Chapter 3Light and Matter

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Units of Chapter 2Information from the Skies

Waves in What?

The Electromagnetic Spectrum

Thermal Radiation

Spectroscopy

The Formation of Spectral Lines

The Doppler Effect

Spectral-Line Analysis2

Information from the SkiesElectromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields

Example: Light

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Information from the Skies

~Light acts as both a wave and particle (photon)

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Light as a Wave

• Light waves are characterized by a wavelength, l,and a frequency, f.

f = c/l

c = 300,000 km/s = 3*108 m/s

• f and l are related through

l

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Information from the Skies

Wave motion: transmits energy without the physical transport of material

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Information from the Skies

Example: water wave

Water just moves up and down

Wave travels and can transmit energy

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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

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Information from the Skies

Wavelength: distance between successive crests

Velocity: speed at which crests move

Relationship:

Velocity = Wavelength / Period

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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

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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

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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:

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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

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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.

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

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The Electromagnetic Spectrum

The visible spectrum is only a small part of the total electromagnetic

spectrum:

Different colors of light are distinguished by their frequency and wavelength.

Longest wavelength

Lowest frequency

Shortestwavelength

Highest frequency

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The Electromagnetic Spectrum

Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.

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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

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The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites

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

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Thermal Radiation

• Temperature = amount of microscopic motion within

• Hotter = higher temp = faster particles =

MORE ENERGY• Intensity- amount/strength of radiation

at any point in space

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Thermal RadiationRadiation Laws

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

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Thermal Radiation

~Blue Star is hotter than Red Star

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Figure 3.11

Thermal RadiationRadiation Laws

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

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More Precisely

Kelvin Temperature scale:

• All thermal motion ceases at 0 K

• Water freezes at 273 K and boils at 373 K

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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.

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SpectroscopyEmission lines: single frequencies emitted by particular atoms

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SpectroscopyEmission spectrum can be used to identify elements:

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

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SpectroscopyAbsorption spectrum of the Sun:

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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

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Kirchhoff’s laws

Spectroscopy

Kirchhoff’s laws illustrated:

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Spectral Line AnalysisUsing spectrum, Astronomers can

determine:• Composition (atoms and molecules)• Temperature• Velocity• Rotation rate• Pressure of gas• Magnetic field

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The Doppler EffectIf one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer

Relationship between frequency and speed:

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Doppler Effect

• Redshift– Net motion away from observer– Shift toward longer wavelength

• Blueshift– Motion toward the observer– Shift toward shorter wavelength

Like train whistle: high pitch (towards) low pitch (away)

Also used for radar guns by police and in baseball

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The Doppler Effect

Depends only on the relative motion of source and observer:

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

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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

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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

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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

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Kirchhoff’s laws

Kirchhoff’s laws