Module 4.5

Question 1

Photons

Answer 1

Discrete energy quanta

Question 2

electrovolt

Answer 2

1 electrovolt is defined as the energy
transferred when an electron travels through a potential difference of 1 volt

Question 3

How to use Led to estimate the value of planks constant

Answer 3

We can use LEDs to determine the Planck constant experimentally. LEDs only emit light when
the potential difference across them exceeds the threshold p.d. required. A potential divider is
set up to vary the voltage through the LED. A small black tube is placed over the LED, to make
it obvious when the LED has lit up. By varying the p.d. across the LED, we can determine the
threshold p.d., V, required to turn it on. As the LED produces light of a specific colour, we know
the wavelength of the light. Each photon from the LED is emitted when a single electron loses
energy. By equating the energy of an individual electron in the LED with an individual photon
produced, we can use the equation eV = hc/λ to determine the Planck constant.
To improve the accuracy of this estimate, the experiment can be repeated with a variety of
different coloured LEDs, which each emit different wavelengths of light. The values of
wavelength and threshold p.d. for each can be recorded, and a graph drawn of V against 1/λ. The
gradient of this graph will be equal to hc/e. As the speed of light and the electron charge are
known constants, we can calulcate the value of h from this.

Question 4

Photoelectric effect

Answer 4

When electromagnetic radiation is shone on to a metal, electrons are released from the surface
of the metal.

Question 5

What is the gold leaf electroscope

Answer 5

a zinc plate on top
of a negatively charged stem, with a negatively charged piece of gold leaf attached to the stem.
Initially, the gold lead and the stem have the same charge, so they repel each other. If UV light is
shone on to the zinc plate, free electrons will be released from the surface of the plate, and the
negative charge will slowly be lost, so the gold leaf will gradually fall back to the stem.

Question 6

What is de broglie equation used for

Answer 6

Particles

Question 7

What is the mass of an electron

Answer 7

The electron rest mass

Question 8

Evidence for the wave particle duality

Answer 8

Electrons are classified as particles, with mass
and charge. They can be accelerated and deflected by magnetic and electric fields, which is
behaviour associated with particles. However, they can also be made to diffract. When a beam of
electrons is fired at a thin piece of polycrystalline graphite (a material containing carbon atoms
spread over many layers) the electrons are diffracted by the gaps between atoms, and produce
a diffraction pattern when they hit a screen. This diffraction is a property of waves.

Question 9

How many photons does each photoelectric absorb prior to emission

Answer 9

1 to 1.
If it does not contain enough energy the electron will re-emit the energy rather than being released

Question 10

How does the photon model of light explain the threshold frequency seen in the photoelectric effect

Answer 10

Each electron absorbs a single photon. This single photon must have enough energy for the electron to be released. The electron cannot build up energy as it could if light was a wave.

Question 11

What experimental evidence appears to show particles behaving as waves

Answer 11

• Electron diffraction
• Electrons will diffract if passed through spaces between atoms in graphite (lite tiny diffraction gratings)
• This would not happen if electrons were behaving as photons
• The spacing between the atoms in the graphite is equal to the wavelength of an electron