The metric (International) system defines a hertz (Hz) as a unit of frequency, while a joule (J) represents a unit of energy. The conversion between these units is based on the Planck's equation written as: Energy = Planck Constant x Frequency. It allows you to calculate an energy of the electromagnetic radiation, such as light, if you know the frequency. Use a convenient online energy converter or a calculator to convert hertz to joules.
Things You'll Need
 Computer with Internet access
 Calculator
Using Online Converter

Open the Energy Converter in any web browser using the link in Resources.

Set the unit "Hz" (hertz) in the column "Frequency."

Enter the frequency value: "20000," for example, in the box under "Hz."

Set the unit "J" (joule) in the column "Energy." Then read the energy in joules in the box under "J." In the example, 20000 Hz converts to 1.3252 x 10^29 J.
Using a Calculator

Retrieve the Planck constant value using the link in Resources: 4.13566733 x10^15 eV s. Note that the Planck constant is usually given in electron volts (eV), which is another unit for energy.

Multiply the frequency and the Planck constant to calculate the energy in eV. For example, if the frequency is 50,000 Hz, then the photon energy is 50,000 Hz x 4.13566733 x 10^15 eV s = 2.07 x 10^ 10 eV.

Multiply the energy in eV by 1.6021765 x 10^19 to convert it to joules (J). In this example, the energy of 2.07 x 10^ 10 eV corresponds to (2.07 x 10^ 10 eV) x (1.6021765 x 10^19) = 3.31 x 10^ 29 J.
References
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