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### Course: Physics library>Unit 17

Lesson 1: Photons

# Photon Energy

Learn what a photon is and how to determine the energy of a photon as you explore the dual nature of light as both a wave and a particle, known as wave-particle duality. Uncover the concept of quantum mechanics, where light deposits energy in discrete amounts called photons. Investigate Planck's constant and how it relates to the energy of a photon. Created by David SantoPietro.

## Want to join the conversation?

• Photons are light particles because they exhibit particle like behavior. But are they actual/physical particles?
• Wave particle duality could better be understood by the fact that all particles on a quantum level are described by wave functions in order to describe their location, which we can't exactly know due to the Heisenberg Uncertainty Principle. Because of the uncertainty, we must describe particles as a function of it's likely locations, so thusly the particles act like waves because we don't know their exact position and must describe them as waves. This is why we can see wave particle duality with things as large as bucky balls(60 Carbon atoms) in experiments such as the double slit experiment. To summarize, photons ARE particles that just show wave behaviour due to us having to describe them as wave functions due to uncertainty of their position. At least this is what I gather after reading QED from Feynman, but I won't pretend to understand anything (Neither did Feynman.)
• At David mentions Planck discovering this smallest, discrete amount of energy that light can deliver (6.626 x 10^-34 (J·s)). My question: how did Planck, in 1900 (!), quantify such a small quantity of energy?
• He calculated it by making the hypothesis that energy comes only in discrete packets (i.e. the photon). He uses this assumption to calculate the frequency distribution of black body radiation with his constant h being the separation between allowed frequency values and it exactly matched what was experimentally observed.
• Just out of curiosity.
Light travels at different speeds in different medium.
Can't we use medium that has extremely high n value to slow speed of light to a level that we can capture, photograph light photons and examine its properties little better?
• 'Energy cannot be created or destroyed' then where did all the initial energy in the big bang come from?
• The big bang created an equal amount of positive and negative energy and mass, so if you were to calculate the mass and energy of the universe, you find that it actually equals 0, so no law is broken.
• Can you give an example problem for E=hf?
• Calculate the wavelength of a photon emitted when an electron in a hydrogen atom undergoes a transition from n = 2 to n = 1.
• At , is frequency a measure of the wave (of light) frequency? If so, how does it fit with the idea that light is a particle? How is a particle supposed to have a frequency?
• Yes, frequency is a measure of the wave of light. This is ok with the understanding that light is a particle because all particles do have frequency. All of the particles in your body technically have wave like properties (frequency and wavelength). Check out the De Brogli wavelength: http://calistry.org/calculate/deBroglieEquation
Ordinary particles have a wavelength so small that we can't see the oscillations (this is because ordinary particles have mass). Photons, on the other hand, do not have mass. Their nonexistent mass and high speed (speed of light, c) cancel out nicely to produce noticeable wave like behavior.
• Why is that Photons are known to be mass less? But at the same time it has MOMENTUM? Then how can it physically interact with matter?
• Photon is the quantum of light and light is electromagnetic wave which carries momentum and energy.
i.e, If the total energy transferred to a surface in time t is U, then p=U/c . So there, is interaction of photon with matter. Take an example you can see the surrounding because photons interact with matter.
• There has been something that has been bothering me ever since I entered the realm of physics and chemistry, it's that how are constants discovered? In school, they just give us the constants as a given and we have to memorize it, but it always nags me that I don't know how it was found out in the first place.
• Most constants are measured. For example when you look at the ideal gas law PV=nRT the constant R is determined by measuring the pressure and volume and then measuring the number of molecules and the temperature and put them in the rearranged equation R=(PV)/(nT).