 Quantum physics has shown that in many experiments it makes sense to view light not as a wave, but as a stream of many particles. Such a light particle is called a photon. The energy of a photon is determined by the light frequency. If we look at light as a wave, the frequency tells us how often the light wave oscillates per second. The unit of frequency is 1 per second, or more compactly, Hertz. An important finding of quantum mechanics was that the energy of a photon is linked to Planck's constant H, one of the most important constants in physics. It always has the same value, 6.6 times 10 to the power of minus 34 joule-second. As you can see, this physical constant is incredibly small and has the unit joule-time-second. Using Planck's constant H and the frequency of light F, Max Planck postulated the energy Wp of a photon. The energy of a photon is equal to Planck's constant times the frequency of light. From this, we can conclude that the higher the light frequency, the higher the photon energy. We therefore expect visible light to have less energy than x-rays, for example. The formula for photon energy only tells us how large the energy of a single photon is. If we want to know what the energy of many photons is, we must of course determine their number, N, and multiply it by the energy of a single photon. The energy of N photons is equal to N times H times F. Sometimes we know the corresponding wavelength lambda instead of the light frequency. In the case of light, it doesn't matter which of the two quantities we know. As soon as we know the frequency, we can determine the wavelength from it and vice versa. The two quantities are linked by the speed of light, C. C is equal to F times lambda. This formula is known as the dispersion relation of light. The wavelength is the distance between two wave crests of the light in the wave model of light, and the unit of wavelength is the meter. The speed of light is a physical constant and has the value 3 times 10 to the power of 8 meters per second. Let's do a simple example of how to convert the two quantities into each other. From the operating instructions for the laser pointer, you have determined that the light emitted by the laser pointer has a wavelength of 500 nanometers. Nano stands for 10 to the power of minus 9. Let's rearrange the dispersion relation with respect to the frequency. F is equal to C by lambda. If we insert the speed of light and the wavelength, we get the frequency of this light, 6 times 10 to the power of 14 hertz. We can also express the photon energy WP with the wavelength. To do this, we replace F with C by lambda. We can conclude that the longer the wavelength of the light, the lower the energy of the photon. The photons of visible light have a color that we can perceive. When you hear something like wavelength of green light, the green refers to a specific wavelength of light. Depending on the wavelength, you perceive the light in a different color. Yellow light in the range of 575 nanometers, green light in the range of 546 nanometers, blue light at approximately 435 nanometers, violet light at 400 nanometers, and starting at around 365 nanometers, UV light begins, which we cannot see with the naked eye. Let's do a quick example of how to calculate the photon energy. We want to calculate the energy of a photon of yellow light. To do this, we insert 575 times 10 to the power of minus 9 meters into the Planck formula. If we also insert the value of Planck's constant and the speed of light, we get a photon energy of 3.4 times 10 to the power of minus 19 joules.