Chapter 5 Part 2

c = ln Practice!

• What is the wavelength of a microwave that has a frequency of 1.56 x 109 Hz?

• The red-colored light in a fireworks display might be produced when Strontium salts are heated. What is the frequency of such red light with a wavelength of 6.5x10-7 m?

• After careful analysis, an electromagnetic wave is found to have a frequency of 7.8x106

Hz. What is the speed of the wave?

Vocabulary to Know• Emit – to give off (energy, light, heat, etc.)

• Electromagnetic radiation – energy that travels through space as a WAVE

• Wavelength – distance from peak to peak of a wave

• Frequency – # of waves that pass a point per second

• Amplitude – height of wave

• Electromagnetic wave relationship – c = ln

• Electromagnetic spectrum – all wavelengths of energy from radio to gamma (visible 400-700nm)

Energy of Waves

• We want to be able to understand the energy given off by these waves…

• Are they harmful to us? (too much energy)

• Can we use them for medicinal imaging?

• Technology?

• Transmission of information?

What is the relationship between l, n and Energy?!

What else is nu?

• Energy, E = hn

• h = Plank’s constant = 6.626 x 10-34 Js

Named for Max Plank – German physicist.

• Whats J? Joule = (kg x m)/s2 unit of energy

How much energy is possessed by a single photon of UV-A electromagnetic radiation with a frequency 9.231 x 1014 Hz?

Calculations

• We know c = ln and E= hn

• Rearrange both equations to solve for n

• We can relate wavelength, energy, and frequency with those relationships!

n = c/l n = E/h

c/l = E/hE = hc/l

Practice Problem

• A Zn salt emits orange light at a wavelength of 450 nm. What is the energy of this light?

1. Identify the appropriate equation.

E = hc/l

2. Rearrange to solve for unknown.

Check (E is unknown)

3. Solve and show unit cancellations.E = 6.626x10-34 Js x 3x108 m

s = 4.42 x 10-19 J 450 x 10-9 m

Warm - Up• n is the symbol for ______________

• l is the symbol for ______________

• The value of c is always ________

• The value of h is always ________

• The unit for wavelength is _____

• The unit for frequency is ______ or _______

• c = ln describes light as a ________

• E = h___/____ and E = h_____

What is the energy for a wave with a frequency of 3.24 x 1014 1/s ?

Warm Up

• What is the energy? (show equations)

• Light with a wavelength of 640 nm.

• Wave with a frequency of 3.24 x 1013 Hz.

Today’s AgendaQuestion of the Day: If light transfers energy like a wave, how does light also act like a particle?

Warm – Up

• Discuss light as a particle – quantum, photoelectric effect,

• Use mathematical relationships to demonstrate the photoelectric effect.

• Compare and contrast a continuous spectrum to an atomic emission spectrum

• Test and Lab Return – correction guidelines

Light as a Wave• Light transfers energy through a wave with a

specific frequency and wavelength.

c = nl, E = hn, E=hc/l

• HOWEVER, because science laughs at us for trying to figure out the universe, light doesn’t ALWAYS act like a wave…

Science

Light as a PARTICLE• Scientists noticed that some of

the time, light acts more like a

particle than a wave.

• Quantum concept: Matter can gain or lose energy in small specific amounts called quanta.

• Quantum: minimum amount

of energy that can be gained or

lost by an atom

Photoelectric Effect - Einstein

• Electrons (photoelectrons) are emitted from a metal’s surface when a certain energy light shines on the surface.

• Photon is a massless particle that carries a quantum of energy.

• If light was a wave,

photoelectric effect

would NOT be observed.

Photoelectric Effect in Practice

Energy = 2.4 x 10-19 J

Energy A

Energy B

Energy C

What is the value of Energy C?

Photoelectric Effect Problems• How to solve:

• Find the energies associated with each l or n.

• Is the energy in question over the threshold for the given quantity of energy?

A photon in the IR region of the electromagnetic spectrum has a wavelength of 4.52 x 10-5 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold frequency of 5.51 x 1014 Hz?

Using the Photoelectric Effect• A photon in the IR region of the

electromagnetic spectrum has a wavelength of 7.23 x 10-9 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold threshold frequency of sodium is 5.51 x 1014 Hz?

• E = hc/l E = (6.626x10-34Js x 3x108m/s) = 2.74 x 10-17 J

7.23x10-9 m

• E = hn E = 6.626x10-34Js x 5.51x 10141/s = 3.65 x 10-19 J

• Yes!

Remember that E = hn & E = hc/l

• Aluminum has a threshold wavelength of 295 nm. If a sample of aluminum foil is exposed to four types of radiation, which radiation source would eject a photon?

• Microwave? n = 3.82 x 1011 1/s

• Optical wave? n = 5.18 x 1014 1/s

• UV wave? n = 1.02 x 1015 1/s

• X ray wave? n = 4.31 x 1016 1/s

295 nmE = ? J

Atomic Emission Spectra

• Neon signs!

Neon gas absorbs energy and

becomes excited. Electrons return to their stable state by emitting that energy as light!

Prism can separate light – not continuous.

Atomic Emission Spectra – set of frequencies of waves given off by atoms of a specific element. Each element has it’s own emission spectrum (because of energy steps).

Atomic Emission Spectra

• Each element’s atomic emission spectrum is unique, and can be used to identify that element.

Question:

• What is the difference between a continuous spectrum and an atomic emission spectrum?

A) Atomic emission gives off light, continuous does not.

B) Continuous spectra contain all l’s light and atomic emission have selected l’s of light.

C) A continuous spectrum requires a prism, atomic emission spectrum does not.

Wave – Particle Duality

• Light can act as a PARTICLE (transfer energy in discrete packets called QUANTA or PHOTONS of energy).

When do we see this? Photoelectric Effect

• Light can act as a WAVE (transfer energy through an oscillating wave with a WAVELENGTH and FREQUENCY)

When do we see this? Crazy quantum mechanics

Practice Problem

c = ln, E = hn, E = hc/l

Calculate the energy needed to eject a single photon of radiation from a metal with a threshold wavelength of 4.74x10-8 m?

Would a photon of n = 6.78 x 1015 1/s eject an electron?

Question of the Day

If light transfers energy like a wave, how does light also act like a particle??

Answer:

Light acts as a particle because intensity of light does not affect the energy. The number of photons aimed at a surface does not change the # of photons emitted. Only the energy (wavelength, frequency) changes the photons ejected.