Physics is a very diverse field with a vast array of different branches to focus on. One of the most interesting areas of physics is optics - the study of light.
It's common to think of light as some super-fast form of matter that just bounces around at 300,000 km/s and never slows down. However, light can actually slow down when it moves through different substances. Imagine dropping a baseball from the air into a deep pool of water. It would slow down, right? Well, what happens for light isn't too different.
We call the air or the water in the previous example 'mediums' (or media). Light moves through each of these mediums differently. For example, light moves close to the speed of light in vacuum, 299 792 458 m/s, in air, but it moves considerably slower in water, closer to 225 000 000 m/s. Take a look at Indices of Refraction for more details on how we can quantify this change in speed and Dispersion for some information on the role that the wavelength of light plays.
So light slows down when it enters a medium with a higher refractive index. It also speeds up when it moves from a higher refractive index to a lower one. But did you know that it also bends? Unlike in the example of the baseball falling into the pool, light that changes speeds moving between mediums will also change direction.
Snell's Law is our way of determining how much light bends between mediums. Try playing around with the values of the indices of refraction and the incident angle and see what effect that has on the refracted ray. Is there a combination of parameters for which the refracted ray disappears? The answer can be found in Critical Angle and Total Internal Reflection.
Want to learn about how principles from optics can be applied in the real world? See Fiber Optics - Main Page for information on one of optics' most impactful applications.