Einstein's theory of relativity changed how we think about light and physics in general. Here are some important points to understand:

1. Constant Speed of Light: Einstein said that the speed of light, which is about 300 million meters per second, is always the same. This means it doesn’t change no matter who is watching it. Light acts like a speed limit that no one can go faster than.

2. Relativity of Simultaneity: Things that happen at the same time for one person might not seem to happen at the same time for someone else. This changes how we think about cause and effect, as well as time and space.

3. Time Dilation: When things move really fast, close to the speed of light, time starts to slow down for them when someone is watching from a still position. Here’s a simple way to think about the formula for this idea:

   • Imagine that the time a moving person feels (t') is different from the time a stationary person feels (t). As the moving speed (v) gets closer to the speed of light (c), the difference becomes bigger.

4. Length Contraction: Objects that are moving look shorter to someone who is not moving. This can be shown with a simple formula:

   • If you measure the length of a moving object (L'), it will appear shorter than its actual length at rest (L₀). Again, the moving speed (v) and the speed of light (c) are what make this happen.

5. Impacts on Electromagnetism: Because the speed of light is constant, it made scientists rethink some of the earlier equations about electricity and magnetism, bringing them together into one main idea.

In short, Einstein’s theory helped us understand how light works with the basic rules of space and time. This has had a huge impact on modern physics and how we understand our universe.