Quantum Physics

Question 1

What is a PHOTON?

Answer 1

Discrete wave packets of energy.

Question 2

What is ELECTRON DIFFRACTION?

Answer 2

Path of electron becomes spread out when passing through a spacing comparable to its de broglie wavelength.

Question 3

What proves the WAVE THEORY of LIGHT?

Answer 3

• When light is shined through a diffraction grating similar to its wavelength, a diffraction pattern occurs with maxima and minima.
• Diffraction only happens in waves.

Question 4

What proves the PARTICLE THEORY of LIGHT?

Answer 4

• Photoelectrons are only emitted when the frequency of light is high enough to have a high enough energy to overcome work function. Wave theory suggests that light can cause a build-up of energy and cause emission even if the energy of a photon is not large enough to overcome work function in 1 to 1 interaction.
• Photons are absorbed by electrons instantaneously, transferring all their energy.
• Increase in intensity is independent of energy of photon.

Question 5

Why is CAESIUM used instead of ZINC in a PHOTOELECTRIC EXPERIMENT?

Answer 5

Zinc requires UV which is harmful. Caesium only needs visible light.

Question 6

What are the MAIN OBSERVATIONS of the PHOTOELECTRIC EFFECT?

Answer 6

• UV light of high intensity causes gold leaf to fall quickly.
• UV light of low intensity causes gold leaf to fall slower than high intensity - less photons incident on plate per second.
• Light source, no matter the intensity, does not cause the gold leaf to fall.
• Metal plate discharges overtime regardless of light incident, so leaf will fall anyway.
• Using a positively charged plate causes no change in gold leaf’s position - Any electrons will be attracted back by positive charge on the surface of the metal plate, even if they are emitted.
• Increasing distance increases intensity, I is proportional to D.

Question 7

How does an ELECTRON GUN WORK?

Answer 7

• Electrons are accelerated to high speeds due to high supply voltage. KEmax = eV = 1/2*mv^2
• Cathode is placed at negative terminal so the electrons will be repelled or they will be attracted back to the cathode.
• Accelerated electrons pass through graphite with spacing comparable to their de broglie wavelength. wavelength = h/mv
• Electrons are diffracted and emerge to form fluorescent rings with a minima and maxima.
• Wavelength is inversely proportional to v, so if velocity increases wavelength decreases.
• Wavelength = ax/D , wavelength is proportional to x, fringe spacing.

Question 8

What is DE BROGLIE WAVELENGTH?

Answer 8

All particles have the ability to show wave-like particles if they are moving.

Question 9

How does the ELECTRON GUN prove ELECTRONS are a PARTICLE?

Answer 9

Electrons have to have a mass in order to be accelerated by the potential difference.

Question 10

What is an ENERGY LEVEL of an ELECTRON?

Answer 10

• Electrons in an atom have discrete energies.
• When electrons down an energy level, a photon is released.
• When electrons move up an energy level, a photon is absorbed.
• Energy of photon is equal to the difference in energy levels.

Question 11

What is the gradient of a graph of maximum photoelectron kinetic energy against frequency?

Answer 11

h, planck’s constant.