Quantum Physics Flashcards
What is the relationship btw the energy E of a photon and its frequency f
E = hf = hc/λ
What is the equation for the power of radiation
P = Etotal/t = (N/t) x hf
What is the intensity of radiation
I = P/A = (N/tA) x hf
What is the photoelectric effect
It is the emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency is shone on it
What are the experimental observations of the photoelectric experiment
- electrons emitted only when frequency of light above threshold frequency
- Emission of photoelectrons almost instantaneous
- Max KE of photoelectron dependent on frequency of light but independent of intensity
- Rate of electron ejection and photocurrent proportional to intensity of light
What are the failures of the classical wave theory
- Fails to explain the existence of threshold frequency
- E proportional to A^2. Classical wave theory predicts that if light intense enough, electrons will be released. There should be no threshold frequency.
- Fails to explain almost immediate emission of photoelectrons\
- electrons can absorb energy over a period of time before it gains enough energy. Should be some delay but don’t have
- Fails to explain the frequency dependence of the maximum kinetic energy
- Classical states higher intensity = greater ke. cannot explain why KEmax independent of intensity but dependent on frequency
What is the work function of a metal Φ
It is the minimum amount of energy required for an electron to escape from the surface of a metal
Φ = hf.
- f. is the threshold frequency
What is Einstein’s photoelectric equation
hf = Φ + KEmax hf = Φ + 1/2 mvmax^2 hf = Φ + eVs
What is the electronvolt eV
1 eV is the energy transferred when an electron travels through a potential difference of one volt
What is de Broglie wavelength
λ = h/p = h/mv
What is the emission line spectrum
It is the spectrum obtained after light emitted from a gas containing isolated atom at low pressure and high voltage in a discharge tube is passed through a diffraction grating
What is the absorption line spectrum
It is the spectrum obtained after white light that has been passed through a tube filled with a low-pressure cool gas has been examined through a diffraction grating.
Electrons in an atom can only exist in __________
Fixed energy states
Electrons at the lowest energy leve are said to be ________
in the ground state
If the electron in the ground state absorbs energy, it can _______
be promoted to a higher energy level. This is known as excitation.
How can the energy for excitation be provided
- By collision with another electron
- By photon absorption
- By heating gaseous atoms
What is the ionisation energy on an atom
It is the energy required to remove the electron completely from the atom
How is the emission line spectrum obtained?
Electrons in excited state jump back to ground state. It loses energy by emitting photons whose energy is the difference in energy levels
Energy of photon = hf = ΔΕ
Electron may not jump back directly rather make series of jumps. As a result, photons of different frequencies emitted giving rise to series of emission spectral lines
How absorption line spectrum is obtained
Photons with right amount of energy to excite electrons in white light absorbed. This photon energy must match the difference btw electron initial energy and higher level. Excited atoms eventually return to lower energy level re-emitting the photons. However, emissions occur in all directions, so only part of the re-emitted light is in same direction of beam. Spectrum of transmitted beam will be coloured spectrum with missing lines
What is braking radiation?
It is the radiation emitted when a high energy charged particle is accelerated or decelerated.
What is the minimum cut-off wavelength in x-ray spectrum
- It depends only on V across tube.
- KE = eV
- hfmax = ev
- λmin = hc/eV
- The cutoff wavelength is independent of the target material but is inversely proportional to the accelerating voltage V
Why is the x-ray spectrum continuous
- electron may lose only part of its initial KE during collision
- λ= hc/(KEinital-KEfinal)
- accounts for wavelengths longer than the minimum cutoff wavelength
What is the characteristic x-ray spectru,
- intense and sharp lines on continuous spectrum called characteristic or discrete X-ray spectrum
- Correspond to the emission line spectrum of the target material
- incoming electrons knock electrons out of inner shell
- Outer shell electron falls into inner shell to fill vacancy
- a single x-ray photon emitted with energy = ΔΕ
- independent of the accelerating voltage
What is the Heisenberg uncertainty principle
ΔxΔp >_ h
