Electromagnetic Radiation and Quantum Phenomena

Question 0

What three conclusions were drawn from detailed experimentation?

Answer 0

Conclusion 1: If the light’s frequency is below a certain value (the threshold frequency) no photoelectrons are emitted
Conclusion 2: The maximum kinetic energy of photoelectrons are dependent only on the light’s frequency and not its intensity
Conclusion 3: The number of photoelectrons per second is proportional to the light’s intensity

Question 1

Explain what the photoelectric effect is

Answer 1

When a light of a high enough frequency is shone onto a metal surface, electrons will be emitted

Question 2

What is meant by the work function energy of a metal?

Answer 2

The energy required to break the bonds holding the electrons in place

Question 3

How is the maximum kinetic energy of a photoelectron calculated?

Answer 3

hf = phi + 0.5(mv^2)

Question 4

Describe line absorption and line emission spectra. How are these two types of spectra produced?

Answer 4

Absorption:
A full spectrum with black lines of absorped frequencies
Caused by the cool gas’ electrons moving up an energy level
Emissions:
A black strip with coloured lines of the frequencies being emitted
Caused by excited gas’ electrons moving down an energy level and/or to ground state

Question 5

Use the size of the energy level transitions involved to explain how the coating on a fluorescent tube converts UV into visible light

Answer 5

The phosphorus coating absorbs the photons emitted by the mercury vapour, this moves the phosphorus’ electrons up to a high energy level which then cascade back down emitting visible frequencies

Question 6

Which observations show light to light to have a ‘wave-like’ character?

Answer 6

Interference and diffraction:
Light produces interference and diffraction patterns - alternating bands of dark and light
This can only be explained by waves interfering constructively or destructively

Question 7

Which observations show light to have a ‘particle’ character?

Answer 7

The photoelectric effect:
Einstein explained the results of the photoelectricity experiments by thinking of the beam of light as a series of particle-like photos

Question 8

What happens to the de Broglie wavelength of a particle if its velocity increases?

Answer 8

The wavelength grows shorter

Wavelength = h/(mv)

Question 9

Which observations show electrons have a ‘wave-like’ character?

Answer 9

Electron diffraction:
Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal