In today's video, we're looking at how waves can be reflected off a boundary, and in doing so, we'll cover how to draw ray diagrams, and the different types of reflection, including specular and diffuse. The first thing we need to say, though, is that waves aren't always reflected. Whenever a wave arrives at a boundary between two materials, or mediums, three different things can happen. One is that the wave might be absorbed by the material, and so the energy from the wave is transferred to that material's energy stores. Option two is that the wave could be transmitted, which is where the wave enters the material but carries on travelling, and so passes out the other side, which often leads to refraction.
The last option is that the wave could be reflected off the surface and so never even enters the material and it's this one that we're going to be focusing on in this video. Which one of these three things happens though depends on the wavelength of the wave involved and the properties of the two materials. In order to show reflection taking place we use something called ray diagrams. as well as one simple rule, which is that the angle of incidence is always equal to the angle of reflection. To draw a diagram, we normally start at the bottom, with the boundary between the two materials.
For example, this area above could be the air, and this area below could be a mirror. So our boundary is effectively the surface of the mirror. Next, we draw our incoming ray of light, and you should add an arrow to show that it's coming down towards the mirror. At this point, we could draw our reflected ray.
The problem, though, is that we can't be sure exactly what angle to draw it at. So instead, we have to draw the normal first. The normal is a dashed line that's perpendicular, so exactly 90 degrees, from the surface.
And once we have the normal, we can measure the angle of incidence, which is this angle between the incoming ray and the normal, so 45 degrees in this case. In the exam, you might have to measure this angle yourself using a protractor. Now according to our rule, which says that the angle of incidence is always equal to the angle of reflection, this means that the angle of reflection must also be 45 degrees.
and so we will draw our reflected ray out like this at 45 degrees from the normal. And finally you need to know that this point here where the incoming ray touches the boundary is called the point of incidence. The last thing we need to look at are the different types of reflection.
Some materials like mirrors are perfectly smooth and so the boundary is completely flat. This means that all the normals are in exactly the same direction, and so all of the incoming light rays will be reflected in the same direction. We call this specular reflection, and it gives us a nice clear image, like you would get if you looked in the mirror.
Other materials though, like paper, have a relatively rough surface when you look at them really closely, which means that the boundary is all bumpy. This time, even though the incoming light rays are all coming down in the same direction, the normals will all be pointing off in different directions, because the surface itself isn't flat. For example, 90 degrees to this surface over here would be different to 90 degrees to this surface over here, so the normals point in different directions, and this means that the light will be reflected in all different directions. We call this type diffuse or scattered reflection and we normally can't see ourselves at all in these types of materials which is why you can't see your reflection in a piece of paper.
Importantly though in both of these types the angle of incidence is always equal to the angle of reflection it's just that the surface of a rough material isn't flat. so the normals, which remember are perpendicular to the surface, will always be pointing in different directions, and so the reflected rays get scattered. Anyway, that's everything for this video, so hope you found it useful.
If you did, then please do tell your friends about us or your teachers, and we'll see you next time.