Topic 5b - Waves And The Particle Nature Of Light Flashcards
Explain why electron accelerates to positive terminal using force and then energy transfers
Give the equation for wavelength in terms of h and two other things and derive it
Lambda = h/m x v
E =m c ^2
E = h x f
Equalise them
What kind of behaviour do electrons exhibit when they pass through a double slit
Diffraction and interference - wave behaviour
In the double slit experiment what are the size of the slits compared to the size of electrons
Similar to wavelength of electron
Explain what electrons diffracting tells us about the properties of electrons
Give plank’s constant
6.63x10^-34 Js
What things is v = f lambda ok for? What equation should be used if v = f l can’t be used?
- things without mass
- lamdba = h/mv (use for electrons as they have mass or particles that exhibit wave behaviour)
Why do electrons need high energies to investigate the structure of protons?
- high energy electrons have a short de broglie wavelength
- the wavelength needs to be less than the proton size
What is the energy of a wave determined by?
Amplitude - only (not frequency)
Define intensity
Power per unit area
What is intensity dictated by for a wave?
Amplitude (as amplitude determines energy)
What is intensity of wave proportional to?
Amplitude ^2
What is a photon \
A DISCRETEpacket of electromagnetic ENEGRY
What is the photoelectric effect evidence for
That light can behave as a particle
Describe the energy transfers in the photoelectric effect
- ENEGRY from the photon is transferred to the electron
- it first uses this ENEGRY to escape the metal surface
Any leftover ENEGRY is converted to kinetic ENEGRY with which the electron moves away
What would be expected by the wave model about the time for the ENEGRY to be transferred from the light to the electron
A wave transfers energy over time (it would be non-instantaneous)
What is the equation for energy of a photon/electron linking h and f
E = h f
What is expected by the wave model about how long it takes for electrons to be released from the metal surface - what is instead observed?
- the energy would take time to be transferred from the light to the electron - the electrons are released instantaneously from the metal surface
How does the observation of electrons being released instantaneously from the metal surface lead to the particle model of light
One electron absorbs one photon, absorbing its energy as a packet
What is expected from the wave model and what is actually observed about the kinetic ENEGRY of the electrons
The kinetic energy of the electron should be dependent on the intensity of the light, as intensity is related to the ENEGRY of the light in the wave model
However the kinetic ENEGRY of the electrons is dependent on the frequency of incident light, not intensity
What does the observation that kinetic energy of photons is dependent on the frequency of incident light rather than its intensity show about our model of light?
It leads to the particle model of light as the ENEGRY of a photon equals hf
What is expected by the wave model and what is instead observed about the frequency of light and its relationship to whether there is emission of electrons in the photoelectric effect
- any frequency of light should release electrons as long as intensity is high enough (ENEGRY in the wave model is dependent on intensity)
- however in reality only light above a certain threshold value will cause electron emission
Why does the observation that only light above a certain threshold frequency value will cause emission of electrons lead to teh particle model of light
The minimum photon ENEGRY to release an electron from the metal surface is equal to the work function (phi) of the metal. This means there is a minimum threshold frequency f = phi/h required to release the electron
What would be expected by the wave model if intensity of light increased in the photoelectric effect and what actually happens?
- should increase ENEGRY of emitted electrons
- in reality it increases the number of electrons emitted per second
Why does the observation that increasing the intensity of light increases the number of electrons emitted per second
- intensity = number of photons x energy of one photon / (time x area)
- intensity = power/area = rate of ENEGRY transfer per second/area
I = Nhf/tA
So intensity is directly proportional to number of photons per second - as one photon is absorbed by one electron this is also proportional to the number of electrons released per second
What is the work function?
The minimum photon ENEGRY to release an electron from the metal surface is
What is the minimum threshold frequency
F = phi/h
What is the equation linking intensity, number of photons
I = Nhf/tA
Intensity = what (in terms of number of photons emitted)
Nhf/tA
Give an equation showing ENEGRY conservation when an incident photon hits a metal surface and an electron is emitted
ENEGRY of incident photon = work function + kinetic ENEGRY of emitted photon
hf = phi + Ekmax
Electrons experience electrostatic forces of attraction to different/the same extent when they are in the surface
Different
What is the work function
The minimum amount of photon ENEGRY for an electron to escape a metal surface
What is threshold frequency
Minimum photon frequency to allow photons to be released from the metal surface
What is an equation linking threshold frequency, work function and planks constant?
F nought = phi / h
A tube containing hydrogen gas is heated. Light is emitted and passed through a spectrometer. Only certain frequencies of light are present. Explain what happens to cause this.
- electrons exist in discrete ENEGRY levels
- electrons within atom excited to higher ENEGRY level when as gas is heated
- the electron then falls back down to lower ENEGRY level
Emitting a photon with energy E equal to the ENEGRY difference delta E between the two electrons levels
E - hf is the energy of the photon
The photon is emitted with a specific frequency f = delta E/h where delta E is the energy difference between the levels
There are only a limited number of ENEGRY differences between levels and so there is only a corresponding limited number of frequencies of photons emitted
An electron in level E3 of neon has 0.05eV more energy than an electron in level E2 of helium. Suggest the source of energy to make up this difference
- the kinetic energy of the atom
Describe how passing a current through a gas can cause photon emission
- moving electrons collide with atoms inside the gas
The kinetic ENEGRY of the moving electron is transferred to the ENEGRY of the electron, causing it to move to a higher ENEGRY level
What is a photon define
A discrete packet of electromagnetic radiation
Discrete line spectra are only found in low/high density substances
Low
Explain origin of absorption spectra at specific frequencies
- electrons exist in discreeter energy levels
Electron within atom excites to higher ENEGRY level when it absorbs a photon - E = hf is the ENEGRY of the photon
Photon absorbed must have a specific freqeuncy f = delta E/h wher delta E is there energy difference between the levels - there are only a limited numbe of ENEGRY differences between levels and only a corresponding limited number of freqeuncies/wavelengths of photons absorbed
- different elements have a different ENEGRY differences between levels so produce different absorption lines at specific frequencies
Each element has a different energy ___between shells
Difference
What are the two ways to create an emission spectrum?
Pass current through and heating gas
How would you measure the REISTSANCE of a thermistor - give a diagram of a circuit
What type of spectrum is a solar spectrum?
An absorption spectrum
