Chapter 5

Electromagnetic Radiation (Light)

Radiation is also known as...

• • •

Light

*pure energy

Electromagnetic Waves

*energy-carrying waves emitted by vibrating electrons

Photons

*particles of light

What is light?

• Light can act either like a wave or like a particle • Particles of light are called photons

Waves

• A wave is a pattern of motion that can carry energy without carrying matter along with it

Properties of Waves

• • Wavelength is the distance between two wave peaks Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency

Wavelength (λ) and Frequency (f)

wavelength x frequency = speed of light ~ 300,000,000m/s λf=c

Light: Electromagnetic Waves

• • A light wave is a vibration of electric and magnetic fields Light interacts with charged particles through these electric and magnetic fields

The Electromagnetic Spectrum

•

A range of light waves extending in wavelength from radio waves to gamma rays

The Electromagnetic Spectrum

• • • • • • •

Radio Waves Microwaves Infrared Visible Light Ultraviolet X-rays Gamma Rays

mnemonic

Raging Martians Invaded Venus (Roy G. Biv ) Using X-rays and Gamma Rays

What is the electromagnetic spectrum?

Thought Question The higher the photon energy… a) the longer its wavelength.

b) the shorter its wavelength.

c) energy is independent of wavelength.

Why be a physicist in late 1800s ?

One scientist lamented Newton and Maxwell had done it all.

Nothing was left to discover. All that remained was to repeat the experiments with greater and greater precision.

But there were some curious problems with “classical physics”

Problems of Classical Physics

Spectra from atoms, cold and hot Blackbody radiation Photoelectric effect What is the structure of matter ?

Problem:The structure of matter

Thompson model of matter had negative electrons stuck in positive pudding.

Rutherford alpha scattering experiment proved positive nucleus had most of the mass in small area. Positive particles repel other positive particles. NUCLEI SHOULDN’T EXIST a new more powerful force is required.

Led to planetary model of atoms. Positive nucleus (protons and neutrons) had negative electrons in orbits around them.

More on Structure Problem

• • • Charged particles have an electric field around them.

Motion of a charged particle causes a disturbance in the field, a wave, i.e. radiation Orbiting electrons should radiate energy, spiral into the nucleus.

Spectra from atoms:The Problem

Positive charges repel other positive charges with inverse square law.

Electrons in classical orbits would emit continuous spectra as they spiral into the nucleus.

Atoms shouldn’t exist.

In 1859 long before atomic structure was understood, Gustav Kirchhoff formulated three rules that describe the three types of spectra.

Kirchhoff

p. 99

Kirchoff’s Laws for Spectra • • Law I: Solids, liquids or dense gases radiate at all wavelengths, a continuous spectrum.

Law II : A low density gas excited to emit light will do so at specific wavelengths, a bright line or emission spectra.

• Law III : If light from a continuous spectrum passes through a cool low density gas a dark line spectra will result.

Line spectra are unique for each element !

Kirchoff’s Laws for spectra

Kirchhoff’s 1 st Law: continuous spectra 1 st law: Solids, liquids or dense gases radiate at all wavelengths… we now know the light is radiated by thermal radiation which is sometimes called blackbody radiation

Properties of Thermal/Blackbody Radiation 1. Hotter objects emit more light at all frequencies per unit area.

2. Hotter objects emit photons with a higher average energy.

Blackbody Radiation

The Problem: Attempts to explain BB radiation failed.

Classical physics leads to an equation that fits for long wavelengths but has a term that results in increasing intensity as the wavelengths get short. This was called the ultraviolet (uv) catastrophe.

Wien’s Law for blackbody radiators

Wien’s Law

l max = 3 , 000 , 000

T

Star Colors

• Reddish • Orange-ish • Yellowish • White  coolest star • Bluish  hottest star

3000 K 4000 K 5000 K 6000 K 7000 K

Star Temperatures

10,000 K 15,000 K 30,000 K

Total Energy from BB

The area under the curve is the total energy emitted by the blackbody.

• • • • Stefan-Boltzmann Law: E = sA T 4 , E is the energy emitted per second σ is a constant 5.67 X 10 object -8 J/m 2 sec/K 4 T is the Kelvin (absolute) temperature of the A is the area the energy is passing through.

Emission Spectrum

Something first has to excite the atoms. The surface of a star or near by hot stars mayexcite nebula out to great distances.

Horsehead Nebula

Absorption Spectra

A continuous spectrum passing through a cooler gas will remove (absorb) the same lines the hot gas emits.

These are called absorption or dark line spectra.

Lab Absorption

Stellar Atmospheric Absorption

Solar Spectra

Next Problem: Photoelectric effect

Photoelectric effect:Experimental

Photoelectric effect Results: KE of the ejected electrons

Photoelectric effect: The Problem

Classical physics would predict the brighter the light the more energy the emitted electrons would have. Increasing the intensity gives more electrons, but they all have the same kinetic energy. Each metal has a different threshold wavelength. Below this threshold no electrons are emitted regardless of the intensity of the light.

