Electromagnetic RadiationElectromagnetic Radiation

Energy can travel through space as a form Energy can travel through space as a form of kinetic energy we call electromagnetic of kinetic energy we call electromagnetic radiation.radiation.

Consists of electric and magnetic fields Consists of electric and magnetic fields that vary repetitively.that vary repetitively.

Most obvious to us: visible lightMost obvious to us: visible light

Emission SpectrumEmission Spectrum

Ideas about arrangements of atoms have Ideas about arrangements of atoms have come mostly from emmision spectra.come mostly from emmision spectra.

Emission Spectra are bright lines Emission Spectra are bright lines produced on film by radiation that has produced on film by radiation that has passed through a glass prism after being passed through a glass prism after being emitted from electrically or thermally emitted from electrically or thermally excited atoms.excited atoms.

EM RadiationEM Radiation

Can be described in terms of waves…Can be described in terms of waves…

WavelengthWavelengthFrequencyFrequencySpeed of LightSpeed of Light

Electromagnetic RadiationElectromagnetic Radiation

All light travels at the same speed: 2.9979 x 10All light travels at the same speed: 2.9979 x 1088 m/s.m/s.

How are wavlength, frequency, and c related?How are wavlength, frequency, and c related?

wavelength (frequency) = speed of lightwavelength (frequency) = speed of light

Symbols…Symbols…

Electromagnetic RadiationElectromagnetic Radiation

Because all photons travel at the same speed, Because all photons travel at the same speed, the difference between two photons corresponds the difference between two photons corresponds to a difference in a quality commonly referred to to a difference in a quality commonly referred to as “wavelength”.as “wavelength”.

We are looking at light particles (photons) that We are looking at light particles (photons) that behave as waves. behave as waves.

Light is a WaveLight is a Wave A DVD has thousands of small ridges only A DVD has thousands of small ridges only

hundreds of nanometers in width. These hundreds of nanometers in width. These ridges diffract light of different wavelengths ridges diffract light of different wavelengths to a different extent.to a different extent.

The rainbow pattern we see is a consequence The rainbow pattern we see is a consequence of the wave nature of light.of the wave nature of light.

Calculations…Calculations…

Light near the middle of the UV region of the Light near the middle of the UV region of the Electromagnetic spectrum has a frequency of Electromagnetic spectrum has a frequency of 2.73*102.73*1016 16 ss-1-1

Yellow light near the middle of the visible spectrum Yellow light near the middle of the visible spectrum has a frequency of 5.26*10has a frequency of 5.26*102424ss-1-1

Calculate the wavelength that corresponds Calculate the wavelength that corresponds to these two frequencies.to these two frequencies.

Continuous SpectrumContinuous Spectrum Light from the Sun and other incandescent Light from the Sun and other incandescent

sources contains a mixture of photons of sources contains a mixture of photons of different wavelengths.different wavelengths.

The different colors we see are a result of this The different colors we see are a result of this difference in energy between two photons.difference in energy between two photons.

When light from the sun is diffracted (separated) When light from the sun is diffracted (separated) it forms a continuous spectrum, which is a it forms a continuous spectrum, which is a rainbowrainbow

Using a spectroscope, look at the projector Using a spectroscope, look at the projector screen. Can you see a continuous spectrum?screen. Can you see a continuous spectrum?

The Complete Electromagnetic The Complete Electromagnetic SpectrumSpectrum

The spectrum visible to the human eye ranges The spectrum visible to the human eye ranges from about 700-400 nmfrom about 700-400 nm

The complete electromagnetic spectrum ranges The complete electromagnetic spectrum ranges thousands of times bigger and smaller. thousands of times bigger and smaller. Only a very small portion is visible to the eyeOnly a very small portion is visible to the eye

The Complete Electromagnetic The Complete Electromagnetic SpectrumSpectrum

In order from low to high energy:In order from low to high energy:Radio WavesRadio WavesMicrowavesMicrowaves Infrared RadiationInfrared RadiationVisible LightVisible LightUltraviolet RadiationUltraviolet RadiationX-RaysX-RaysGamma RaysGamma Rays

Particle Behavior of PhotonsParticle Behavior of Photons

According to Max Planck each photon of According to Max Planck each photon of light has a particular quanta of energy.light has a particular quanta of energy.

This energy depends on the frequency.This energy depends on the frequency.

Energy = Planck’s Constant ( frequency)Energy = Planck’s Constant ( frequency)

Symbols…Symbols…

Calculation Cont. Calculation Cont.

We have already calculated the wavelength. We have already calculated the wavelength. Now, calculate the energy, in Joules, of an Now, calculate the energy, in Joules, of an individual photon of each.individual photon of each.

Compare the two energies by calculating the Compare the two energies by calculating the energy of each.energy of each.

UV vs. Visible Light EnergyUV vs. Visible Light Energy

UV Light = 1.81*10UV Light = 1.81*10-17-17 J J

Visible Light (yellow) = 3.49*10Visible Light (yellow) = 3.49*10-19-19 J J

Ratio = ERatio = Euvuv/ E/ Evisvis

Ratio = 1.81*10Ratio = 1.81*10-17-17 J/ 3.49*10 J/ 3.49*10-19-19 J J

Ratio = 51.9Ratio = 51.9

So, why might UV light be so harmful to our skin?So, why might UV light be so harmful to our skin?

Bohr’s Atomic Model Bohr’s Atomic Model (aka Planetary model)(aka Planetary model)

Bohr used his observations to argue that the Bohr used his observations to argue that the energy of an electron is "quantized”energy of an electron is "quantized” Meaning only certain quantities of energy are Meaning only certain quantities of energy are

allowed therefore restricting its location of the allowed therefore restricting its location of the electron to certain regions around the nucleuselectron to certain regions around the nucleus

Since only certain energy levels are allowed it is Since only certain energy levels are allowed it is actually possible to diagram the atom in terms of actually possible to diagram the atom in terms of its energy levels.its energy levels.

Electron TransitionsElectron Transitions

e- “Rest (Ground) State”

Electron TransitionsElectron Transitions

e-

photon (energy) absorbed

“Excited State”

Electron TransitionsElectron Transitions

e-“Rest (Ground) State”

photon (energy) released

Gas Emission TubesGas Emission Tubes

The Chords of NeonThe Chords of NeonThe individual lines correspond to the energy difference between two orbitals. Using the entire set of lines, we can figure out the relative arrangement of the energy levels and therefore the element.

Gas TubesGas Tubes

NeonNeonHydrogenHydrogenChlorineChlorine

Make your own EM spectrum…Make your own EM spectrum…

Obtain a piece of paper and draw your own Obtain a piece of paper and draw your own EM Spectrum…EM Spectrum…