Module 4 Section 3 Light

Question 1

What was light initially believed to be and what changed scientists mind (don’t need to learn by good to know)

Answer 1

In the late nineteenth century, if you asked what light was, scientists would happily show you lots of nice experiments showing how light must be a wave

Then came the photoelectric effect which mucked up everything.
The only way you could explain this was if light acted as a particle - called a photon.

Question 2

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Answer 2

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Question 3

Who discovered quanta

Answer 3

When Max Planck was investigating black body radiation he suggested that EM waves can only be released in discrete packets, called quanta.

Question 4

What is the equation to calculate the energy carried a quantum (single packet of EM radiation is called a quantum) (photon)

Answer 4

E= hf = hc/λ

h = Planck constant = 6.63 x 10-34 Js
f= frequency (Hz)
λ= wavelength (m)
c= speed of light in a vacuum = 3.00 x 10° ms

Question 5

What does a higher frequency of EM waves mean for its quanta

Answer 5

So, the higher the frequency of the electromagnetic radiation, the more energy its wave-packets carry.

Question 6

What are photons

Answer 6

Einstein went further by suggesting that EM waves (and the energy they carry) can only exist in discrete packets. He called these wave-packets photons.

Spec: photon as a quantum of energy of electromagnetic radiation (the energy of which is directly proportional to its frequency)

Question 7

Sort out difference between photon and quantum

Answer 7

A quantum (plural: quanta) is the smallest discrete unit of a phenomenon. For example, a quantum of light is a photon, and a quantum of electricity is an electron. (Don’t learn just for understanding)

Question 8

How do photons interact with particles, in terms of energy transfer

Answer 8

He believed that a photon acts as a particle, and will either transfer all or none of its energy when interacting with another particle, e.g. an electron.

Question 9

Charge of photons

Answer 9

Neutral

Question 10

What is the electron volt and why is it used

Answer 10

The energies involved when you’re talking about photons are so tiny that it makes sense to use a more appropriate unit than the joule.
Bring on the electronvolt

An electronvolt is defined as:
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.

Question 11

What is the equation for an electron volt

Answer 11

eV = 1/2mv^2

Question 12

What is one electron volt in joules

Answer 12

1 eV = 1.60 x 10^-19j

Question 13

What happens when When you accelerate an electron between two electrodes?

Answer 13

it transfers some electrical potential energy (eV) into kinetic energy.

Question 14

Describe the theory behind how you can find Planck constant

Answer 14

Current will only pass through an LED after a minimum voltage is placed across it - the threshold voltage V(0)

This is the voltage needed to give the electrons the same energy as a photon emitted by the LED. All of the electron’s kinetic energy after it is accelerated over this potential difference is transferred into a photon.
So by finding the threshold voltage for a particular wavelength LED, you can estimate the Planck constant.

Question 15

Equation to find Planck constant

Answer 15

h = eV(0) λ / c

Where V(0) is threshold voltage as a pose to energy

Question 16

Describe the practical used to find planks constant

Answer 16

Connect an LED of known wavelength in the electrical circuit shown.
2) Start off with no current flowing through the circuit, then adjust the variable resistor until a current just begins to flow through the circuit and the LED lights up.
3) Record the voltage (V) across the LED, and the wavelength of light the LED emits.
4) Repeat this experiment with a number of LEDs of different colours that emit light at different wavelengths.
5) Plot a graph of threshold voltages (V) against 1/λ (where λ is the wavelength of light emitted by the LED in metres).
6) You should get a straight line graph with a gradient of hcle - which you can then use to find the value of h (see CGP page 95).

Question 17

What happens when you shine EM waves of a high enough frequency onto the surface of a metal
(And why does this occur)

Answer 17

The metal surface will instantaneously eject electrons. For most metals, this frequency falls in the U.V. range.
1. Free electrons on the surface of the metal absorb energy from the light.
2. If an electron absorbs enough energy, the bonds holding it to the metal break and it is emitted from the surface.
3. This is called the photoelectric effect and the electrons emitted are called photoelectrons.

Question 18

How can the photo electric effect be demonstrated

Answer 18

The electroscope plate is initially negatively charged, so the gold leaf is repelled.
2) The zinc plate is then exposed to ultraviolet light and the photoelectric effect causes its free electrons to be ejected. This causes it to lose its negative charge - the gold leaf is no longer repelled and so drops down.

(See CGP page 96 for diagram)

Question 19

What 3 conclusions can be drawn from the gold leaf electroscope

Answer 19

Conclusion 1: For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value - called the threshold frequency.

Conclusion 2: The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value. This value of maximum kinetic energy increases with the frequency of the radiation, and is unaffected by the intensity of the radiation.

Conclusion 3: The number of photoelectrons emitted per second is proportional to the intensity of the radiation.

Note 1 and 2 can’t be explained using wave theory.

Question 20

Why can’t the photoelectric effect be explained by wave theory ?

