Module 4.5 (Quantum Physics)

Question 1

What is the history of ideas about light?

Answer 1

1. -Isaac Newton proposed light was a stream of tiny particles
   -backed up by his own laws as light was reflected and refracted and travelled slower in denser mediums
   2.-Christian huygen said light was a wave which explained all above and also that light would be diffracted when passing through a small slit
2. Thomas Youngs two slit experiment showed that light behaved like a wave as two coherent light sources produced dark and light fringes

Question 2

What did Max Planck discover

Answer 2

-In 1900 he discovered that radiated energy doesn’t have continuous values and is instead released in packets or quantum of fixed energy
-Einstein used this idea to show that atoms absorb and emit these individual packets
-this opened up the idea that light could behave as both waves and particles

Question 3

What is a photon?

Answer 3

-A quanta or packet of light or other electromagnetic radiation

Question 4

What is the value of planck constant

Answer 4

6.626 *10^-34 Js

Question 5

How can we determine a value for plancks constant?

Answer 5

1. Adjust a variable resistor to determine the current as the voltage increases for each LED when they start to emit light, get around six values of V and I
   2.Plot a graph of V and I and use this to determine the activation voltage- maximum voltage that current increases linearly
   3.Plot a graph of activation voltage against 1/wavelength of led light and determine gradient
2. Multiply by charge of electron and divide by speed of light to get a value for Planck constant

Question 6

Why can we use LEDs to determine plancks constant

Answer 6

-The colour or wavelength of light is related to photon energy, E=hc/wavelength

Question 7

What is an electronVolt? and the equation for energy gained by an electron?

Answer 7

-An electron volt is the amount of kinetic energy gained by an electron when accelerated through 1V
-Energy gained=charge * Voltage
the charge of an electron is 1.602*10^-19 J
so an electron volt is that but with joules
-If we increase the particle’s charge or potential difference the kinetic
energy gained will increase proportionally

Question 8

What is the photoelectric effect?

Answer 8

When EM radiation of a particular frequency is shone at the surface of a metal electrons are emitted (photoelectrons)

Question 9

What are the observations from the gold leaf electroscope?

Answer 9

-When UV light and visible light is shone on the positively charged cap nothing happens and the stem and leaf still repel
-Same for when visible light is shone on negatively charged cap
-However when UV light is shone on the negatively charged cap the leaf falls which shows the cap has lost is charge due to emission of electrons.
-The loss of electrons can’t be due to ions as the plate is in a vaccum

Question 10

What is Threshold frequency?

Answer 10

-The lowest frequency of radiation that causes electrons to be emitted from the particular metal. This is different for different metals but usually around UV radiation.

Question 11

What is the work function of a metal?

Answer 11

-The minimum amount of energy required to release the electron from its surface and overcome the electrostatic attraction between the electron and the positive metal ion.

Question 12

How can we determine atomic spacing?
and if all objects have a wavelength how come we don’t show wave like properties?

Answer 12

-Electron diffraction can be used to determine the atomic spacing
- For macroscopic objects the wavelength is believed to be so low that the effects are negligible

Question 12

What is the wavelengths of the EM spectrum

Answer 12

Radio=10^3
Microwave=10^-2
Infrared=10^-5
Visible=10^-6
Ultraviolet=10^-8
x-ray=10^-10
Gamma rays=10^-12