# Chapter 3 Light and Matter

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Chapter 3 Light and Matter

Units of Chapter 2 Information from the Skies Waves in What?
The Electromagnetic Spectrum Thermal Radiation Spectroscopy The Formation of Spectral Lines The Doppler Effect Spectral-Line Analysis

Information from the Skies
Electromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields Example: Light

Information from the Skies
~Light acts as both a wave and particle (photon)

Light as a Wave l c = 300,000 km/s = 3*108 m/s Light waves are characterized by a wavelength, l,and a frequency, f. f and l are related through f = c/l

Information from the Skies
Wave motion: transmits energy without the physical transport of material

Information from the Skies
Example: water wave Water just moves up and down Wave travels and can transmit energy

Information from the Skies
Frequency: number of wave crests that pass a given point per second Period: time between passage of successive crests Relationship: Period = 1 / Frequency

Information from the Skies
Wavelength: distance between successive crests Velocity: speed at which crests move Relationship: Velocity = Wavelength / Period

Waves in What? Diffraction: the bending of a wave around an obstacle
Interference: the sum of two waves; may be larger or smaller than the original waves

Waves in What? Diffraction: purely wave phenomenon
-If light were particles, there would be no fuzziness and light would be same size as the hole

Waves in What? Water waves, sound waves, and so on, travel in a medium (water, air, …) Electromagnetic waves need no medium Created by accelerating charged particles:

Waves in What? What is the wave speed of electromagnetic waves?
c = 3.0 x 108 m/s It can take light millions or even billions of years to travel astronomical distances

Waves in What? Magnetic and electric fields are inextricably intertwined. A magnetic field, such as the Earth’s shown here, exerts a force on a moving charged particle.

Waves in What? Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa

The Electromagnetic Spectrum
Different colors of light are distinguished by their frequency and wavelength. The visible spectrum is only a small part of the total electromagnetic spectrum: Longest wavelength Lowest frequency Shortest wavelength Highest frequency

The Electromagnetic Spectrum
Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths. Note that the atmosphere is only transparent at a few wavelengths – the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band. This means that our atmosphere is absorbing a lot of the electromagnetic radiation impinging on it, and also that astronomy at other wavelengths must be done above the atmosphere. Also note that the horizontal scale is logarithmic – each tick is a factor of 10 smaller or larger than the next one. This allows the display of the longest and shortest wavelengths on the same plot.

The Electromagnetic Spectrum
atmosphere is only transparent at a few wavelengths (opacity=thickness) the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band atmosphere absorbs a lot of the electromagnetic radiation impinging on it astronomy at other wavelengths must be done above the atmosphere

The Electromagnetic Spectrum
Wavelength Frequency High flying air planes or satellites Need satellites to observe

Thermal Radiation Blackbody Spectrum: radiation emitted by an object depending only on its temperature

Temperature = amount of microscopic motion within Hotter = higher temp = faster particles = MORE ENERGY Intensity- amount/strength of radiation at any point in space

1. Peak wavelength is inversely proportional to temperature. Wien’s Law = λmax = 0.29 cm/T

Thermal Radiation ~Blue Star is hotter than Red Star

Figure 3.11 © 2014 Pearson Education, Inc.

2. Total energy emitted is proportional to fourth power of temperature. Stefan’s Law

More Precisely Kelvin Temperature scale:
All thermal motion ceases at 0 K Water freezes at 273 K and boils at 373 K

Spectroscopy Spectroscope: splits light into component colors
Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum.

Spectroscopy Emission lines: single frequencies emitted by particular atoms

Spectroscopy Emission spectrum can be used to identify elements:

Spectroscopy Absorption spectrum: if a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit

Spectroscopy Absorption spectrum of the Sun:

Spectroscopy Kirchhoff’s Laws:
Continuous spectrum- Luminous solid, liquid, or dense gas produces Emission spectrum- Low-density hot gas Absorption spectrum- Continuous spectrum through a cool, thin gas

Kirchhoff’s laws

Spectroscopy Kirchhoff’s laws illustrated:

Spectral Line Analysis
Using spectrum, Astronomers can determine: Composition (atoms and molecules) Temperature Velocity Rotation rate Pressure of gas Magnetic field

The Doppler Effect If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer Relationship between frequency and speed:

Doppler Effect Redshift Blueshift Net motion away from observer
Like train whistle: high pitch (towards) low pitch (away) Redshift Net motion away from observer Shift toward longer wavelength Blueshift Motion toward the observer Shift toward shorter wavelength Also used for radar guns by police and in baseball

The Doppler Effect Depends only on the relative motion of source and observer:

The Doppler Effect The Doppler effect shifts an object’s entire spectrum either towards the red or towards the blue:

Summary of Chapter Wave: period, wavelength, amplitude
Electromagnetic waves created by accelerating charges Visible spectrum is different wavelengths of light Entire electromagnetic spectrum: radio waves, infrared, visible light, ultraviolet, X-rays, gamma rays Can tell the temperature of an object by measuring its blackbody radiation

Summary of Chapter Spectroscope splits light beam into component frequencies Continuous spectrum is emitted by solid, liquid, and dense gas Hot gas has characteristic emission spectrum Continuous spectrum incident on cool, thin gas gives characteristic absorption spectrum

Summary of Chapter Spectra can be explained using atomic models
Emission and absorption lines are distinct to different atoms and molecules Doppler effect can change perceived frequency of radiation Doppler effect depends on relative speed of source and observer

Kirchhoff’s laws

Kirchhoff’s laws