Waves And The Particle Nature Of Light, From Exam Questions Flashcards
Measuring the distance between the two 2nd order maxima would produce a smaller percentage uncertainty in the value of wavelength, explain why
The resolution would be the same but the distance measured is greater
Or
The uncertainty would be the same but is divided by a greater length
Describe how the distance of the (moon from earth) can be determined using a pulse echo technique
Pulses of light are emitted from (laser/ earth or pulses of light sent to the moon/ mirror)
Light reflects (from the mirrors)
Measure the time taken for the reflected light to return
Calculate distance= speed x time
Use speed of (light)
Measured time must be divided by 2 or calculated distance must be divided by 2
Electrons in an atom can only exist at discrete energy levels
Explain how this standing wave model can account for this
(One model of atomic electrons suggests that the wave associated with an atomic electron forms a standing wave that fits exactly into the diameter of the atom)
A discrete number of wavelengths fit into the diameter of the atom
E=hc/wavelength to link wavelength to discrete energy levels
The light incident on the camera lens is unpolarised. The photographer places a polarising filter in front of the camera lens
Explain how the polarising filter affects the intensity of light incident on the camera lens
Unpolarised light oscillates in many planes
The light perpendicular to the plane of polarisation of the filter is absorbed
Or
Only light parallel to the plane of polarisation of the filter is transmitted
So polarised light oscillates in only one plane
So light intensity is reduced
is observed that, for a given metal,
• no electrons are emitted if the frequency of the incident radiation is below a certain threshold frequency.
• electrons are emitted instantaneously if the frequency of the incident radiation is above a certain threshold frequency.
• the kinetic energy of the emitted electrons depends only on the frequency of the incident radiation.
Discuss how the photon model of electromagnetic radiation can explain these observations and why the wave model of electromagnetic radiation cannot.
Particle model
• One photon interacts with one electron
• And each photon has energy proportional to the frequency, Or reference to E=hf
• The electron is emitted (instantly)
only if the energy of the photon is greater than the work function (of the metal)
Or The electron is emitted (instantly)
only if the energy of the photon is greater than the energy needed for an electron to break free (from metal surface
• Any photon energy over and above the work function is gained by the electron as kinetic energy
Wave model
• It would be expected that the energy of the electron would build up and eventually be emitted.
• The (kinetic) energy of the (emitted)
electrons would depend on the intensity of the wave (and not the frequency)
When the light from a star is dispersed to form a spectrum, dark lines are seen at a number of frequencies. This is known as an absorption spectrum and is caused by the presence of certain elements in the star.
Explain how the absorption spectrum is created
if photon energy equal to an energy level difference in the elements present (1)
• then the photon can be absorbed by an electron and the electron is excited/moves to higher level (1)
• so the absorption spectrum is created because the frequencies of the absorbed photons are missing from the continuous spectrum produced by the star (1)
Describe the particle model of ultraviolet radiation that explains how it can “knock electrons out of atoms”.
(UV radiation consists of) photons
• One photon interacts with one electron
Or energy of photon depends on frequency
• Electrons released if energy (of photon) greater than work function
Or frequency is greater than threshold frequency
