Photon Rate

Power is energy transferred each second. A beam of power P transfers energy at a known rate. Dividing this energy rate by the energy of one photon gives the number of photons emitted each second.

This relation applies to a monochromatic beam. All photons then have the same frequency and the same energy.

For a monochromatic beam, substitute the photon-energy relation from the previous section.

• At fixed frequency, greater power gives more photons each second.
• At fixed power, higher frequency gives fewer photons each second.
• If power and frequency both change, compare their factors. If both double, the photon rate remains unchanged.
• If a question only says that both increase, there is not enough information to decide how the photon rate changes.

The figure now holds power fixed. The markers count photons; their drawn size does not represent photon energy.

Swipe left or right to view the whole figure →

• Higher frequency → more energy per photon → fewer photons each second.
• Lower frequency → less energy per photon → more photons each second.

Name the fixed condition before making a comparison. Frequency sets energy per photon, power sets total energy per second, and intensity includes the beam area.