Atom and Light

ASTR 14: Exploring the Universe

2

Outline

• Nature of Light

• Basic Properties of Light

• Atomic Structure

• Periodic Table of the Elements

• Three Types of Spectra

• Interaction of Light and Matter

• Useful Application – Doppler Effect

3

Nature of Light

• What is the “Nature” of Light?

• Observed Properties of Light– reflection, refraction, diffraction, & interference

• Wave Properties– reflection, refraction, diffraction, & interference

• Particle Properties– reflection, refraction

• Particle-Wave Duality

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Wave Properties of Light

• Wave Properties– diffraction– Interference

• Young’s Double-Slit Experiment (1801)

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Wave Properties of Light

• Electromagnetic Waves– wavelength (meter)– frequency (cycles per

second = Hertz = Hz)

• Speed of Light– constant within a

medium– absolute speed– 3.0 x 108 m/s in

vacuum

Fizeau-Foucault Method (1850)

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Electromagnetic Spectrum

• Speed of Light = wavelength x frequency

• C = x

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Particle Properties of Light

• Max Planck (1900)– Blackbody Radiation– Electromagnetic energy

(radiation) is emitted in discrete, particlelike packet.

• Albert Einstein (1905)– Re-interpret Planck’s result– Photoelectric Effect– Light as particles --

photons• Energy of a Photon

– The energy of a photon is proportional to its frequency.

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Blackbody Radiation

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Blackbody Radiation

• Wien’s Law– The wavelength of

maximum emission of a blackbody is inversely proportional to its temperature.

• Stefan-Boltzmann Law (Box 5-2)– The energy flux (power

per unit area) of a blackbody is proportional to the 4th power of its temperature.

)(

0029.0)(max KTm

42842 1067.5;)( KWmxTWmFlux

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Atomic Structure

• Rutherford’s Experiment (1910)

• Rutherford’s Model of the Atom

• Atomic Structure– cloud of electrons– nucleus

• protons• neutrons

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Atomic Structure

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Three Types of Spectra

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Interaction of Light and Matter

• Kirchoff’s 1st Law– A hot opaque body, such as a perfect blackbody, or a hot,

dense gas produces a continuous spectrum.

• Kirchoff’s 2nd Law– A hot, transparent gas produces an emission line spectrum.

• Kirchoff’s 3rd Law– A cool, transparent gas in front of a source of a continuous

spectrum produces an absorption line spectrum. The absorption lines in the absorption line spectrum of a particular gas occur at exactly the same wavelengths as the emission lines in the emission line spectrum of the same gas.

• Types of spectra of matter can produce is dictated by the physical condition/state of the matter.

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Interaction of Light and Matter

• Bohr Model of Hydrogen Atom– energy levels or states

• Quantum Rules– Electron can only occupy at discrete energy level.– Electron can jump from one energy level to another only if it

gains or loses a specific amount of energy equal to the differences of the levels.

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Interaction of Light and Matter

Electron Transitions in Hydrogen

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Interaction of Light and Matter

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Interaction of Light and Matter

States of Matter

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An Example

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Doppler Effect

• Christian Doppler (1842)• The apparent change in wavelength or frequency of radiation due to relative

motion between the source and the observer along the line-of-sight.• If the relative motion between the source and the observer is moving away

from each other, the observed spectral line is longer than the lab wavelength, it is called a redshift. If the relative motion between the source and the observer is moving towards each other, the observed wavelength is shorter than the lab wavelength, it is called a blueshift.

c

v

o

= wavelength shift

o = lab wavelength

V = radial velocity

c = speed of light

+ = redshift

- = blueshift

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Doppler Effect

Stationary Source Moving Source

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An Example