Goal: To understand light Objectives: 1)To learn about the Properties of light 2)To learn about Diffraction 3)To learn about Polarization 4)To learn how to create your very own Holograms!

Just like with other waves Light waves have wavelengths, periods, frequencies, and amplitudes. For the first 3 we find them the same way that we found them for other waves. Light waves have a wave speed, which in a vacuum is the speed of light. However, there are two things to add for light:

Energy There is a way to easily calculate the energy (or amplitude) of the wave! Energy = h * speed of light / wavelength h is just a constant here which you will use in recitation and on the homework.

Light particle? One change is that unlike waves, light can also act as a particle! This is called a photon. Light has momentum and is affected by gravity just like normal matter.

Light spreads out Just like with waves, a single light photon will spread out over distance. This occurs both from the point it is emitted as well as through any obstructions it might encounter.

Diffraction This leads us to diffraction. Imagine a pair of slits that light passes through on route to a screen behind it. On each part of that screen there will be light waves coming from each slit. The waves from each slit will travel a different distance. The difference in distance will create a diffraction pattern.

Young’s Double Slit Experiment In 1801 Thomas Young passed light through 2 parallel slits. Below would have been the result:

Why? Light going through both slits spread out in all directions. On a screen at any point waves going through different slits travel different distances. The wavelength of light is pretty small, so even a small change in distance can have big effects. From wikipedia:

So: Where the waves add in phase you get a bright area. Where they add out of phase you get a dark area. Conceptual question! If the wavelength of your light is 600 nm then what is the path length differences between the two slits for the first dark region?

So: Where the waves add in phase you get a bright area. Where they add out of phase you get a dark area. Conceptual question! If the wavelength of your light is 600 nm then what is the path length differences between the two slits for the first dark region? Since dark regions are out of phase, there must be a half wavelength difference, so the difference is 300 nm.

Polarization A light photon is a 2 dimensional wave (i.e. a wave in a 2 dimensional plane) with some velocity in some direction. Usually each photon will come in at some random plane. This is unpolarized light. However, if the light is emitted preferentially in some plane, or is forced into it, then it will become polarized.

Linear polarization One way to polarize light is linearly. This means that some percentage of light comes in at some specific plane. The % is the % polarization.

Circular polarization If the plane rotates with time in some way it is circularly polarized.

Polarizers There are filters called polarizers. These devices absorb all the parts of light not polarized in a specific plane. The parts in the plane get through (sort of like a door for people not paying attention to it).

Holograms Holograms are pretty cool as their image appears to move and be 3 dimensional. They are not as hard to create as you might think though.

Create your own hologram! Have the object that you want to make a hologram of. Shine a laser towards that object. Have half of the laser light reflect off the object to your film. Have the other half of the laser light reflect off of a mirror towards the film. The interference pattern this will cause will give you your 3 D image!

Conclusion We have learned about the properties of light. We have learned what we can do with light and what we can use it for, namely diffraction, polarization, and holograms.