight from an extended object comes in at different anglesoAn extended object appears as an image in the focal planeTwo types of telescopesoRefracting telescopesoReflecting telescopesGalileo’s telescope was a refracting telescope

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