Light & Matter

Question 1

What is de Broglie’s wavelength?

Answer 1

The wavelength associated with a particle of matter (anything that has mass), proving that it has wave-like properties.

Question 2

What happens if the de Broglie wavelength of an electron is the same as a photon’s wavelength in terms of diffraction?

Answer 2

The diffraction patterns will be the same.

Question 3

Why do we use electron microscopes instead of optical (photon) ones?

Answer 3

Because in most cases the electron’s de Broglie wavelength is shorter, this means it will have less diffraction.

When there’s less diffraction, the images produced will be much clearer.

Question 4

What’s observed in the double-slit experiment with one particle?

Answer 4

That after a long period of time the electrons passing through the slits will create an interference pattern as if it were a wave.

Question 5

What can we interpret from the double-slit experiment with a single photon/electron?

Answer 5

• The object leaves the source with particle-like properties (discreteness) and wave-like properties (wavelength & momentum)
• The immediate measurement on the screen consists of small discrete spots, which suggests the object is still behaving as a particle.
• The pattern over time is an interference pattern which suggests the object is still behaving as a wave.

Question 6

What is the single particle double-slit experiment evidence for?

Answer 6

The dual nature of light and matter.

Question 7

What can we call electrons/photons?

Answer 7

A quantum object.

Question 8

What provides evidence that photons have momentum even though they have no mass?

Answer 8

Compton scattering.

Question 9

What can we say if the diffraction pattern of an electron and photon are the same?

Answer 9

This means that they have the same wavelength.
This also means that they have the same momentum.

For this to occur, the photons and electrons must have different amounts of energy.

Question 10

If the diffraction pattern of an electron and photon are the same, what does this mean?

Answer 10

That the wavelength and momenta of the photons and electrons are the same.

Question 11

What is a continuous spectrum?

Answer 11

Contains many different colours, or wavelengths, with no gaps.
Perfectly white light shined through a prism causes dispersion, and we see a rainbow. This is a continuous spectrum

Question 12

What is an absorption spectrum?

Answer 12

This is seen where certain frequencies of white light that comes from the sun are absorbed due to gases in the atmosphere.
This creates dark lines on the spectrum where the colours are missing.

The specific set of wavelengths absorbed by an element.

Question 13

What is an emission spectrum?

Answer 13

Where elements are heated and produce light, and their light passes through a prism.
This has a black base with the colours produced by the element when heated.

The specific set of wavelengths produced by heating an element.

Question 14

What happens when electrons move down to a lower energy level?

Answer 14

They emit light. This is emission.

Question 15

What happens when electrons move up to a higher energy level?

Answer 15

They absorb light. This is absorption.

Question 16

What is an ion? How do atoms ionise?

Answer 16

An atom or molecule with a net negative charge due to loss or gain of an electron.

This is typically done by removing an electron.

Question 17

What is ionisation energy?

Answer 17

The energy required to remove the most loosely bounded electron from an atom.

They can also absorb more energy than the ionisation energy.

Question 18

What is observed when heating a gas in metal vapor lamps?

Answer 18

Each metal emits only certain wavelengths of light and each metal emits a different set of wavelengths.

Question 19

What did Louis de Broglie suggest with quantised energy levels of electrons?

Answer 19

• They have a de Broglie wavelength and they exist as circular waves around a nucleus.
• An electron is only stable if it forms a standing wave in its orbit. So the circumference of orbit must be integer multiple of the electron’s de Broglie wavelength.
• So only certain wavelengths are allowed and only form a specific energy level.

Question 20

In terms on Heisenberg’s Uncertainty Principle, what occurs when there’s a larger slit?

Answer 20

Uncertainty for the photon’s x-axis position is larger.

The diffraction pattern is less spread out which means that uncertainty in the photon’s x-axis momentum is smaller.

Question 21

In terms on Heisenberg’s Uncertainty Principle, what occurs when there’s a smaller slit?

Answer 21

Uncertainty for the photon’s x-axis position is smaller.

The diffraction pattern is more spread out which means that uncertainty in the photon’s x-axis momentum is larger.

Question 22

What does Heisenberg’s Uncertainty Principle state?

Answer 22

It is impossible to exactly define a particle’s position and momentum at the same time.

The more exactly a particle’s position is determined, the more uncertainty there is in its momentum.

The more exactly a particle’s momentum is determined, the more uncertainty there is in the particle’s position.

Question 23

Outline the results of the photoelectric effect that would be expected under the wave model of light.

Answer 23

• Ek of electrons should be proportional to the light intensity.
• Light of any frequency should be able to eject an electron above a minimum frequency.
• There will be a time delay observed, as energy is gradually transferred to the metal through light waves.

Question 24

Explain why the visible spectrum of light emitted from a hydrogen vapour lamp gives discrete spectral lines.

Answer 24

• Electrons orbit nuclei in shells with discrete energy levels
• Electrons can transition between these shells by absorbing or emitting discrete amounts of energy equal to the difference between the two shells
• Since transitioning electrons can only release discrete amounts of energy, only discrete spectral lines will be observed