5: Electromagnetic Waves (Chapters 33 & 34) Snapshot of a light wave. Wave vs Particle. A Brief History of Light

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A Brief History of Light 5: Electromagnetic Waves (Chapters 33 & 34) Phys130, A01 Dr.

Thomas Young, 1800 s: No, no, check this out. (Shines light through two slits.) Interference! It s a wave!

Albert Einstein, 1905: No, no, check this out. (Describes the photoelectric effect with photons.) It s quantized!

) Prince Louis debroglie, 1924: Hey, everybody. Maybe particles are waves! Everything s both! Scientists:... Woah.

1 A Brief History of Light 5: Electromagnetic Waves (Chapters 33 & 34) Phys130, A01 Dr. Robert MacDonald Isaac Newton, 1600 s: Light is like little bullets. Scientists: Okay, right, that makes sense! Thomas Young, 1800 s: No, no, check this out. (Shines light through two slits.) Interference! It s a wave! Scientists: Oooooh, you re right, it can t be little bullets if it can do that! Albert Einstein, 1905: No, no, check this out. (Describes the photoelectric effect with photons.) It s quantized! It must be particles! Scientists: Oooooh, you re right! Waves don t work that way! (Einstein gets a Nobel prize in 1921.) Prince Louis debroglie, 1924: Hey, everybody. Maybe particles are waves! Everything s both! Scientists:... Woah. (debroglie gets a Nobel Prize in 1929.) 2 Wave vs Particle Snapshot of a light wave Shape of the wave at a certain moment in time. electric field... photon P Pirate and Ninja Bunnies from Bunny (

2 History graph of a light wave History of the fields at point P as the wave passes. E = electric field B = magnetic field Models of Light Light is obviously very important to us generally as human beings. It s also very important to us as engineers and scientists. So there are many different models of light some more simplified or abstract than others that we can use depending on the situation. The main types of models are particles (photons), waves, or rays. We ll be starting with the ray model. This branch is often called geometric optics. We ll come back to the wave model later. That branch is often called physical optics. Rays A light ray represents an imaginary line along the direction of travel of the light. (Or maybe the path of a single photon, if you want to think of it that way.) A light ray will continue to travel along a straight line until it interacts with something else. Possible interactions include: Reflection happen at an interface between two materials Refraction (bending) Scattering Absorption 7 happen within a single material A point source of light: Rays go out uniformly in all directions.

3 Extended source of light, such as a flood light. (A source that has some shape to it). An object that isn t glowing. Light radiates uniformly in all directions from each point on the surface of the light source. Reflected light radiates in all directions from each point on the surface of the light source. No real difference between this and the floodlight! Light from a point source going past an object. Light from a point source going past an aperture (hole). point source Some rays are blocked. Shadow matches the object. Shadow s edges are sharp. point source Some rays are blocked. Light patch matches the hole. Light patch edges are sharp.

4 Light from an extended source going past an aperture. Light from an extended object going past a pinhole. extended source Some rays are blocked. Light patch matches the hole. Light patch edges are blurred. image! Most rays are blocked. Only one ray from each place gets through the pinhole. Image forms on! Note that if the hole isn t of zero size, the image will blur. Images To get an image on a, the light from a given point on the object (or other source of light) must strike exactly one point on the. Seeing an Object image! 15 not an image Light rays from each point on the object go everywhere. Some light from each point reaches the. 16

5 fog As long as only one ray from each point on the object reaches the, we see the object clearly. 17 If the light from the same point reaches the from several different directions e.g. it s scattered by fog then we see each point fuzzed out, and object appears blurry. 18 projector projector Light from an image on a is scattered in all directions. We see the image in the same way as we see any object. It s a sort of copy of the object. The sees no difference between an object and an image. 19 Light from an image on a is scattered in all directions. We see the image in the same way as we see any object. It s a sort of copy of the object. The sees no difference between an object and an image. 20

6 object virtual image So now you know: What the ray model of light is. When we look in a mirror, the rays that reach the appear to come from an object behind the mirror. What we see is called a virtual image. As far as the is concerned, it s the same as a real image (or an object). But the light that we see is not coming from the virtual image, unlike a real image. 21 How the ray model can describe what will appear on a. What an image is, and how an image is formed. What a real image and a virtual image are. 22 Polarization A string can have transverse waves in the up-anddown (vertical) direction, or in the side-to-side (horizontal) direction. Waves that are only waving in one direction are said to be polarized. Up-and-down waves are vertically polarized. Side-to-side waves are horizontally polarized. If you sent a twist along the string, that would be an unpolarized wave. If you ran the twisty wave through a vertical slot, the string on the other side could only wave vertically. Light is a transverse wave, as well. Can also be horizontally or vertically polarized (or neither). Light s polarization is the direction its electric field is oscillating. Most visible light is unpolarized when created. It can be polarized by a polarizing filter!like the slot for the string wave. Unpolarized light is made up of a mixture of all possible polarizations. Be warned: lots of vocabulary ahead! 24

