ISP 205 - Astronomy Gary D. Westfall1Lecture 6 The Nature of Light Light and other forms of radiation carry information to us from distance astronomical

1ISP 205 - Astronomy Gary D. Westfall Lecture 6

The Nature of LightThe Nature of Light• Light and other forms of radiation carry

information to us from distance astronomical objects

• Visible light is a subset of a huge spectrum of electromagnetic radiation

• Maxwell pioneered the theory of electromagnetic radiation (and light) Electric fields Magnetic fields Oscillating charges produce electric and magnetic

fields Famous 4 equations (outside the scope of this course)


Light as a WaveLight as a Wave• Wave

Diffraction Interference

• Can describe waves in terms of wavelength and frequency

WavelenthCrestTroughMoving at the speed of light


Electromagnetic RadiationElectromagnetic Radiation• Light differs from other forms of electromagnetic

radiation by its wavelength• Visible light has wavelengths between 400 and

700 nanometers• EM radiation with wavelengths just longer than

visible light is called infrared radiation (heat)• EM radiation with wavelength just shorter than

visible light is called ultraviolet radiation (UV)• Radio waves have long wavelengths (WKAR FM

is 3 meters)• Microwaves have about 3 cm wavelength


EM Radiation SpectrumEM Radiation Spectrum• The frequency/wavelength varies dramatically

• Most EM radiation cannot penetrate the Earth’s atmosphere


View of the Sky with X raysView of the Sky with X rays• If we could “see” with X rays instead of visible

light and we were above the Earth’s atmosphere the sky would look like:


Light as a ParticleLight as a Particle• Light (and all EM radiation) exists in quantized

units called photons

• A photon carries a specific amount of energy

• High frequency EM radiation has high energy photons Gamma rays

• Low frequency EM radiation has low energy photons Long-wave radio

• Described by Quantum Mechanics


Radiation and TemperatureRadiation and Temperature• The temperature

of an object determines what wavelength of EM radiation it will emit

• The wavelength of the maximum energy emission is given by Wien’s LawmaxT = 2.9 x 10-3 mK


Energy Emitted by StarsEnergy Emitted by Stars• The higher the temperature of an object, the more

energy is radiated at all wavelengths

• The higher the temperature, the “bluer” the star looks

• The total energy radiated is given by the Stefan-Boltzmann law E = T4 where E is the emitted energy, T is the

temperature, and is a constant


Spectroscopy in AstronomySpectroscopy in Astronomy• EM radiation carries information about the nature

of astronomical object• Visible light is the most used• Light can be

Reflected From a mirror

Refracted Through a lens

Dispersed Separated by wavelength

Prism Spectrometer


Continuous SpectrumContinuous Spectrum• When white light (a superposition of light with all

wavelengths) is dispersed with a prism or a spectrometer, all colors (wavelengths) are visible

• Wavelengths shorter than 400 nm are invisible (UV)

• Wavlengths longer than 700 nm are invisible (IR)


Discrete Emission SpectraDiscrete Emission Spectra• When atoms are heated, they emit light at specific

wavelengths characteristic of those atoms


Discrete Absorption SpectraDiscrete Absorption Spectra• When white light passes through atoms light is

absorbed at specific wavelengths

• Several elements were first observed in absorption spectra from the sun


Probing the AtomProbing the Atom• The electron was discovered by J.J. Thomson in 1897

Related to electricity, lightning

• In 1911, Ernest Rutherford bombarded a thin foil of gold with alpha particles from naturally occurring radioactive radium


Rutherford’s Model of the AtomRutherford’s Model of the Atom• Rutherford’s results showed that most of the mass

of the atom was concentrated in the nucleus

• Rutherford proposed a model similar to the solar system with negative electrons orbiting a positive nucleus


The Hydrogen AtomThe Hydrogen Atom• The simplest atom is the hydrogen atom• Composed of 1 electron and 1 proton

Electron has charge -1 Proton has charge +1 Proton is 2000 times heavier

• The electron is bound to the proton in its ground state

• We know now that the electron does not orbit the proton like the Earth orbits the Sun

Heisenberg Uncertainty Principle We cannot simultaneously know the position and energy of a particle to

arbitrary precision


Other AtomsOther Atoms• The next most simple atom is

helium

• A helium atom has 2 neutrons and 2 protons in its nucleus with 2 electrons orbiting the nucleus The neutron and proton have almost the same mass but

the neutron has not charge Neutron not discovered until 1930 by Chadwick

• The helium atom is much more complicated than the hydrogen atom because the 2 electrons interact with each other


IsotopesIsotopes• The chemical properties of atoms are determined by the number

protons and the number of electrons

• Light nuclei have roughly the same number of neutrons and protons

• Atomic nuclei can have different number of neutrons Isotopes

• Hydrogen has 3 naturally occurring isotopes

Hydrogen, 1H Stable

Deuterium, 2H Stable

Tritium, 3H Radioactive


The Bohr AtomThe Bohr Atom• Rutherford’s model of the atom had some tragic

flaws

• Orbiting electrons are accelerating and should radiate energy Lifetime of the atom should be 10-10 seconds!

• Neils Bohr proposed that the electrons in the hydrogen atom could only exist in certain quantized orbits

• Jumping between the orbits required the emission or absorption of photons of a specific wavelength


Radiation and AbsorptionRadiation and Absorption• Whenever a hydrogen atom changes from one stationary state to another, energy

is emitted or absorbed. When that energy takes the form of electromagnetic radiation then it has a frequency f (or as it is often called, ) given by h = |Ef - Ei|

Ef and Ei are the final and initial energies respectively.

• If Ei > Ef, then radiation occurs while if Ef > Ei, then absorption takes place.

• In the diagram the first six levels are shown as well as the zero energy level (n = ).

• If the transition takes place from any n to n =1, it is referred to as a Lyman line.

• Transitions to n = 2 are called Balmer lines and so on.

• Four of the Balmer lines are in the visible range.


Photon EnergiesPhoton Energies• Visible light has wavelengths between 400 and

700 nm

• Photons have energy E = hf = hc/• Photons from visible light then have energies

between 700 nm

E = 6.62 x 10-34 * 3 x 108 / 700 x 10-9 = 2.8 x 10-19 J = 1.8 eV 400 nm

E = 6.62 x 10-34 * 3 x 108 / 400 x 10-9 = 5.0 x 10-19 J = 3.1 eV


Three Kinds of SpectraThree Kinds of Spectra• We will consider three kinds of spectra

Continuous Light bulb or other source

Emission Heated cloud of gas

Absorption Continuous spectra passing through a cold cloud of gas


Doppler ShiftDoppler Shift• Relative motion affects

waves

• If a source of waves is moving toward you, the frequency is higher and the wavelength is shorter

• If a source of waves is moving away from you the frequency is lower and the wavelength is longer

• Toward, shorter wavelength, blue shift

• Away, longer wavelength, red shift

• A familiar example is sound


Red ShiftRed Shift• Most of the objects in the universe seem to be

moving away from us Evidence for Big Bang Red shift v = c/

• We observe the red shift of specific emission lines from known atoms Hydrogen or calcium have distinctive lines and are

almost always present

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ISP 205 - Astronomy Gary D. Westfall1Lecture 6 The Nature of Light Light and other forms of radiation carry information to us from distance astronomical