Quantum Phenomena

Question 1

Define 1 electronvolt (1 eV).

Answer 1

The energy gained by an electron when it moves through a potential difference of 1 volt.

Question 5

What is 1 eV in joules?

Answer 5

1 eV = 1.60 × 10⁻¹⁹ J

Question 5

How do you convert from eV to J?

Answer 5

Multiply by 1.6 × 10⁻¹⁹

Question 6

How do you convert from J to eV?

Answer 6

Divide by 1.6 × 10⁻¹⁹

Question 7

What equation relates energy, charge, and potential difference?

Answer 7

V = E / Q
(Where V = potential difference, E = energy, Q = charge)

Question 8

What happens when a charged particle is accelerated through a potential difference?

Answer 8

It gains kinetic energy equal to the energy transferred (in eV).

Question 9

What is the kinetic energy gained by an electron in terms of eV?

Answer 9

eV = ½mv²

Question 10

What is the photoelectric effect?

Answer 10

The emission of electrons (photoelectrons) from the surface of a metal when electromagnetic radiation is absorbed.

Question 11

What does the photoelectric effect demonstrate about light?

Answer 11

That light behaves as a particle, with energy carried in discrete packets (photons)

Question 12

Why must a photon have a minimum frequency to emit an electron?

Answer 12

Because an electron can only absorb one photon, and it must have enough energy to overcome the work function.

Question 13

Define threshold frequency (f₀).

Answer 13

The minimum frequency of electromagnetic radiation required to release a photoelectron from a metal surface.

Question 14

Define threshold wavelength (λ₀).

Answer 14

The maximum wavelength of radiation that will emit a photoelectron.

Question 17

Relationship between frequency and wavelength?

Answer 17

c = fλ
Where c is the speed of light, f is frequency, λ is wavelength.

Question 17

Define the work function (Φ).

Answer 17

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

Question 18

What happens if a photon’s energy is less than Φ?

Answer 18

No photoelectron is emitted.

Question 19

What happens to excess photon energy after overcoming Φ?

Answer 19

It becomes the kinetic energy of the emitted photoelectron.

Question 20

Equation linking energy, work function, and kinetic energy?

Answer 20

hf = Φ + Ek(max)

Question 21

Why do alkali metals have lower work functions?

Answer 21

Their surface electrons are less tightly bound due to weaker attraction.

Question 22

Define stopping potential (Vₛ).

Answer 22

The minimum potential difference required to stop all photoelectrons from reaching the collector plate.

Question 23

Equation linking stopping potential and kinetic energy?

Answer 23

Ek(max) = eVₛ Where e = electron charge

Question 24

What does stopping potential measure?

Answer 24

the maximum kinetic energy of emitted photoelectrons.

Question 25

Does increasing light intensity affect Vₛ?

Answer 25

No – intensity affects photoelectric current, not kinetic energy or Vₛ.

Question 26

What does increasing intensity do?

Answer 26

Increases number of photons, hence more photoelectrons, but not their energy.

Question 27

What happens if you increase frequency while keeping intensity constant?

Answer 27

- Photon energy ↑ - Photon number ↓ - Photoelectric current ↓ - Kinetic energy ↑

Question 28

What are energy levels in an atom?

Answer 28

Discrete energy states that electrons can occupy.

Question 29

Which energy level do electrons occupy by default?

Answer 29

The lowest possible energy level (ground state), which is the most stable.

Question 30

What is excitation?

Answer 30

When an electron absorbs energy and moves to a higher energy level.

Question 31

How does an electron become excited?

Answer 31

By absorbing a photon with exactly the right amount of energy.

Question 32

What is de-excitation?

Answer 32

When an electron falls to a lower energy level, emitting a photon.

Question 33

What happens to the energy of the emitted photon during de-excitation?

Answer 33

It equals the difference in energy between the two levels.

Question 34

What is ionisation?

Answer 34

The removal of an electron from an atom, causing it to become charged.

Question 35

What is ionisation energy?

Answer 35

The minimum energy required to remove an electron from the ground state.

Question 36

Can an electron be removed from any energy level?

Answer 36

Yes, but the ionisation energy is defined from the ground state.

Question 37

What is a fluorescent tube?

Answer 37

A low-pressure mercury vapor tube with a phosphor coating that emits visible light.

Question 38

What is the role of mercury atoms in a fluorescent tube?

Answer 38

They get excited by colliding electrons and emit UV photons upon de-excitation.

Question 39

What does the phosphor coating do in a fluorescent tube?

Answer 39

It absorbs UV photons and re-emits visible light photons during de-excitation

Question 40

How is excitation achieved in a fluorescent tube?

Answer 40

By electron collisions with mercury atoms, not just photon absorption.

Question 41

What causes the visible light in a fluorescent tube?

Answer 41

The de-excitation of phosphor electrons, not mercury directly.

Question 42

What causes emission spectra?

Answer 42

Electrons de-exciting to lower energy levels, emitting photons.

Question 43

What does an emission spectrum look like?

Answer 43

Coloured lines on a black background.

Question 44

What does each line in an emission spectrum represent?

Answer 44

A specific photon energy from a particular electronic transition.

Question 45

What causes absorption spectra?

Answer 45

Electrons absorbing photons and jumping to higher energy levels.

Question 46

What does an absorption spectrum look like?

Answer 46

A continuous spectrum with dark lines (missing wavelengths).

Question 47

Why are specific wavelengths missing in absorption spectra?

Answer 47

Those photons were absorbed by electrons for excitation.

Question 48

How are emission and absorption spectra related?

Answer 48

The same wavelengths appear in both — emitted in one, absorbed in the other.

Question 49

What evidence do line spectra provide about atoms?

Answer 49

Electrons can only occupy discrete energy levels.

Question 50

Why does each element have a unique line spectrum?

Answer 50

Each element has a unique arrangement of energy levels.

Question 51

What happens to absorbed photons in an absorption spectrum?

Answer 51

Re-emitted in random directions, so some wavelengths are missing from original beam.

Question 52

What is wave-particle duality?

Answer 52

The concept that light and matter exhibit both wave and particle properties.

Question 53

What experiment supports the wave nature of light?

Answer 53

Young’s Double-Slit Experiment (shows diffraction and interference).

Question 54

What phenomenon supports the particle nature of light?

Answer 54

The Photoelectric Effect (light behaves as photons).

Question 55

Why is graphite film used in electron diffraction experiments?

Answer 55

Its atomic lattice provides gaps acting like slits for electron waves.

Question 56

What is the de Broglie wavelength?

Answer 56

It is the wavelength associated with a moving particle, showing wave-particle duality.

Question 57

What happens when electrons pass through a slit similar in size to their de Broglie wavelengthThey exhibit diffraction,

Answer 57

They exhibit diffraction, forming a pattern

Question 58

How does increasing the kinetic energy of electrons affect their de Broglie wavelength?

Answer 58

Increasing kinetic energy increases momentum, which decreases the de Broglie wavelength.

Question 59

What is the relationship between electron momentum and the size of the diffraction pattern?

Answer 59

Smaller momentum → longer wavelength → larger diffraction pattern radius Larger momentum → shorter wavelength → smaller radius.