Photons

Photons

1. The photon model

   • In 1900, German scientist Max Planck proposed that electromagnetic radiation had a particulate nature as opposed to a continuous wave nature.
     • Planck discovered that EM radiation was emitted as discrete packets of energy, which could only exist in certain values, called quanta Einstein described these quanta as streams of particles, which he called photons.

2. Energy of a photon

   • The energy of a photon is directly proportional to its frequency.
   • E=hf
     • Where E is the energy of the photon in J, f is the frequency of the EM radiation in Hz and h is the Planck constant ( 6.63 x 10^-34 Js).
     • This equation can also be combined with the wave equation c=fλ to find the energy of a photon in terms of its wavelength. This gives a new equation of E=hc/λ where c is the speed of light in a vacuum (3.00 x 10^8 m/s) and λ is the wavelength of the photon.
   • The energy of a photon is inversely proportional to its wavelength. This means that photons with a shorter wavelength have a higher energy than those with a longer wavelength, which explains why, for example, gamma rays (short wavelength) are much more dangerous than radiowaves (long wavelength).

3. The electronvolt (eV)

   • The electronvolt is a unit of energy and is defined as the energy transferred to an electron when it passed through a potential difference of 1 V
   • The value of the electronvolt can be found with the equation W=VQ. Which means the work done on an electron is equal to the pd multiplied by the elementary charge. Giving 1 V x 1.6 x 10^-19. Showing that 1 eV is equal to 1.6 x 10^-19 J.

4. LEDs and the Planck constant

   • LEDs can be used to estimate the value of the Planck constant. When the pd across an LED reaches its threshold voltage it begins to emit photons of a specific wavelength. At this point the work done (W=VQ) is roughly equal to the energy of the emitted photon. If the wavelength of the photons emitted by the LED is also known, then h can be estimated by rearranging the equation eV=hc/λ,
   • The Planck constant can be determined with more accuracy by using multiple LEDs with different wavelengths (colours).. LEDs with different wavelengths have different threshold pd's so a graph of V against 1/λ can be plotted. The equation of the straight line is equivalent to V=hc/λe. The constant can be determined from the gradient of the graph which is equivalent to hc/e.