Fall 2014 Astron 1 Instructor: Babar Ali

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Visible spectrum of the sun.

Lecture 10

Astron 1Instructor: Dr. Babar Ali

Book: Chapter 5

Fall 2014 Astron 1 Dr. Ali

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

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?

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