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 90o 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:

Question 1  What evidence is there that light is a wave?  

Answer

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 × 108 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 × 1013 Hz.  What is its wavelength?  What region of the electromagnetic spectrum is this?

Answer

 

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.

Question 3

What do you understand by the term photon?

Answer

The energy of each photon is given by the simple equation:

E = hf

 

 

Question 4

Write down the formula that links photon energy with frequency.  Explain each term and give the correct units.

Answer

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 × 108 = 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?                                                                                                                          

Answer

Did you forget to convert MHz to Hz?