Quantum Theory

Question 1

What are the units of wavelength?

Answer 1

m

Question 2

What are the units of frequency?

Answer 2

Hz = s^-1

Question 3

What is the symbol and what are the units of wave number?

Answer 3

K, m^-1

Question 4

What is the equation for frequency?

Answer 4

ν = speed of light (c) / wavelength (λ)

Question 5

What is the equation for wave number?

Answer 5

k= 2π/λ

Question 6

What is a photon?

Answer 6

Electromagnetic radiation made up of a stream of particles

Question 7

What speed does a photon travel at?

Answer 7

The speed of light (c)

Question 8

How do you calculate the amount of energy that a photon is carrying?

Answer 8

Energy (E) = h (Planck’s constant) x ν (frequency)

Question 9

What is the value of Planck’s constant?

Answer 9

6.62607004 × 10^(-34) m^2 kg / s

Question 10

What is the wavelength of visible light?

Answer 10

400nm-700nm

Question 11

Which colour of light has the longest wavelength?

Answer 11

Red (740nm)

Question 12

Which colour of light has the shortest wavelength?

Answer 12

Violet (390nm)

Question 13

As wavelength increases, what happens to frequency?

Answer 13

It decreases due to equation c=λν

Question 14

What is the photoelectric effect?

Answer 14

When UV radiation strikes a metal surface, electrons are ejected. Electrons are only ejected when the frequency of the UV radiation is above a certain threshold which is specific to the metal

Question 15

Why aren’t electrons ejected in the photoelectric effect until they pass a threshold frequency?

Answer 15

If an incoming photon doesn’t transfer enough energy to the surface of the metal, the electron will not be ejected. A higher frequency means a higher energy, hence the frequency threshold

Question 16

What is the equation to work out the energy of an ejected electron in the photoelectric effect?

Answer 16

hν = Φ + EKE

• Where Φ is the minimum energy required to remove an electron, known as the work function
• EKE is the energy of the ejected electron

Question 17

What is the evidence for a EM radiation behaving as a wave?

Answer 17

When light passes through two closely spaced slits, each slit gives rise to a circular wave and these waves interact with each other

Question 18

What is constructive interference?

Answer 18

When light passes through two closely spaced slits 2 circular waves are formed. When the peak of the first wave coincides with the peak of the second wave, this is constructive interference. The amplitudes of the two waves is added together to form one big wave.

Question 19

What is destructive interference?

Answer 19

When light passes through two closely spaced slits 2 circular waves are formed. When the peak of the first wave coincides with the trough of the second wave, destructive interference occurs. The amplitudes of the two waves cancel out to give a wave with a reduced amplitude or zero amplitude

Question 20

What is the quantisation of energy?

Answer 20

Atoms can only take certain values of energy. Electrons can’t lie between orbits in the atom and when an electron absorbs energy it can move to a higher energy level

Question 21

What is the Lyman series?

Answer 21

Where an electron moves from a higher orbit down to n=1 orbit, the Lyman series is in the UV region

Question 22

What is the Balmer series?

Answer 22

Where an electron moves from a higher orbit down to the n=2 orbit, the Balmer series is in the visible region

Question 23

What is the Paschen series?

Answer 23

Where an electron moves from a higher orbit down to the n=3 orbit, the Paschen series is in the infrared region

Question 24

How is the emission spectrum of Hydrogen obtained?

Answer 24

When a spark or electric discharge passes through H2 gas, the gas splits into H atoms and energy is provided to the H atoms, the atoms release energy in the form of EM radiation. The emitted light is examined by passing it through a prism. The resulting spectrum is not a continuous spectrum which demonstrates the fact that the atom can only exist at certain energy levels.

Question 25

Which equation would you use when working out the exact energy of a specific energy level?

Answer 25

Hν = E2 - E1 | - Where E2 is the upper energy level and E1 is the lower energy level

Question 26

What is the de Broglie equation?

Answer 26

de Broglie suggested that all matter has wave properties associated with it - The equation: λ = h/mv - Wavelength = Planck's constant / momentum - The wavelength is inversely proportional to the momentum of the object

Question 27

What is the Si unit of a joule?

Answer 27

Kgm^2s^-2

Question 28

If all matter has wave like properties, why is this not visible in larger objects?

Answer 28

The wavelength of larger objects is so small that their wavelike properties is not detectable

Question 29

What is the Heisenberg uncertainty principle?

Answer 29

- ΔpΔq ≥ 4π - (Uncertainty in momentum) x (Uncertainty in position) ≥ 4π - Since objects such as electrons have wavelike properties, it is impossible to know their momentum and their position simultaneously

Question 30

What is the Rydberg equation?

Answer 30

ν (frequency) = RH [ (1/n1^2) / (1/n2^2) ] | - Where RH is the Rydberg constant (3.29x10^15)Hz and n1 is the lower energy level and n2 is the upper energy level

Question 31

What is an absorption spectrum?

Answer 31

Samples are irradiated with a continuous spectrum of EM radiation, photons with energies corresponding to energy levels in the atom are absorbed, this produces dark lines on a light background

Question 32

What is the Rayleigh-Jeans law and why does it fail?

Answer 32

The Rayleigh-Jeans law states that as wavelength decreases, so does light intensity. However this fails as when the wavelength decreases and we move into the UV region, the intensity begins to decrease. This is why we don't get burned when opening an oven, the intensity of the UV radiation isn't too high