Answer 20

According to wave theory:
1) For a particular frequency of light, the energy carried is proportional to the intensity of the beam.
2) The energy carried by the light would be spread evenly over the wavefront.
3) Each free electron on the surface of the metal would gain a bit of energy from each incoming wave.
4) Gradually, each electron would gain enough energy to leave the metal.
SO… The higher the intensity of the wave, the more energy it should transfer to each electron - the kinetic energy should increase with intensity. There’s no explanation for the kinetic energy depending only on the frequency.
There is also no explanation for the threshold frequency. According to wave theory, the electrons should be emitted eventually, no matter what the frequency is.

Question 21

How does the photon model explain the photoelectric effect ?

Answer 21

According to the photon model (idea of light being particles):
1) When light hits its surface, the metal is bombarded by photons.
2) If one of these photons is absorbed by a free electron, the electron will gain energy equal to hf.
Before an electron can leave the surface of the metal, it needs enough energy to break the bonds holding it there. This energy is called the work function energy (symbol , Φ) and its value depends on the metal.

Question 22

What is intensity in terms of photons

Answer 22

The number of photons per second

Question 23

What happens if the required work function is more or less than the energy gained by an electron

Answer 23

If more, the electron won’t be emitted but the metal may heat up. Note that electrons cannot accumulate energy from multiple photons.

If less If the energy gained by an electron (on the surface of the metal) from a photon is greater than the work function, the electron is emitted.

Question 24

How to calculate threshold frequency

Answer 24

Since, for electrons to be released, hf ≥ Φ the threshold frequency must be: f= Φ/h

Question 25

What is Einstein’s photoelectric equation:

Answer 25

hf = ф + KEmax where KEmax = 1/2mV(max)^2

Question 26

What is the kentic energy a electron has when it leaves the metal surface determined by and when is it maximized.

Answer 26

1) The energy transferred to an electron is hf.
2) The kinetic energy it will be carrying when it leaves the metal will be hf minus any energy it’s lost on the way out (there are loads of ways it can do that, and so the emitted electrons have a range of energies).
3) The minimum amount of energy an electron can lose is the work function energy hence (maximum kinetic energy is given by Einstein’s photoelectric equation)

Question 27

Why does intensity not effect the kinetic energy of a election

Answer 27

The kinetic energy of the electrons is independent of the intensity of the radiation, because they can only absorb one photon at a time.
A higher intensity just means more photons hitting a given area per second.

Question 28

What is the effect of intensity when it’s above the threshold frequency

Answer 28

the rate of photoelectron emission is directly proportional to the intensity of radiation provided it’s above the threshold frequency - more photons per second means more collisions.

Question 29

What are interference and diffraction patterns and what must this show about light

Answer 29

alternating bands of dark and light.

These can only be explained using waves interfering constructively (when two waves overlap in phase) or intertering destructively (when the two waves are out ot phase).

Question 30

What happens on a graph of frequency against Ke in term of what the gradient and y intercept are equivalent to?

Answer 30

Gradient will be equal the h (planks constant p)
Y intercept will be equal to (-) work function

Question 31

What is evidence that light has wave like properties

Answer 31

Light produces interference and diffraction patterns — alternating bands of dark and light.
2)
These can only be explained using waves interfering constructively (when two waves overlap in phase) or interfering destructively (when the two waves are out of phase).

Question 32

What is evidence that light has particle properties

Answer 32

Einstein explained the results of photoelectricity experiments by thinking of the beam of light as a series of particle-like photons.
If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one-to-one way.
All the energy in the photon is given to one electron.

Question 33

What is De Broglie’s wave-particle duality theory

Answer 33

If ‘wave-like’ light showed particle properties (photons), ‘particles’ like electrons should be expected to show wave-like properties.

Question 34

What is the De Broglie equation

Answer 34

λ= h/p

The de Broglie equation relates a wave property (wavelength, λ) to a moving particle property (momentum, p). h = Planck constant = 6.63 × 10^-34 Js.

Question 35

Explain how electrons were diffracted

Answer 35

Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces between carbon atoms in polycrystalline graphite.

This confirms that electrons show wave-like properties. According to wave theory, the spread of the lines in the diffraction pattern increases if the wavelength of the wave is greater.

In electron diffraction experiments, a smaller accelerating voltage, i.e. slower electrons, gives widely spaced rings. Increase the electron speed and the diffraction pattern circles squash together towards the middle. This fits in with the de Broglie equation above — if the momentum is higher, the wavelength is shorter and the spread of lines is smaller.

Question 36

What is the wavelength of accelerated electrons

Answer 36

Around 1x10^-10m

Question 37

Why do you not see diffraction of particles occurring often

Answer 37

You only get diffraction if a particle interacts with an object of about the same size as its de Broglie wavelength.

Question 38

Try CGP example

Answer 38

Pg 99

Question 39

How is the de broglie wave length utilized in an electron microscope ?

Answer 39

Diffraction effects blur detail on an image. If you want to resolve tiny detail in an image, you need a shorter wavelength. Light blurs out detail more than ‘electron-waves’ do, so an electron microscope can resolve finer detail than a light microscope.