Fig. 33.9 Picturing polarization Fig. 33.10 Polarizing Filters A good, and very common, polarizing filter is Polaroid, used in sunglasses and camera filters.

The electric field in the light pushes electrons back and forth on long molecules in the Polaroid (like a little tiny BBQ grill), absorbing the light s energy and producing heat.

An ideal filter passes 100% of the light with the right polarization, and none of the perpendicularly-polarized light. Fig. 33.

7 Fig Picturing polarization Fig Polarizing Filters A good, and very common, polarizing filter is Polaroid, used in sunglasses and camera filters. Horizontally-polarized light is absorbed by the Polaroid. The electric field in the light pushes electrons back and forth on long molecules in the Polaroid (like a little tiny BBQ grill), absorbing the light s energy and producing heat. The vertically polarized light has nowhere to push the electrons, so it just goes right through. An ideal filter passes 100% of the light with the right polarization, and none of the perpendicularly-polarized light. Fig Intensity After Polarization The electric field vector of a photon can be decomposed into vertical and horizontal components. Unpolarized light is made up of all possible polarizations. So on average the horizontal and vertical components are equal. Polarizers select one component and absorb the other. So half the light gets through, and the resulting light beam has half the intensity of the incident light beam. This is only true if the incident light is unpolarized. Polarized Incidence What happens when polarized light hits a polarizer? A polarizer is often called an analyzer when it s used with already-polarized light. If the incident light is polarized in the same direction as the analyzer s polarization axis, the light goes right through (for an ideal filter!). If the incident light s polarization is perpendicular to the analyzer s polarization axis, the light is entirely blocked. So the important relationship is the angle between the light and the polarizer s axis

For incident light at some angle θ, we can decompose the incident polarization. Only the component aligned with the analyzer is passed. Fig. 33.

8 For incident light at some angle θ, we can decompose the incident polarization. Only the component aligned with the analyzer is passed. Fig If the amplitude of the electric field of the incident light is E, then the amplitude of the electric field that makes it through the analyzer is E cosθ. θ=0 means the light is polarized in the same direction as the analyzer.! It all gets through. cos(0) = 1 θ=π/2 (90º) means the incident light is perpendicular to the analyzer,.! It s all absorbed. cos(π/2) = 0 The analyzer is called an analyzer because you can turn it until you get zero light coming through; then the polarization of the incident light is perpendicular to the analyzer s polarization axis Intensity after two polarizers Just like with other waves, the intensity of an electromagnetic wave (a light wave) is proportional to the square of the electric field amplitude: I α E 2. We can use this to compare the intensity before and after polarization. Take a ratio to get rid of constants: Example: Two Polarizers Incident light with intensity I0 shines through two consecutive ideal polarizers. The angle between the polarization axes of the two polarizers is π/4 (45º). What is the intensity of the light after each polarizer? Malus s Law for polarized light passing through another polarizer. (a.k.a. the cosine-squared law ) (Remember that for unpolarized light, I = I0/2.) 32

$Reflection and Refraction Reflection is when light bounces off of a surface. A surface here means the interface between any two media (air and glass, for example).$

9 Example: Three polarizers Angle between the first and second sheets: θ12 = 60!0 = 60 Angle between the second and third sheets: θ23 = 90!60 = 30 Liquid Crystal Display (LCD) Reflector or back light Horizontally polarizing filter Electrodes Vertically polarizing filter Electrodes Liquid crystals Diagram from Wikipedia. Reflection and Refraction Reflection is when light bounces off of a surface. A surface here means the interface between any two media (air and glass, for example). Refraction is when light enters a new medium. Its path is usually bent in the process. Usually you get a little bit of both at the same time. Consider light travelling through air (a medium) and striking a glass window (another medium)... 36

Chapter 24. Wave Optics

Chapter 24. Wave Optics Chapter 24 Wave Optics Diffraction Huygen s principle requires that the waves spread out after they pass through slits This spreading out of light from its initial line of travel is called diffraction