Dr. Chin ChuRiver Dell Regional High School

Unit 3 – Electron ConfigurationsPart A – Electromagnetic Waves

What had we learned so far?Atomic Structure –

NucleusElectrons

Essential question: how are those electrons surrounding nucleus arranged?

Experimental EvidenceDischarging Tubes

The Flame TestThe light coming out of

the excited atomic entities is very specific to

particular element!Results are quite

reproducible.

Experimental EvidenceThere has been no radioactive decay

going on. Hence the nucleus does not change when the atomic entity gets excited either by

electricity or heat.

So the colored light must have come from those electrons.

Light emitted from excited atomic entities is the tool used to probe how

electrons are arranged.

What is light?Lights, both visible and invisible to human eyes, are electromagnetic waves. Time Out!

Before we go any further,

what is a Wave?

What is a wave?A wave is a means to transfer energy from

point A to point B.

Waves in water Sound

wavesTypical mechanical waves such as those in water and sound waves DO need medium in

which they propagate. Water and air are the prerequisites

for waves to travel.

Waves – in more the abstract form

Note: great link to an online simulation of waves. http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html -

Wavelength - distance from crest to crest abbreviated Greek letter, , l pronounced “lambda”. Also can be defined as how far the wave travels in a cycle.

Waves – in more the abstract form

Frequency – the number of complete waves passing any given point per second.• SI unit for

frequency is Hertz (Hz), or cycles/sec.

• Abbreviated Greek letter, , npronounced “nu”.

The graph shows that the top wave passes any given point 4

complete wave forms every second; the middle one 2

complete wave forms; and the bottom one 1 complete wave

form.

Waves – in more the abstract formWavelength - defined as how far the wave travels in a cycle.

Frequency – the number of complete waves passing any given point per second.

Wavelength x Frequency = how far the wave travels in a second (speed of the

wave)s = lns: speed of wave

l: wavelengthn: frequency

Exercise I

1. What is the frequency of a wave in water where the speed of the wave is 3.4m/s and the wavelength is 0.5 m?

2. What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 103 Hz)

3. What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz?

Pause and complete the following exercise

before proceeding.

Exercise I

1. What is the frequency of a wave in water where the speed of the wave is 3.4m/s and the wavelength is 0.5 m? [6.8 Hz]

2. What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 103 Hz) [1.62 x 105 m/s]

3. What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz? [15.3 m]

Answers to the Questions.

Electromagnetic Waves (Lights)

Disturbance in a magnetic field is perpendicular to a disturbance in an electric field.

• Can travel in vacuum. No need for medium!

• Travels at 3 x1010 cm/second (or 3.00 x 108 m/s)in vacuum. Known as the “Speed of Light”, which does not vary with frequency nor wavelength

• Varying in wavelength and frequency.

Electromagnetic Waves (Lights)

c = lnc: speed of light (3.00 x

108 m/s)l: wavelengthn: frequency

Since the speed of light in vacuum does not change with frequency nor

wavelength, frequency and wavelength are inversely proportional.

• For an electromagnetic wave, frequency goes up, then wavelength has to come down proportionally and vies versa.

Electromagnetic Waves (Lights)

E = hnc: energy of the photon

h: Planck’s constant, 6.626 x 10-34 J.s

n: frequency

Now you have all the parameters and two relationships (see boxes below). Then you should be able to solve problems related to lights. Keep in mind how to manipulate parameters and variables.

Good luck!

c = lnc: speed of light (3.00 x

108 m/s)l: wavelengthn: frequency

Exercise II – Electromagnetic Waves

1. What is the frequency of an electromagnetic wave with wavelength of 3.4m?

2. What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 106 Hz)

3. What is the energy of a photon that has a frequency of 4.23 x 107 Hz?

4. What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10-19 J)

Pause and complete the following exercise

before proceeding.

Exercise II – Electromagnetic Waves

1. What is the frequency of an electromagnetic wave with wavelength of 3.4m? [8.82 x 107 m/s]

2. What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 106 Hz) [8.33 m]

3. What is the energy of a photon that has a frequency of 4.23 x 107 Hz? [2.80 x 10-26 J]

4. What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10-19 J) [4.85 x 10-7 m]

Answers

Electromagnetic Waves (Radiations)Examples:radio wavesmicrowavesinfraredwhite light (visible spectrum)ultraviolet lightX-raysgamma radiation

Radiowaves

Microwaves

Infrared .

Ultra-violet

X-Rays

GammaRays

Low Energy

High Energy

Low Frequency

High Frequency

Long Wavelength

Short Wavelength

Visible Light

---------------- > decreasing energy -------------------------------------> decreasing frequency ----------------

>---------------> increasing wavelength ----------------

>

Diagram Showing Wavelength and Frequency

Types of SpectraContinuous – all wavelengths within a

given range are included.

Electromagnetic – all electromagnetic radiation arranged according to increasing or decreasing wavelength.

a.unit for wavelength ranges from meters to nanometers

b.unit for frequency is hertz (Hz) (# waves per second)

Types of SpectraVisible spectrum - light you can

see (ROY-G-BIV) a.Red has the longest wavelength and

the smallest frequency.b.Violet has the shortest wavelength and

the greatest frequency.

Bright Line spectrum (emission spectrum)Bands of colored light emitted by

excited electrons when they return to the ground state.

Passing Light Through a Prism

White light is made up of all the colors of the visible spectrum.

Passing it through a prism separates the colors in white light.

If the light is not white,By heating a

gas with electricity we can get it to give off colors.

Passing this light through a prism does something different.

If the light is not white,Each element

gives off its own characteristic colors.

Can be used to identify the atom.

This is how we know what stars are made of.

Spectroscopy1. Emission spectra of a substance is

studied to determine its identity.

2. Spectroscope – instrument that separates light into a spectrum.

3. Spectral lines – represent wavelength of light emitted when excited electrons fall back to the ground state.

How Does a Spectroscope Work?

Emission Spectrum (Line Spectrum)

Emission Spectrum