Quantum phenomena

Question 1

What is photon energy equal to?

Answer 1

E = HF, or E = HC / wavelength

Question 2

What happens when light is incident on a metal surface?

Answer 2

An electron absorbs a single photon from the incident light, equal to hf.

Question 3

What is required for photoelectric effect to occur? What is the threshold frequency and what does it depend on?

Answer 3

Incident light frequency has to be greater than the threshold frequency. This is minimum frequency at which the photon has energy equal to the work function of the metal. This depends on type of metal being used, not the light source.

Question 4

What is the threshold frequency equation?

Answer 4

fmin = work function / planks constant. Equate e = hf to work function to work it out.

Question 5

When can an electron leave the surface of a metal?

Answer 5

If the energy it gains from a single photon exceeds the work function.

Question 6

Define work function

Answer 6

The minimum energy needed by an electron to escape from a metal surface.

Question 7

What happens to excess energy gained by the photoelectron? What is the equation used to find this value?

Answer 7

It becomes the photon’s kinetic energy. Ekmax = hf - work function. Equate e = hf to work function plus KE.

Question 8

What is proportional to the intensity of the light?

Answer 8

The number of electrons emitted per second.

Question 9

What is proportional to the frequency of the light?

Answer 9

When the frequency of the light increases, the maximum K.E of the photo-electrons increases.

Question 10

Why is it a max kinetic energy?

Answer 10

Energy gains energy equal to hf. Some electrons deeper in metal need more energy to leave the metal, and therefore will have less kinetic energy when out of the metal. Electrons closer to surface require less energy to get out, and therefore have more kinetic energy in the end.

Question 11

What is stopping potential?

Answer 11

The minimum potential required to stop photoelectric emission. Equate E = 1/2mv^2 to QV.

Question 12

How do electrons get attracted back to the plate?

Answer 12

By giving the plate a sufficient positive charge.

Question 13

What occurs at the stopping potential?

Answer 13

The max kinetic energy of the emitted electron s reduced to zero.

Question 14

What is ionisation?

Answer 14

The process of creating ions

Question 15

What is excitation?

Answer 15

Gas atoms absorbing energy from colliding electrons or by absorbing photons without being ionised.

Question 16

What are excitation energies?

Answer 16

The energy values at which an atom absorbs energy.

Question 17

What occurs during excitation?

Answer 17

A colliding electron makes an electron inside the atom move from an inner shell to an outer shell.

Question 18

Why is energy needed for the process of excitation?

Answer 18

The atomic electron moves away from the nucleus of the atom

Question 19

Why is excitation energy always less than the ionisation energy?

Answer 19

Because atomic electron isn’t completely removed from the atom when excitation occurs.

Question 20

What is the ground state?

Answer 20

The lowest energy state of an atom.

Question 21

What is the excited state?

Answer 21

When an atom in its ground state absorbs energy, one electron moves to a shell of higher energy, so atom is now in its excited state.

Question 22

Why do atoms de-excite?

Answer 22

Atoms don’t retain the absorbed energy permanently. The electronic configuration is unstable in an excited atom, as there is a vacancy in the shell it leaves from.

Question 23

What happens during de-excitation?

Answer 23

An electron from an outer shell transfers to a lower energy level and the electron emits a photon

Question 24

What is the energy of the photon equal to when released?

Answer 24

The energy lost by the electron and therefore by the atom.

Question 25

When can an atom de-excite via several energy levels?

Answer 25

If there are intermediate energy levels present.

Question 26

What is the energy of the emitted photon equal to?

Answer 26

HF = E1 - E2, when an electron moves from energy level E1 to a lower energy level E2

Question 27

When can excitation by photons occur?

Answer 27

Electron in an atom can absorb a photon and move to an outer shell where vacancy exists, only if the energy of the photon is exactly equal to the gain in the electron’s energy.

Question 28

What is fluorescent tube?

Answer 28

A glass tube with fluorescent coating on the inner surface. The tube contains mercury vapour at low pressure. When the tube is on it emits visible light.

Question 29

Explain how the fluorescent tube emits visible light?

Answer 29

Mercury atoms collide with each other and with electrons, so ionisation and excitation occurs. Mercury atoms emit UV photons, and visible photons when they de-excite. The UV photons are absorbed by atoms of the fluorescent coating, causing excitation of atoms. The coating atoms then de-excite in steps and emit visible photons.

Question 30

How is the wave like nature of light observed?

Answer 30

During the diffraction of light. Light passes through a narrow slit and spreads out.

Question 31

How is the particle like nature of light observed?

Answer 31

During the photoelectric effect.

Question 32

What is evidence for the particle like nature of light?

Answer 32

Electrons are deflected in an electric field.

Question 33

What is the de broglie wavelength?

Answer 33

wavelength = planck’s constant / momentum (MV)

Question 34

What is evidence for De Broglie’s theory that matter particles also have a wave - like nature?

Answer 34

It’s demonstrated by a beam of electrons being diffracted. A beam of electrons is aimed at a sheet of metal foil. the rows of atoms on metal foil cause the electrons in the beam to be diffracted. Electrons then form a pattern of rings on a fluorescent screen, and diffraction pattern is seen. Constructive interference occurs where rings are seen, and destructive interference at the gaps.

Question 35

How is the beam of electrons produced in electron diffraction ?

Answer 35

By attracting electrons from a heated filament wire to a positively charged metal plate, which has a small hole at its centre. Electrons which pass through the hole form a beam.

Question 36

How can the speed of the electrons be increased in the electron diffraction process?

Answer 36

By increasing the p.d between the filament and the metal plate.

Question 37

How does increasing the speed of the electrons in electron diffraction affect the size of the diffraction rings?

Answer 37

It makes the rings smaller, because the increase in speed makes the De broglie’s wavelength smaller, so less diffraction occurs.

Question 38

What is the photoelectric effect?

Answer 38

The process by which a metal with photons incident on it emits electrons (photoelectrons).

Question 39

What do emission and absorption spectra tell us?

Answer 39

They are evidence for discrete energy levels within an atom. In an emission spectra you can see the frequency of photons that certain elements emit. In absoprtion spectra you can see what frequency of photons certain elements absorb. These correlate to energy levels within the atom. These frequencies proportional to energy since E = HF for a single photon. Emission spectra appears as bright lines whereas absorption spectra appear as black lines.