Particle Physics Tutorial 3 - Photons
The electromagnetic spectrum is a family of waves that have the following properties:
They are made up of an electric field and a magnetic field at 90^{o} to each other. The electric field (E) and the magnetic field (B) are always in phase (in step) with each other.
The main parts of the electromagnetic spectrum are shown below:
Light shows wave behaviour:
It refracts, and reflects;
It is diffracted;
As a transverse wave, it can be polarised.
Question 1 | What evidence is there that light is a wave? |
Like all waves, EM waves follow the wave equation:
The symbol l is lambda, a Greek letter ‘l’. It is the physics code for wavelength, measured in metres.
Speed of light (3 × 10^{8} m s^{-1}) is given by the code c, while f is the frequency (Hz). Light waves are often given in nanometres (nm) where 1 nm = 1 × 10^{-9} m.
Question 2 |
(a) What is the frequency of radio waves of wavelength 247 m? (b) An electromagnetic radiation has a frequency of 2 × 10^{13} Hz. What is its wavelength? What region of the electromagnetic spectrum is this? |
Photons
Physicists now believe that light travels in packets of waves called photons. (We will look at the evidence for this in the photo-electric effect.) Each photon is a train or burst of waves. Photons are given out when charged particles lose energy. These travel in random directions from a light source. Once they have left the light source, the photons travel in straight lines until reflected or refracted.
What do you understand by the term photon? |
The energy of each photon is given by the simple equation:
E = hf
E – energy per photon (J);
h – Planck’s constant, 6.63 × 10^{-34} J s (joule-seconds, NOT joules per second)
f – frequency (Hz)
Write down the formula that links photon energy with frequency. Explain each term and give the correct units. |
Wavelengths are often given rather than frequency, so we have to convert to frequency using:
It doesn’t take a genius to see that this relationship can be substituted to give us:
Worked Example
What is the photon energy of red light of wavelength 600 nm? |
Use E = 6.63 × 10^{-34} × 3 × 10^{8} = 3.32 × 10^{-19} J 600 × 10^{-9} |
Power of a Beam of Light
If we have n photons, each of energy E, passing a particular point, we can easily work out the power P (energy per second).
P = nhf
If we know the power of a laser beam, we can work out many photons it gives out every second.
Question 5 |
Aeroplanes approaching to land at Leeds Bradford Airport (Yeadon Aerodrome) are guided in to the runway by a beam of radio waves transmitted at a frequency of 110.90 MHz. (a) What is the wavelength of the radio waves? (b) What is the energy per photon? (c) If the transmitter has a power of 100 W, how many photons are given out every second? |
Did you forget to convert MHz to Hz?