4.5 Quantum physics

State and explain which part of the visible spectrum has photons with the highest energy

Violet has the highest frequency, and according to E = hf, the greater the frequency the greater the energy.

How do you calculate number of protons per second?

power / energy

What is the name for a quantum of light energy?

A photon

Describe the photon model of light

It treats electromagnetic radiation as ‘packets’ of energy (E = hf)

Describe the photoelectric effect

The emission of electrons from the surface of a metal when elecrtomagnetic radiation above a certain frequency is incident on it

What is the name given to electrons emitted due to the photoelectric effect?

Photoelectrons

Define the work function of a metal

The minimum energy an electron needs to break bonds and escape from the metal surface

Define the threshold frequency of a metal

The minimum frequency of photos needed for the photoelectric emission of electrons from the metal

State the photoelectric effect equation

hf = φ + KEₘₐₓ

If a graph has KEₘₐₓ of photoelectrons plotted on the y-axis and frequency of incident light for a particular metal, what will the y-intercept represent?

-φ

If a graph has KEₘₐₓ of photoelectrons plotted on the y-axis and frequency of incident light for a particular metal, what will the gradient represent?

h

How does the photon model explain why no electrons are emitted if the frequency of the incident light is below the threshold frequency?

An electron in the metal can only absorb energy from a single photon, gaining energy (hf). The electron is only emitted if this energy is greater than the metal’s work function

How does the photon model explain why KEₘₐₓ of the emitted electrons does not change with intensity, but does increase with frequency?

Increasing the intensity increases the number of photons arriving per second, but each photon has the same energy and can only interact with one electron, however if the frequency is increased beyond the threshold frequency, excess energy gained by the photoelectrons goes to the electron’s kinetic energy store, increasing KEₘₐₓ

What is the relationship between intensity of light and rate of photoelectron emission, as long as the frequency of the light is above the threshold frequency?

Rate of photoelectron emission ∝ intensity BECAUSE intensity ∝ number of photons arriving per second

How does the photon model explain why photoelectrons are emitted immediatly once the appropriate frequency light is incident on the metal?

One electron interacts with one photon (one-to-one interaction), so an electron either gains enough energy to be emitted or it doesn’t

Define an electron volt

The energy gained by an electron accelerated through a potential difference of 1 volt (1 ev = 1.6 × 10 ⁻¹⁹)

Define the de Broglie wavelength of a particle

λ = h / p

What is λ in the equation λ = h / p?

Wavelength

What is h in the equation λ = h / p?

Plank’s constant

What is p in the equation λ = h / p?

The momentum of the particle

Name two wave behaviours that electrons have been observed to exhibit

Diffraction and interference

How can electron diffraction be observed?

By firing a beam of electrons at a thin slice of polycrystalline graphite in a vacuum

What will happen to the de Broglie wavelength of an electron fired from an electrongun if the accelerating p.d is doubled?

Increase in V causes an increase in p², decreasing λ by a factor of √2

What is the photoelectric equation?

hf = φ + KEₘₐₓ

What is the y intercept when KEₘₐₓ is plotted against frequency?

-φ

What is the gradient when KEₘₐₓ is plotted against frequency?

h

How can the photoelectric effect be shown?

Through a gold leaf electroscope, a torch and a UV light.

The zinc plate is initially discharged and the gold leaf hangs loose.

Shining UV light causes photoelectrons to be emitted because the frequency of UV light is above the threshold frequency for zinc, but shining torch light doesn’t cause photoelectrons to be emitted because the frequency of the light is below the threshold frequency for zinc.

This means that the gold leaf is positively charged so it repelles away from the metal plate for UV light.

When does energy increase for the photoelectric effect?

When frequency is increased (NOT INTENSITY)

Define electronvolt

An eV is the energy acquired by an electron accelerated through a p.d. of 1V