A photon is the basic particle of light and all electromagnetic waves. it is what carries energy in light, radio waves, X-rays, gamma rays—basically every form of electromagnetic radiation.

• It’s not like a tiny ball.

• It’s a quantum particle that behaves both like a particle and a wave.

• In quantum electrodynamics (QED), the photon is the “messenger” particle of the electromagnetic force.

  
  

🔹 Energy and momentum of a photon:

• A photon always moves at the speed of light (c) in vacuum.

• It carries energy (E) and momentum (p).

• Its energy depends on its frequency (color for visible light):

  
  

So, even though it has no rest mass, it can still “push” things because of its momentum.

🔹 Does a photon have mass?

This is a subtle but important point.

1️⃣ Rest mass:

• A photon’s rest mass is exactly zero.

• This means if you try to “stop” a photon and measure its mass at rest, you can’t, because a photon can never be at rest.

• It only exists while moving at the speed of light.

  
  

2️⃣ Relativistic mass:

• Even though rest mass = 0, photons carry energy.

• Einstein’s relativity says energy and mass are connected:

• For a photon, the energy it has can be seen as a kind of effective mass when it interacts with gravity.

• This is why light bends near massive objects (gravitational lensing). Gravity doesn’t need rest mass; it curves spacetime and photons follow that curve.

✅ So photons are massless in rest mass, but they act like they have “moving mass” because of their energy.

🔹 Why must photons be massless?

If a photon had even a tiny rest mass:

• It wouldn’t travel at exactly the speed of light.

• The laws of electromagnetism (Maxwell’s equations) would change.

• Long-distance light, like from distant stars, would spread out in weird ways.

Scientists have tested this. The upper limit for a photon’s mass is unimaginably tiny, essentially zero for all practical purposes.

🔹 Quantum nature:

• Photons are bosons, meaning many can exist in the same state (that’s why lasers can have many identical photons).

• They have spin = 1, but no electric charge.

• They are the force carriers of the electromagnetic field in the Standard Model of particle physics.

  
  

🔹 Everyday effects of photons:

• Vision: Your eyes detect photons. Each photon excites a molecule in your retina.

• Photosynthesis: Plants use photons to create energy.

• Technology: Solar panels, fiber optics, lasers—all work by controlling photons.

  
  

✅ In deep simple words:

A photon is a quantum packet of light energy that always moves at the speed of light .It has zero rest mass, but it carries energy and momentum, which makes it able to exert forces and be affected by gravity.Its masslessness is what allows light to move at the universal speed limit and makes electromagnetism work the way it does.