Topic 5 : Waves and Particle Nature of Light

Question 1

state the evidence to support the wave nature of EM waves.

Answer 1

Diffraction and interference providing evidence for this model.

Question 2

State what the energy of a photon is proportional to.

Answer 2

Energy of a photon is directly proportional to the frequency of the electromagnetic radiation.

Question 3

Define 1 eV.

Answer 3

One electronvolt is the energy transferred when an electron travels through a potential difference of 1 volt.

Question 4

State the equation for work done which involves potential difference.

Answer 4

W = V Q

Question 5

Explain the reason why LED light is used to investigate Plank’s constant.

Answer 5

LED only emit light when the potential difference exceeds the threshold potential difference required.
As the LED will be producing a specific colour of light, we can determine the wavelengths of light emitted.

Question 6

Explain how we can determine the threshold frequency of LED lamp.

Answer 6

By varying the potential difference, we can determine the threshold voltage required to turn on the LED.

Question 7

State your assumption about photons emitted by the LED.

Answer 7

Each photon of light from the LED emitted would be due to a single electron losing energy.

Question 8

Describe the equation you can use to determine Plank’s constant.

Answer 8

By equating the energy of an individual electron in the LED with an individual photon produced, we can use the equation eV – h c/ λ to determine the Plank’s constant.

Question 9

Describe how we can make the Plank’s constant experiment more accurate. Also state the variables to consider when drawing the graph.

Answer 9

To improve the accuracy of this estimation, the experiment can be repeated using a variety of different coloured LEDs, which will emit different wavelengths of light. The values of wavelength and threshold potential difference for each can be recorded, and a graph of V against 1/ λ can be drawn.

Question 10

Define the photoelectric effect.

Answer 10

When electromagnetic radiation is shone on to a metal, electrons are released form the surface of the metal. This is known as the Photoelectric effect.

Question 11

Describe how the photoelectric effect is investigated.

Answer 11

The photoelectric effect can be demonstrated using a gold leaf electroscope – a zinc plate on top of a negatively charged stem, with a negatively charged piece of gold leaf attached to the stem. Initially, the gold lead and the stem have the same charge, so they repel each other. If UV light is shone on the zinc plate, free electrons will be released from the surface of the plate, and the negatively change will be lost, so the gold leaf will gradually fall back to the stem.

Question 12

Explain why visible light would not induce the photoelectric effect on a metal surface.

Answer 12

When visible light is used, it doesn’t matter how intense the radiation is, no electrons will be removed from the plate. When UV light is used, even at very low intensity, electrons are instantaneously removed from the plate.

Question 13

Describe what happens to excess energy absorbed by an electron during the photoelectric effect.

Answer 13

any excess energy above the minimum energy required to escape becomes kinetic energy of the electron.

Question 14

Define the work function.

Answer 14

The work function, Φ, of a metal is the minimum energy required to free an electron from the surface of the metal.

Question 15

Define the threshold frequency.

Answer 15

As the photon’s energy is directly proportional to its frequency, there is a threshold frequency for the electromagnetic radiation, which is the minimum frequency required to free an electron form the surface of a metal.

Question 16

State the photoenectric equation.

Answer 16

hf = Φ + KE max

Question 17

Define KE max.

Answer 17

KE max is the maximum kinetic energy of the released electron.

Question 18

Why is KE max the maximum kinetic energy?

Answer 18

It is a maximum because some electrons may be closer to the nucleus, requiring more kinetic energy than the work function to be released, leaving less energy that can be converted into kinetic energy.

Question 19

Describe the relationship between frequency and the photoelectric effect.

Answer 19

If the frequency of the radiation is equal or greater than the threshold frequency, the intensity will increase or decrease the rate of electron emission from the surface of the metal.

Question 20

Describe how to increase the KE of an electron during the photo electric effect.

Answer 20

The only way to increase kinetic energy of the electron is to increase frequency of the radiation above the threshold frequency, so that more energy is left over to be converted into kinetic energy.

Question 21

State the formula for calculating intensity.

Answer 21

I = P/A

Question 22

State the different and compare techniques used to prove the wave/ particle nature of photons.

Answer 22

Diffraction and superposition of light relies on the radiation acting as a wave, but the photoelectric effect relies on it acting as a discrete photon.

Question 23

State debroglie’s postulate.

Answer 23

De Broglie realised that all matter can exhibit both wave and particle properties, and the wavelength associated with a particle is inversely proportional to its momentum.

Question 24

State the equation liking wavelength and plank’s constant.

Answer 24

λ = h/p = h/mv

Question 25

When can the wavelength equation be applied?

Answer 25

This equation can be applied to all particles with mass. As the mass of the particle increases, the wavelength decreases, so it becomes harder to observe wavelike properties.

Question 26

Describe the wave and particle nature of electrons.

Answer 26

They can be accelerated and deflected by an electric and magnetic fired, which is behaviour associated with particles, however, they can also be diffracted.

Question 27

How can you produce a diffraction grating of a beam of electrons.

Answer 27

When a beam of electrons is fired at a thin polycrystalline graphite, the electrons are diffracted by the gap between atoms, and produce a diffraction pattern when they hit a screen. The diffraction is a property of waves.

Question 28

How do scientists distinguish EM spectra of different elements?

Answer 28

Each element has its own set of energy levels.

Question 29

Describe and explain why the energy associated with energy levels are negative.

Answer 29

All energy level values are negative, with the ground state being the most negative. An electron is completely free form an atom when it has energy equal to zero. This negative sign is used to represent the energy required to be inputted to remove the electron from the atom.

Question 30

Define the emission line spectra.

Answer 30

Each element produces a unique emission line spectrum because of the unique set of energy levels associated with its electrons. It appears as a series of coloured lines on a black background.

Question 31

Define the absorption line spectra.

Answer 31

A series of black spectral lines on a coloured background, which corresponds to the wavelengths of light used to excite the electrons in that atom’s element. The black lines are the same wavelength of light emitted when the electrons are de-excited.

Question 32

define a continuous spectra.

Answer 32

All visible wavelengths of light are present. They are produced by atoms of solid heated metals or by atoms under high pressure.

Question 33

State the formula for calculating the energy of a photon.

Answer 33

The energy of a photon is given by the formula E = hf.

Question 34

Define spectroscopy.

Answer 34

Spectroscopy is a technique used to identify elements based on the wavelengths of light emitted when the atoms in a gas are de-excited.

Question 35

Define ionisation energy.

Answer 35

Ionisation energy is the minimum amount of energy required to ionise an atom of hydrogen in its ground state.