The energy depends linearly on the frequency.

E= hf = hc/λ

What can we find out from the light from a distant source? A.

Summary so far: 1. Temperature from BB peak: Wien’s Law 2. Total energy from the BB temperature: Stefan Boltzmann Law 3. Composition: Hot gases, emission nebula Cooler gases, stellar atmospheres, nebula All are descriptive only, not understood until …

Problems resolved

Unexplained events are due to either poor experiments or standing on the edge of a great discovery.

The Birth of Modern Physics Max Planck (c 1900) explained BB radiation by applying discrete energy levels to the oscillators in a BB cavity, Planck’s constant, h.

Einstein took Planck’s idea a “quantum leap” forward by saying the energy itself was emitted in packets of specific energy, photons. Energy given by E = hf. Light has particle properties too. [ This won him the Nobel prize].

It also explains spectra as well.

Atoms are mostly empty

If hydrogen nucleus were the size if a grape seed the first electron orbital would be out 225 yards away.

Bohr , Schrodinger and others developed “quantum mechanics” to describe the structure of matter.

We still use the idea of orbitals with associated energy levels, but it’s better to call these probability distributions.

Take a chemistry class.

Fig. 6-2, p. 94

Spectra explained,1

Quantum mechanics, uses particle and wave properties to explain structure of atoms.

Electrons in energy levels do not emit energy.

Only photons with the exact energy to match differences in energy levels interact, going or coming, with atoms.

The lowest energy level is called the ground state. Higher levels are called excited states.

Energy Level Transitions

I’m Free • The only allowed changes in energy are those corresponding to a transition between energy levels Not Allowed Allowed

Photon absorption

The excited atom is unstable and within a fraction of a second returns to a lower energy level, reradiating the photon in a random direction.

Hydrogen Absorption

Fig. 6-4, p. 96

Spectra explained , 2

Hydrogen Energy levels

Transitions between levels lead to different series of spectra.

To n=1 level Lyman series .. High energy we can’t see these.

To n=2 Balmer series… These are what we see To n= 3 , Paschen series, in infrared.

p. 100

Energy levels for different atoms

Increasing atomic number, more protons pulls the energy levels closer and we get more electrons too, Spectra become more complex.

Each element has its signature spectrum. Levels have limits on number of electrons in them (Pauli Exclusion Principle).

Fig. 6-3, p. 95

Chemical Fingerprints

• Each type of atom has a unique spectral fingerprint

Spectra,3

Bright line, emission spectra result from electron transitions to lower energy levels. Conservation of energy requires the energy to be carried away. Photons have energy E = D E =hf, D E is the energy difference between the levels in the atom.

Dark line , absorption, spectra result when an electron is hit by a photon with energy matching a transition to a higher level.

The electron jumps to a higher state. The energy is later emitted in either a single photon or a series of photons. Emission is in a random direction. This lowers the light intensity in the original direction.

Stellar Spectral

Fig. 6-CO, p. 92

Solar Spectra

Fraunhoffer in early 1800’s found 600 dark lines in solar spectra. We now know the source.

Doppler Effect

• The apparent change in wavelength or frequency of a wave when the source, observer, or both is in motion.

Everyday Doppler Effect : Sound

Figure 6.11: The Doppler effect. (b) The clanging bell on a moving fire truck produces sounds that move outward (black circles). An observer ahead of the truck hears the clangs closer together, while an observer behind the truck hears them farther apart.

Doppler Effect: Light

True Velocity Radial Velocity Tangential Velocity Radar

Optical Doppler Shift

Figure 6.11: The Doppler effect. (a) A blue shift appears in the spectrum of a star approaching Earth (top spectrum). A red shift appears in the spectrum of a star moving away from Earth (bottom spectrum). (The

Observatories of the Carnegie Institution of Washington

Fig. 6-11a, p. 106

Radial Velocity from Doppler Data !!!!

V r c V r c = D l l The ratio of the radial velocity to the speed of the wave is equal to the ratio of the shift in the wavelength to the original wavelength. Or V r = Δλ c/ λ

What can we find out from the light from a distant source? B.

Summary so far: 1. Temperature from BB peak: Wien’s Law 2. Total energy from the BB temperature: Stefan Boltzmann Law 3. Composition 4. Radial velocity Still more to come…..

How does light tell us the rotation rate of an object? • Different Doppler shifts from different sides of a rotating object spread out its spectral lines

Spectrum of a Rotating Object

• Spectral lines are wider when an object rotates faster

Collisions cause shifts Higher pressure higher probability of a collision

Higher Temperatures also cause more broadening

Thin cool gas in nebula give sharp lines

Demonstrations for Lab

1.View light from a single slit or from a light bulb filament.

Vary the voltage changing the temperature. Law I 2. Observe light from selected gas discharge tubes. Law II 3. Observe a solar spectrum by sunlight reflected off a cloud Law III