Module 4 - Section 3 - light - Wave Or Particle

Question 1

How does light behave ?

Answer 1

Light behaves as a stream of particles or a wave

Question 2

What caused light to be perceived as a particle ?

Answer 2

The photoelectric effect caused people to think of light as a particle - called a photon

Question 3

What is light called if it is perceived as a particle ?

Answer 3

A photon

Question 4

What is a quantum ?

Answer 4

In simple terms it is the smallest unit for any phenomena - for example the quantum for light is a photon and the quantum for electricity is an electron

Question 5

What is a photon ?

Answer 5

A photon is a quantum of EM radiation

Question 6

Whats a single packet of EM radiation called ?

Answer 6

It’s called a quantum - and Max Planck said that EM radiation could only be released ion discrete packets called quanta - so basically its the smallest unit of EM radiation that there can be

Question 7

What is the equation for the energy of a photon ?

Answer 7

E = hf = hc/landa

E = energy of a photon
H= Plancks constant = 6.63x10-34 Js
F = frequency
C = speed of light in a vacuum - 3.00x108

Question 8

The higher the frequency the higher the ….

Answer 8

The higher the frequency , the higher the energy carried

Question 9

Do photons have charge ?

Answer 9

Photons have no charge - they are neutral like neutrons

Question 10

What happens when a photon interacts with another particle ?

Answer 10

It will transfer all or none of its energy

Question 11

How do you change Joules into electron volts ?

Answer 11

J / e

j=joules
E=charge of an electron

Question 12

What are the photon energies usually given in ?

Answer 12

Photon energies are usually given in electron volts

Question 13

Why do we use electron volts to measure the photon energies rather than joules ?

Answer 13

Because the energies involved with photons are so small therefore we need a more appropriate unit —> electron volts

Question 14

What is he equation for an electron volt ?

Answer 14

eV = 1/2mv*2

Question 15

What an electron volt defined as ?

Answer 15

An electron volt is defined as the kinetic energy gained by an electron when it is accelerated through a potential difference of 1V

Question 16

how much joules is in 1eV ?

Answer 16

1eV = 1.60x10*-19

Question 17

What is the threshold voltage used to find ?

Answer 17

Threshold voltage is used to find the Planck Constant

Question 18

What is the threshold voltage ?

Answer 18

It is the minimum amount of voltage needed for current to pass through a component

This is the voltage needed to give the electrons the same energy as a photon emitted by the LED - all of the electrons kinetic energy after it has been accelerated by the potential difference is transferred into a photon

Question 19

How can you estimate the Plancks Constant ?

Answer 19

You can estimate the Planck constant by finding the threshold voltage for a particular wavelength LED

Question 20

How can you use LEDs to estimate the Planck Constant ?

Answer 20

1) connect an LED of a known wavelength in the electrical circuit which has a variable resistor, voltmeter, milliammeter and a battery - the voltmeter is over the LED

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 LEDs light up - when the light just about lights up is when a current has flown through it - this means that there is a small current and it means that the voltage that the variable resistor was changed to is the threshold voltage as its the minimum amount of voltage that allows current to flow

3) record the threshold voltage across the LED , and the wavelength of light that the LED emits

4) repeat this experiment with a umber of LEDs of different colours that emit light at different wavelengths

5) Then plot a graph of threshold voltage against 1/wavelength - the threshold voltage is on the y-axis and the 1/wavelength is on the x-axis

6) You should get a straight line with a gradient of hc/e - which you can then use to find the value of h

7) so e.g. gradient = hc/e = 1.24x10*-6

So h = ( 1.24x10-6 ) x ( 1.6x10-19 ) / 3x10*8 —> as c is the speed of light - e is the charge of an electron and to find h you have to x e to the other side and you have to divide c to the other side

Question 21

What is a milliammeter used for ?

Answer 21

It is used for measuring small currents

Question 22

What happens if you shine electromagnetic waves of a high enough frequency onto the surface of a metal ?

Answer 22

If your shine EM waves of a high enough frequency onto the surface of metals, the metal will instantaneously eject electrons