fall 2014astron 1instructor: babar ali visible spectrum of the sun
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Fall 2014 Astron 1 Instructor: Babar Ali
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Visible spectrum of the sun.
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Lecture 10
Astron 1Instructor: Dr. Babar Ali
Book: Chapter 5
Fall 2014 Astron 1 Dr. Ali
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Today's Topics• Light
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Light is important to astronomers
• “Light 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.
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James Clerk Maxwell (1831-79)
• Scottish physicist.• Investigated the
phenomena of Electricity and Magnetism.
• Mathematically unified both in 4 elegant equations.
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• When charged particles oscillate,• They create electromagnetic
disturbances• That move through space in the form of
waves.• Light!
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James Clerk Maxwell’s Equations
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James Clerk Maxwell’s Equations
• Synthesized:– laws of Electricity, – laws of Magnetism– laws of the behavior
of light.
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Magnetic cause Electric Fields Current
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Light is Radiation
• Radiation 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.
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Light behaves like a wave
• Maxwell’s findings were consistent with the commonly held beliefs of the time.
• The observed experiments and behavior indicated that light behaves as a wave:– Interference– Diffraction
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Image: courtesy of wiki commons
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Properties of a light wave
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Peak = Crest
Trough
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Properties of a light wave
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Wavelength
Wavelength is a distance. Units of wavelengths are units of distance.
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Properties of a light wave
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Direction of propagation at the speed of light
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Properties of a light wave
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Frequency = how many peaks pass by a fixed point in one second.Unit of frequency is Hertz (Hz), which is inverse seconds (1/second).
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Properties of light wave
• Wavelength is related to frequency:
• The wavelength/frequency determines the color of light.
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l X n = c
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Light as a particle
• Earth 20th century experiments showed that light also acts like a moving particle
• Photoelectric effect– Photons provide discreet quanta of energy
which can be absorbed by electrons.• Compton scattering
– Photons carry momentum = a property of particles, not waves.
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Light as a particle
• PhotonA massless particle that moves like wave.
Light = E&M wave stream of photons.
• Energy of photon:
E = hn• Quantum physics and later refinements
explain the wave-particle duality of all objects.
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Propagation of light• Apparent Brightness of a
source decreases with increasing distance in proportion to d2 = Inverse Square Law
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B ~ 1/d2B1AU ~ 1/(1)2= 1/1
B2AU ~ 1/(2)2= 1/4
B3AU ~ 1/(3)2= 1/9
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Propagation of light
• Principle 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!
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E&M Spectrum
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E&M Spectrum• g (Gamma) Rays: l < 0.01 nm (1 nm = 10-9 m)
– Gamma = 3rd letter of Greek alphabet, 3rd kind discovered from radioactive atoms
– Dangerous: 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 nm– Mostly blocked by Ozone– Causes sunburn & skin cancer
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E&M Spectrum• Visible: 400 nm < l < 700 nm (1 nm = 10-9
m)– where gives off greatest amount of radiation– Penetrates ’s atmosphere effectively– In 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 band– Absorbed by H2O & CO2 in ’s atmosphere
• Radio Waves: few mm < l < 100’s Km– (microwaves, radar, FM & TV, AM)– communication, ovens, airport, military, AM related to IonosphereFall 2014 Astron 1 21
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E&M Spectrum
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Review Topics
• Properties of light: wavelength, frequency, speed.
• What determines the color of light.• E&M spectrum. All parts of it.• How does light propagate?
Fall 2014 Astron 1