Quantum Physics

Question 1

What is the relationship btw the energy E of a photon and its frequency f

Answer 1

E = hf = hc/λ

Question 2

What is the equation for the power of radiation

Answer 2

P = Etotal/t = (N/t) x hf

Question 3

What is the intensity of radiation

Answer 3

I = P/A = (N/tA) x hf

Question 4

What is the photoelectric effect

Answer 4

It is the emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency is shone on it

Question 5

What are the experimental observations of the photoelectric experiment

Answer 5

1. electrons emitted only when frequency of light above threshold frequency
2. Emission of photoelectrons almost instantaneous
3. Max KE of photoelectron dependent on frequency of light but independent of intensity
4. Rate of electron ejection and photocurrent proportional to intensity of light

Question 6

What are the failures of the classical wave theory

Answer 6

1. 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.
2. 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
3. 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

Question 7

What is the work function of a metal Φ

Answer 7

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

Question 8

What is Einstein’s photoelectric equation

Answer 8

hf = Φ + KEmax
hf = Φ + 1/2 mvmax^2
hf = Φ + eVs

Question 9

What is the electronvolt eV

Answer 9

1 eV is the energy transferred when an electron travels through a potential difference of one volt

Question 10

What is de Broglie wavelength

Answer 10

λ = h/p = h/mv

Question 11

What is the emission line spectrum

Answer 11

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

Question 12

What is the absorption line spectrum

Answer 12

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.

Question 13

Electrons in an atom can only exist in __________

Answer 13

Fixed energy states

Question 14

Electrons at the lowest energy leve are said to be ________

Answer 14

in the ground state

Question 15

If the electron in the ground state absorbs energy, it can _______

Answer 15

be promoted to a higher energy level. This is known as excitation.

Question 16

How can the energy for excitation be provided

Answer 16

1. By collision with another electron
2. By photon absorption
3. By heating gaseous atoms

Question 17

What is the ionisation energy on an atom

Answer 17

It is the energy required to remove the electron completely from the atom

Question 18

How is the emission line spectrum obtained?

Answer 18

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

Question 19

How absorption line spectrum is obtained

Answer 19

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

Question 20

What is braking radiation?

Answer 20

It is the radiation emitted when a high energy charged particle is accelerated or decelerated.

Question 21

What is the minimum cut-off wavelength in x-ray spectrum

Answer 21

• 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

Question 22

Why is the x-ray spectrum continuous

Answer 22

• electron may lose only part of its initial KE during collision
• λ= hc/(KEinital-KEfinal)
• accounts for wavelengths longer than the minimum cutoff wavelength

Question 23

What is the characteristic x-ray spectru,

Answer 23

• 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

Question 24

What is the Heisenberg uncertainty principle

Answer 24

ΔxΔp >_ h