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Please turn off your phone. Visible spectrum of the sun. Lecture 10 Astron 1 Instructor: Dr. Babar Ali Book: Chapter 5. Today's Topics. Light. Light is important to astronomers. “Light from the sky is a treasure that links you to the rest of the Universe” - PowerPoint PPT Presentation



Fall 2014Astron 1Instructor: Babar AliPlease turn off your phoneVisible spectrum of the sun.

Lecture 10

Astron 1Instructor: Dr. Babar Ali

Book: Chapter 5Fall 2014Astron 1Dr. Ali2Today's TopicsLight Fall 2014Astron 13Light is important to astronomersLight from the sky is a treasure that links you to the rest of the Universe-- taken from an older textbook,All objects in the Universe emit, interact with, or affect light.Modern observational astronomy is primarily the study and analysis of light from extra-terrestrial sources.Solar System is the only place where direct material from extra-terrestrial sources is available.It is important to understand the nature of light.Fall 2014Astron 14James Clerk Maxwell (1831-79)Scottish physicist.Investigated the phenomena of Electricity and Magnetism.Mathematically unified both in 4 elegant equations.

Fall 2014Astron 15When charged particles oscillate,They create electromagnetic disturbancesThat move through space in the form of waves.Light!Fall 2014Astron 16James Clerk Maxwells Equations

James Clerk Maxwells EquationsSynthesized:laws of Electricity, laws of Magnetismlaws of the behavior of light.

Fall 2014Astron 17Magnetic cause Electric Fields CurrentLight is RadiationRadiation is the transport of energy in the form of Electromagnetic waves.Light is electromagnetic radiation.Maxwell established light as a wave that requires no medium to be present for its propagation.Fall 2014Astron 18Light behaves like a waveMaxwells findings were consistent with the commonly held beliefs of the time. The observed experiments and behavior indicated that light behaves as a wave:InterferenceDiffraction

Fall 2014Astron 19Image: courtesy of wiki commonsProperties of a light waveFall 2014Astron 110Peak = CrestTroughProperties of a light waveFall 2014Astron 111WavelengthWavelength is a distance. Units of wavelengths are units of distance.Properties of a light waveFall 2014Astron 112Direction of propagation at the speed of lightProperties of a light waveFall 2014Astron 113Frequency = how many peaks pass by a fixed point in one second.Unit of frequency is Hertz (Hz), which is inverse seconds (1/second). Properties of light waveWavelength is related to frequency:

The wavelength/frequency determines the color of light.Fall 2014Astron 114l X n = cLight as a particleEarth 20th century experiments showed that light also acts like a moving particlePhotoelectric effectPhotons provide discreet quanta of energy which can be absorbed by electrons.Compton scatteringPhotons carry momentum = a property of particles, not waves.Fall 2014Astron 115Light as a particlePhotonA massless particle that moves like wave.Light = E&M wave stream of photons.Energy of photon:E = hnQuantum physics and later refinements explain the wave-particle duality of all objects.Fall 2014Astron 116Propagation of lightApparent Brightness of a source decreases with increasing distance in proportion to d2 = Inverse Square Law

Fall 2014Astron 117

B ~ 1/d2B1AU ~ 1/(1)2= 1/1B2AU ~ 1/(2)2= 1/4B3AU ~ 1/(3)2= 1/9Propagation of lightPrinciple of least time.Light will choose a path that takes the least time to move from point A to point B in space.This is not always a straight line!Fall 2014Astron 118

E&M SpectrumFall 2014Astron 119E&M Spectrumg (Gamma) Rays: l < 0.01 nm (1 nm = 10-9 m)Gamma = 3rd letter of Greek alphabet, 3rd kind discovered from radioactive atomsDangerous: brakes up living tissue(generated in interior of s, death of s, merger of s, galaxies

X Rays: 0.01 nm < l < 20 nm penetrates soft tissue to image shadow of bones.Stopped by large # of atoms in s atmosphere

UV (Ultraviolet): 20 nm < l < 400 nmMostly blocked by OzoneCauses sunburn & skin cancerFall 2014Astron 120

E&M SpectrumVisible: 400 nm < l < 700 nm (1 nm = 10-9 m)where gives off greatest amount of radiationPenetrates s atmosphere effectivelyIn 1672, Newton found (w/prism) that sunlight, (white) = made up of colors in rainbow

IR (infrared=Heat): 1000 nm < l < 106 nm Nerve endings = sensitive to this bandAbsorbed by H2O & CO2 in s atmosphere

Radio Waves: few mm < l < 100s Km(microwaves, radar, FM & TV, AM)communication, ovens, airport, military, AM related to IonosphereFall 2014Astron 121

E&M SpectrumFall 2014Astron 122Review TopicsProperties of light: wavelength, frequency, speed.What determines the color of light.E&M spectrum. All parts of it.How does light propagate?Fall 2014Astron 123


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