Lecture Videos

• Treatment of Light:

  • particle: emission/absorption of a photon
  • ray: particle moving in a straight line like reflection and refraction
  • wave: interference and diffraction and polarization
• Huygen's Principle: every wave front is made of many smaller spherical wave fronts
• Wave fronts = peaks of EM waves
• What happens when the gap or obstacle is about the same size as the wavelength of the light?


  Diffraction. Light bending as it passes the edge of something
• Huygen's Principle states that all wave fronts are comprised of many points, each of which is it's own spherical wave front.
• This very simple principle means that when a wave front interacts with something we can look at it on a point by point basis and figure out what happens next.
• As seen in the previous video, Huygen's principle results in light bending after going through a hole in an obstacle. Basically, since wave fronts are comprised of a bunch of tiny spherical wave fronts, they all cancel out their horizontal direction, while moving forward together in one direction.
• When you take away some of those wave fronts, the ones next to the hole are able to spread out horizontally without being cancelled. This means that light bends around corners- aka "diffraction."
• Diffraction is light spreading out as it passes an obstacle: wave fronts exist as many smaller spherical waves but their horizontal parts are cancelling. As you take away some of those mini-wave fronts then they no longer cancel parts of each other, so they spread out gradually filling in the area after the obstacle. This results in the direction of wave fronts spreading out after passing through a hole or around an object.
• The double slit experiment, also called "Young's Double Slit," is one of the definitive experiments that proves that light behaves as a wave. It involves setting up a screen with two tiny slits in it that are very close together. These slits and their separation distance need to be approximately the same size as the wavelength of light used. You then shine a laser on to the double slit, and an interference pattern will appear on a screen behind the slits.
• If light only traveled as a particle, then we might see it move through the two slits and leave only two bright spots on the screen.
• When light strikes a thin film, part of it reflects, bouncing off, and part of it refracts, transmitting through. Upon hitting the far side of the film that happens again, some of the light reflects, and some of it refracts.
• As you've probably noticed by now, the cause of interference in light when using a plane wave front source is a difference in path length. If someone were to observe the ray that is reflected off the top and bottom of the thin film, they might realize that one path is slightly longer than the other. If we label the thickness of the thin film as d, then the second ray travels about 2d farther than the first one! This can result in interference.
• Recall that different wavelengths result in different colors when it comes to visible light. When we have a thin film it is usually not perfectly uniform. Any variance in thickness results in a different wavelength (different color) having constructive interference. As the thickness varies so does the color of the most constructively interfering light at that point, resulting in a rainbow effect.
• X-ray diffraction uses the same idea as thin films where light bounces off of different layers of the crystal structure. When the light is received it will have different constructive interferences depending on what angle the light hits the crystal. This technique can be used to figure out the shape/structure of a crystal! We use x-rays for this because they are just the right wavelength for most crystal structures.
• Polarized light is light where all of the electric field waves are restricted to one direction, or the same plane. If we are talking about just the electric field, as we've done all week, you could say that light were polarized if the electric field lines were all up and down. Or if they were all left and right. Or if you impose a coordinate system, if the electric field lines were restricted to just the x-axis or just the y-axis or just the z-axis.
• We are going to sketch unpolarized light as a sort of star or asterisk shape at the tip of a ray. For clarity, the red ray is the whole ray of light and it is traveling in the direction of the red ray. The green arrows indicate orientations of electric fields (recall that E and B fields wiggle perpendicular to the direction that light travels). This representation shows us that unpolarized light travels in a direction, but the E and B fields can wiggle in any direction perpendicular to the direction that the ray of light is moving.
• The last thing for this week is that polarization can occur from reflections as well. This reflected polarized light is what your polarized sunglasses help with! When light bounces off of a surface the electric field lines that are perpendicular to the surface tend to get absorbed by that surface as they enter the surface and wiggle some electrons.