Particle Nature of Light

What is the de Broglie equation?

Lamda = h/p, h= planks constant, p=momentum, lamda = wavelength

Explain why the photoelectric effect demonstrates light behaving as a particle, not as a wave

• There is a minimum frequency for electron release

• electrons are released instantaneously

• changing intensity does not affect KE or the release of an electron

• The energy of a photon increases as frequency increases

• Photon energy has to be greater than the work function

• Each photon only interacts with one electron

• in wave model, kinetic energy depends on the intensity not the frequency

What is intensity

• power per unit area

• the distance between lamp and LDR is inversely proportional to intensity

• Also called the radiation flux density when talking about radiation from the sun falling on earth

• If we are at a distance r from point of radiation. Intensity = power/4pi*r²

How to demonstrate the photoelectric effect?

• a freshly cleaned zinc plate is given a negative charge by e.g. using a polythene rod

• In visible light, zinc does not discharge but when shine using a if light, zinc begins to discharge immediately.

• This is detected using a nano-colombmeter

• If zinc is given a positive charge nothing would be observed as any photoelectric transmitted would be attracted back again by the positive charge

• Energy from the light is given to the electrons in zinc, some electrons near the surface of zinc gain enough energy to escape from attraction of positively charged in nucleus. These electrons are called photo electrons

What is the photoelectric effect?

What is a photon?

• light causing the emission of electron.

• a discrete packet of energy

Why can we use uv light not visible light?

• electron would only be emitted if a single quantum of light has]d enough energy for the electron to escape. The frequency of visible light(E=hf) is not enough for a photon to provide the necessary energy for electron to escape. For uv light, the quanta have sufficient energy to release an electron

What is work function

• the minimum energy needed to remove an electron from the surface of a metal.

• The remaining energy is the kinetic energy.

• Hence total energy = work function + ke

What is the threshold frequency

• the frequency that is just large enough to liberate electrons f0.

How do we get an emission spectrum

• electron is given energy

• They get excited and jump up,from ground state

• Electron drops down from high level of energy level and emit energy as photon.

• This energy = hf

• Electron have discrete energy levels

• Hf = E2 - E1 (frequency is determined by the difference in energy level)

• there are limited number of possible energy levels, so only certain frequencies are emitted

What is an absorption spectrum?

• photon energy equals to an energy level difference between two quantum states in an element

• photon then absorbed by electrons and excites the electron

• frequencies of the absorbed photons are missing from the continuous spectrum produced by stars

Why are photoelectrons emitted with a range of kinetic energies

• electrons are at different depths in the metal and need different amount of energy to be liberated. The excess energy from a photon once an electron has been liberated is the ke of electron

Why is the observation of a threshold frequency not consistent with the wave theory of light

• in wave theory, energy builds up, photoelectron would be emitted any frequency

• particle model - photon transfers energy to electron - specific frequency

Compare and contrast the photoelectric effect with the effect of radiation incident on an LDR

• both are evidence for particle nature of light

• both increases intensity increases the number of electrons

• in both electrons gain energy from photons

• in both photons need a minimum amount of energy, so light must be above a certain frequency

• and in both electrons are liberated from the surface

• but in LDR, electrons stay in LDR, whereas electrons are released from metal surface

• photoelectric effect occurs in metals, but LDR is a semiconductor

Explain why different elements produce spectra containing different specific wavelengths of light

• electrons jump to higher energy state

• when they drop down they emit photons with the same energy as the difference in energy pf energy levels

• energy level are discrete

• there are only limited number of energy differences

• different elements have different energy level, so they produce different wavelengths

why temperature increases in a resistor when voltage increases

electrons collide with lattice ions

transfer of energy to ions so they vibrate with larger amplitude and temperature increases

photocell

• vacuum

• uses photoelectric effect

• light removes electrons from metal surface