Energy takes many forms including light, sound and heat. Different colors of light are given by photons of various wavelengths. The relationship between energy and wavelength are inversely proportional, meaning that as the wavelength increases the associated energy decreases. A calculation for energy as it relates to wavelength includes the speed of light and Planck’s constant. The speed of light is 2.99x10^8 meters per second and Planck’s constant is 6.626x10^-34joule second. The calculated energy will be in joules. Units should match before performing the calculation to ensure an accurate result.

## Energy in Joules

Identify the wavelength of light you are calculating. This term is measured in meters; visible light is usually on the scale of nanometers (nm).

Convert the wavelength to meters. Multiply the wavelength by 10^-9 for nanometers or10^-6 for micrometers.

Multiply Planck’s constant (6.626x10^-34) and the speed of light (2.99x10^8).

Divide the product of Planck’s constant and speed of light by the wavelength (in meters). The result is energy in joules.

## Energy in Electronvolts

Identify the wavelength of light you are calculating and convert to micrometers using conventional metric conversion rules.

Divide 1.24 by the wavelength in micrometers to calculate energy.

The result of this calculation will be given in electronvolts (eV). This is the energy needed to raise an electron through one volt.

#### Tip

Convert the wavelength to the proper units depending on the equation used before starting the calculation.