1. Photons and Mass:
* Rest Mass: Photons do not have rest mass. This means they have no mass when they are at rest, which they never are because they always travel at the speed of light.
* Relativistic Mass: In some older physics texts, you might see the concept of "relativistic mass," which increases with velocity. However, modern physics favors the idea that mass is an intrinsic property of an object and doesn't change. So, while photons have energy and momentum, they don't have mass in the traditional sense.
2. How Gravity Affects Photons:
* Curvature of Spacetime: Gravity, according to Einstein's theory of general relativity, is not a force but a curvature of spacetime. Massive objects warp the fabric of spacetime around them.
* Photon Trajectories: Photons, though massless, still travel along the curved paths of spacetime. This is why light bends around massive objects like stars and galaxies, a phenomenon called gravitational lensing.
* Redshift: As photons travel through a gravitational field, they lose energy and their wavelengths increase (they redshift). This is because they are essentially "climbing" out of the gravitational well, losing energy in the process.
3. Photons in a Medium:
* Interaction with Atoms: When a photon enters a medium like glass or water, it interacts with the atoms in the material. This interaction causes the photon to be absorbed and re-emitted by the atoms.
* Slower Speed: This absorption and re-emission process takes time, which effectively slows down the overall speed of the photon through the medium. The light appears to travel slower, but it's actually the interaction with the atoms that causes the delay.
* Return to Light Speed: Once the photon exits the medium, it is no longer interacting with atoms and travels at the speed of light again.
Key Points to Remember:
* Photons are massless, but they have energy and momentum.
* Gravity affects photons by bending their paths and causing them to redshift.
* Photons slow down in a medium due to interactions with atoms, but they regain their speed once they exit the medium.
Let me know if you have any more questions!
