Quantum physics

Question 1

Photon

Answer 1

A quantum of energy in the form of electromagnetic radiation

Question 2

Formula for energy

Answer 2

hf

Question 3

Momentum of a photon formula

Answer 3

P= E/c
where c is the speed of light

Question 4

When a charged particle is accelerated in an electric field, its v is calculated how

Answer 4

by equating eV= 1/2 mv²

Question 5

Photoelectric effect?

Answer 5

When electromagnetic radiation of threshold frequency falls on a metal surface, electrons are emitted

Question 6

Threshold frequency

Answer 6

Minimum frequency required to release electrons from the surface of a metal

Question 7

Threshold wavelength

Answer 7

Maximum wavelength required to release electrons from the surface of a metal

Question 8

Work function

Answer 8

Minimum amount of energy required by an electron to escape from its metal surface

Question 9

Work function formula

Answer 9

∅ = hfo

= Plank’s constant * threshold frequency

Question 10

Rules of photoelectric effect (3)

Answer 10

• A single photon can release a single electron
• Energy is conserved during the interaction
• If energy is sufficient, surface electrons are released instantaneously

Question 11

Einstein’s photoelectric equation

Answer 11

Energy of a photon= Work function+ Max. Ke

hf = hfo + ½ m(vmax)2

Question 12

Stopping potential

Answer 12

P.d which stops electrons with max. ke from reaching the anode in a photocell

Question 13

Factors affecting the ke of photoelectrons are (2)

Answer 13

• frequency of incident photons
• work function of the metal surface

Question 14

Photoelectric emission

Answer 14

is the release of electrons from surface of the metal when e.m radiation is incident to the surface

Question 15

Intensity formula in terms of photons

Answer 15

Rate of arrival of electrons * energy of each photon

Question 16

Why do electrons have a high range of KE values (2)

Answer 16

• If the frequency is greater than threshold, surface electrons will escape and have surplus energy in the form of KE- KEmax
• If the photon interacts with electrons below the surface, some energy is used in bringing the electron to the surface so is emitted with less KE

Question 17

Evidence for particle theory

Answer 17

Photoelectric effect as light wave energy is transferred in the form of photos and interacts with other matter such as electrons

Question 18

Light is a wave evidence

Answer 18

Light is propagated through space as a wave.
Evidence is in interference and diffraction of waves

Question 19

Three suggestions of wave theory of light

Answer 19

1. any frequency of light can give rise to photoelectric emission if exposure time is long enough
2. Energy absorbed by each electron will increase gradually with each wave
3. KE of emitted electrons will increase with radiation

Question 20

Wave theory suggests;
any frequency can release electrons

This is wrong because:

Answer 20

Photoelectrons will be released immediately if frequency is above threshold frequency of the metal

Question 21

Wave theory suggests;
Energy absorbed by electrons increases

This is wrong because:

Answer 21

Energy is absorbed instantaneously-
photoelectrons are either emitted or not
emitted after exposure to light.

Question 22

Wave theory suggests;
KE will increase with intensity

This is wrong because:

Answer 22

If the intensity of light is increased more
photoelectrons are emitted per second.

Question 23

When does electron diffraction occur? (2)

Answer 23

• electrons are accelerated to high speeds
• pass through a gap similar to their size

Question 24

De broglie wavelength formula

Answer 24

= h/mv

Question 25

Emission line spectra (2)

Answer 25

• composed of light emitted from hot gases
• dark background with coloured lines

Question 26

Absorption line spectra (2)

Answer 26

• observed when white light is passed through cool gases
• coloured background with dark lines

Question 27

Ground state

Answer 27

condition where the atom possesses the minimum possible energy

Question 28

Excited state

Answer 28

atoms absorb energy so the energy of the atom incerases

Question 29

When electrons move from higher to lower energy level, what happens

Answer 29

they emit energy in the from of photon of me radiation

Question 30

For electrons to move from lower to higher energy level, what needs to happen

Answer 30

Absorption of a photon

Question 31

Use the concept of discrete energy levels to

explain the existence of dark lines (5)

in the absorption spectra of white light passing through cool gas

Answer 31

• photons give energy to electron
• electron moves to higher energy level
• energy is equal to difference in energy levels
• electron de-excites giving off photon of same energy
• photons emitted in all directions

Question 32

Pieces of evidence provided by photoelectric effecet for particulate nature of electromagnetic radiation

5

Answer 32

1. • max. energy of emitted electrons depends on frequency not intensity
2. • rate of emission of electrons depends on intensity
3. • existence of frequency below which no emission of electrons
4. • instantaneous emission of electrons
5. • increasing frequency at constant intensity decreases rate of emission of electrons

Question 33

what is de broglie wavelength

Answer 33

wavelength associated with a moving particle