Quantum mechanics. Some people might get a headache by just hearing it. I sort of got interested in it and have read a few books about it, but it's pretty hard to comprehend. But, the general idea is easy to understand;
It is a new type of physics that deal with things smaller than atoms, contrary to classic mechanics which people are more familiar with and probably learned a lot about in school.
In the world of quantum mechanics, weird things (compared to classic mechanics) happen. (which I'll write more about later, if I have the time to write pt. 2)
Thankfully, it's pretty easy to understand this.
When you hold 2 pencils together like this and look through it, you can see interference patterns (horizontal lines). This happens only with a wave, so now you know light is a wave.
BUT, then this won't make sense. Why are gamma rays and x rays, which have very short wavelengths more powerful than red lights, which has a far longer wavelength?
That's where the idea "lights are particles" comes in.(and when we think of lights as particles we call them photons. Gamma and X rays are like bullets, but red lights are like feathers so it doesn't make much damage to things like your body.
It's pretty confusing, but it's important to keep in mind that light is both a wave and a particle.