Calculating photon energy
- The energy of a photon can be calculated using the formula:
- Where:
- E = energy of a photon (J)
- h = Planck's constant(J s)
- f = frequency (Hz)
Photon representation
A photon in a particle of light carrying discrete packets of energy
- Using the wave equation , energy can also be equal to:
- Where:
- c = the speed of light (m s-1)
- λ = wavelength (m)
- This equation tells us:
- The higher the frequency of EM radiation, the higher the energy of the photon
- The energy of a photon is inversely proportional to the wavelength
- Therefore, a long-wavelength photon of light has a lower energy than a shorter-wavelength photon
Worked example
Light of wavelength 490 nm is incident normally on a surface, as shown in the diagram.
The power of the light is 3.6 mW. The light is completely absorbed by the surface.
Calculate the number of photons incident on the surface in 2.0 s.
Answer
Step 1: Write down the known quantities
- Wavelength, λ = 490 nm = 490 × 10-9 m
- Power, P = 3.6 mW = 3.6 × 10-3 W
- Time, t = 2.0 s
Step 2: Write the equation for photon energy in terms of wavelength
Step 3: Calculate the energy of one photon
Step 4: Calculate the number of photons hitting the surface every second
Step 5: Calculate the number of photons that hit the surface in 2 s
Examiner Tip
The values of Planck’s constant and the speed of light are both given on your data sheet, however, it helps to memorise them to speed up calculation questions.
Remember this equation for E is only for the energy of a photon, not the energy for any other particle!