Chapter 19: Quantum Physics

Question 1

Photoelectric Effect

Answer 1

The emission of photoelectrons from a metal surface when electromagnetic radiation of a sufficiently high frequency is incident on it

It demonstrates particulate nature of electromagnetic radiation

Question 2

Photon

Answer 2

a quantum of energy of electromagnetic radiation

where the energy of a single photon is directly proportional to its frequency, E= hf, where h is the Planck Constant

Question 3

Experimental Observations of photoelectric emission

Answer 3

1. Photoelectrons are emitted only when frequency of EM radiation exceeds a certain minimum, threshold frequency (emission independent of intensity)
2. Photoelectrons are ejected almost instantaneously
3. Stopping potential (Max KE of photoelectrons ejected) is independent of intensity of incident radiation
4. current proportional to intensity of incident light

Question 4

Work function energy

Answer 4

The minimum energy required to remove an electron from the surface of a metal

electrons are held in a metal lattice by electrostatic force, hence minimum amt of energy is required to remove electrons

Question 5

Threshold freq.

Answer 5

Minimum frequency of incident photons for photo electrons to be emitted from a metal surface

Energy of a photon is proportional to the frequency of the radiation

Question 6

Stopping potential

Answer 6

Minimum potential difference between the metal and the collector that will prevent an ejected photoelectron from reaching the collector

same Vs, same freq, dependent on freq

Question 7

Electron Energy levels in isolated atoms

Answer 7

The quantized energy corresponding to an allowed state (or orbital) of the electron within the atom

Question 8

Excitation by particle collisions vs Photo Absorption/Emission

Answer 8

1. Excitation through transfer of energy from a colliding particle to the atom: colliding particle must have kinetic energy greater or equal to energy diff between 2 discrete energy levels
2. Excitation through Photon Absorption/Emission : Photon must have exact amt of energy that corresponds to energy diff btw 2 discrete energy levels

Question 9

Emission spectrum

Answer 9

A series of separate, differently coloured lines on a dark background. These lines are specific photon energies emitted by atoms when electrons in the excited atoms return to lower energy levels

Fast electrons cause excitation followed by deexcitation causing release of characteristic photons

Question 10

Absorption spectrum

Answer 10

A series of separate, dark lines on a continuous spectrum (coloured bg)
These lines are specific photon energies absorbed by atoms of cold gas causing electrons to move to higher energy levels
Cool gas absorbs photons which cause excitation to higher states. The light emerging from cool gas has characteristic photons missing.
Electrons within the cool cloud of gas, in their ground state, absorbs the photon energy from the incident white light,
where photon energy corresponds to the energy difference between two discrete energy levels, causing electrons to become excited and transit to a higher energy level.
The excited electrons, being unstable, de-excite to a lower energy level and emit photons, but in all directions.
Hence, those frequencies of the continuous spectrum which were absorbed and re-emitted in all directions appear as dark lines.

Question 11

Xrays

Answer 11

EM waves with wavelenghts 0.001nm to 10nm

Question 12

Continuous spectrum

Answer 12

When fast bombarding electrons collides with metal atoms in the target, they can lose any fraction or all their KE in a single interaction. With varying KE lost, Xray photons of various wavelengths are emitted, resulting in continuous spectrum

Question 13

Minimum Wavelength

Answer 13

When all of the KE of a very fast bombarding electron is lost in a single collision to produce one photon, the most energetic photon with the highest freq. & lowest wavelength is produced

Question 14

Characteristic Wavelengths

Answer 14

Bombarding electrons cause the target atoms to be ionized by removing the inner electrons from target atoms, so the outer electrons will decay to the empty level and emit x-ray photons characteristic of the target atoms

Question 15

Heisenberg Uncertainty Principle

Answer 15

it is impossible to measure the exact position and momentum at the same time

Question 16

How does electron show wave nature? ( act like particles )

Answer 16

Electrons are diffracted, forming a diffraction pattern of concentric circles on fluorescent screen.

Question 17

De broglie wavelength

Answer 17

A particle/electron has a wavelength which is dependent on its momentum.

Question 18

Why electron diffraction will b observed on the screen? (concentric circles)

Answer 18

separation of ___ is in the same order of magnitude as the de broglie wavelength of the electron, hence effect of diffraction is significant.

Electrons exhibit wave properties and spread/diffract when they pass through the slits by carbon firm. The electron waves interfere constructively to form bright concentric circles on the screen

Question 19

Line spectra

Answer 19

The existence of line spectra provides convincing evidence for the existence of discrete electron energy levels in isolated atoms.
As each line on the line spectrum corresponds to a single wavelength, and hence a single frequency of light, this implies that the energies of the photons emitted are discrete. This is only possible if the energy levels of electrons within the atom are discrete as well

Question 20

Explain an evidence provided by the photoelectric effect experiment for the failure of the wave theory of light.

Answer 20

1. Wave theory predicts that the photoelectric effect should occur for any frequency of the monochromatic incident light. If frequency of light is too low, its intensity can be increased to cause photoelectron emission. However, no electrons were emitted at all unless the frequency of the monochromatic incident light was greater than a minimum value (i.e. threshold frequency).
2. Wave theory predicts that at very low intensities, there will be a time delay in Emission of Photoelectrons as the electrons would need time to accumulate sufficient energy in order to escape from the metal surface.
3. Wave theory predicts that if the incident light intensity is increased, a greater amount of energy will be incident on the surface and hence electrons will escape with more energy from the surface of the metal. However, maximum kinetic energy of the photoelectrons and hence stopping potential is independent of the incident light intensity.

Question 21

Threshold wavelength

Answer 21

the longest wavelength of electromagnetic radiation that can eject electron from a particular metal surface

Question 22

Einstein’s Particle theory

Answer 22

1. Photon energy is proportional to the frequency of radiation
2. One photon interacts with one electron with entire energy of photon transferred to the electron during the interaction

Question 23

energy levels shown as negative

Answer 23

implying electron is bounded by attractive force and that energy must be provided in order for electron to move to higher state

Question 24

describe how you wld produce an emission line spectrum of hydrogen in the laboratory

Answer 24

-Connect a voltage across the hydrogen discharge tube
-Clamp a diffraction grating to a retort stand and place it infront of the discharge tube
-Switch off background lights
-Look through the diffraction grating and slowly move to one side
-Distinct coloured lines will be seen

Question 25

explain how emission line spectrum demonstrate existence of discrete energy levels in isolated atoms

Answer 25

1. Each coloured line is a single frequency emitted by the atom
2. Energy of a photon is proportional to frequency
3. So atoms emit discrete energies when electrons in the atom undergo energy change, implying there must exist discrete energy levels within atoms

Question 26

explain how absorption line spectrum demonstrate existence of discrete energy levels in isolated atoms

Answer 26

1. Each coloured line is a single frequency absorbed by the atom
2. Energy of a photon is proportional to frequency
3. this implied atom absorb discrete energies when electrons in the atom undergo energy change, implying there must exist discrete energy levels within atoms