questions Flashcards
Define the intensity of light in the wave model of light
- Intensity is power/area
- Intensity is proportional to the amplitude of the wave squared.
Define a photon
A discrete packet of electromagnetic energy
What is meant by the wave particle duality of light
- Light exhibits wave behaviour
- diffraction
- polarisation
- interference
- superposition - Light exhibits wave behaviour
- absorption line spectra
- photoelectric effect
- Light exhibits both of these, therefore it has wave particle duality
Define the work function
minimum amount of energy for an electron to be released from the surface of a metal.
Define threshold frequency
minimum frequency of the photon required for an electron to be released from the surface of a metal. BY ABSORBING A PHOTON
Why is the kinetic energy of an electron the maximum kinetic energy
- The work function is the minimum amount of energy required for the electron to be released.
- Due to the equation hf = work function + ke max, the kinetic energy is the maximum possible kinetic energy.
- Most electrons will have less kinetic energy than this because they will lose more energy to leave the surface of the metal as they require more energy than the minimum energy to overcome the electrostatic forces of attraction
How does the photoelectric effect indicate that light is a particle
- In the photoelectric effect, we can see that only EM above a certain frequency will release electrons, doesn’t depend on intensity.
- E = hf, energy of the photon is directly proportional to the frequency. This means that the kinetic energy of the electron only depends on the frequency of the EM radiation.
- In the wave model, energy is directly proportional to intensity, so in the wave model, if intensity was increased, for any frequency of light, electrons would be released. This is not observed.
- In the wave model, energy is directly proportional to intensity, so if intensity of light is increased, the kinetic energy of electrons would increase, but this is not the case.
- electrons are always released from the surface of the metal if the energy of the photon is larger than the work function.
- electrons are instantaneously released, indicating that the electrons absorb the energy instantaneously, one photon by one electron.
- In the wave model, energy would be absorbed over time and the electrons would not be released instantly.
- In the particle model, as intensity increases, more photons are absorbed per second, so more electrons are released per second
What is the intensity in the particle model of light
- Intensity = power/area
- Intensity = energy/area x time
- Intensity = h x frequency x number/area x time
- I = Nhf/tA
- Intensity is proportional to the number of photons per second
State the effect of the number of photoelectrons emitted per second when the intensity of the light increases?
The number of photoelectrons emitted per second increases in proportion to the intensity increasing.
Define stopping potential
- The minimum p.d. at which electrons do not have enough energy to cross the gap between the two plates.
- Vs = KEmax/e
What is the de broglie wavelength?
the wavelength of a particle with momentum
λ = h/p
What is an energy level
The discrete allowed energy of an electron within an atom
Why are energy levels of electrons in atoms negative?
- A free electron has zero energy
- in order for the electron to move up energy levels to be released, it must gain energy.
What is refraction
the change in direction of a wavefront as it changes speed due to the change in density of the medium it is travelling into.
What is the refractive index
n = c/v
ratio of the speed of light in a vacuum compared to the speed of light in the medium
