Atreyi SahaPhysics 132Prof. HatchFebruary 8, 2016Lecture #9: Intro to Ray Optics (continued)- Index of refraction: it is the ratio between the speed in vacuum over the speed in a mediumo this is because light travels different speeds in different mediao light travels the fastest in a vacuumo n = speed in vacuum/speed in mediumo Light that propagates through the boundary changes speed- If light comes in perpendicular to the surface, it will propagate straight. If light comes in at an angle, it will change direction, or refract. It changes direction because the speed changes. Let's say, if light is moving more slowly, it will change direction.o Refraction: light changes direction when it enters a new medium unless it is headed in at a normal incidence- Demo: Light comes out of the laser, hits the boundary (water) and it then changes directiono Water acts as a hard boundary since it is going from air to water. Therefore some of the light is reflectedo For the light propagating through, frequency is not allowed to change so wavelength has to change wave speed changes so light ray bends - If going from low index of refraction (n = 1) to high index of refraction (n = 2), then the light ray travelling through tends to bend TOWARD the normal- If going from high index of refraction (n = 2) to low index of refraction (n = 1), then the light ray travelling through tends to bend AWAY from the normal- Practical example: If you put a stick in the water, itlooks like it bends. The stick did not bend - it is thelight!- Fish example: Fish is actually, in the non circled spotbut light hits the fish and is reflected back. But, youreyes can only follow light in a straight line so itthinks that the fish is at the circled spoto we measure the angle off of the normal, usethe normal as a reference point- Snell's Law provides the precise relationship between the angles(beginning) n1sin(θ1) = n2sin(θ2) (end)1 and 2 are in sequence; 1 is first and 2 is secondo By figuring out the angle, we can figure out the change in direction using this law'o IMPORTANT....most common mistake: where is the light coming from = 1 & where is the light going to = 2- Question: A 1-cm thick layer of water stands on a horizontal slab of glass. A light ray in the air incident on the water 60 degrees from the normal. After entering the glass, what is the ray's angle from the normal?o index of refraction changes speedo follow Snell's law through the series of steps - for this step, light goes from air to water to glass that's why we have to do snell's law twice- Any transparent object that is placed in a liquid with the same index of refraction appears invisibleo If light is going through two media that have the same index of refraction, the second medium becomes invisibleo This is why you cannot see air - light travels through the air but chunks of air at different locations have the SAME index of refraction. However, hot air has different index of refraction so there is a change and light bends/changes directions - this is why you see shimmers of light on hot dayso Demo: Oil and pyrex glass havesame index of refraction Oil is in a beaker when you put pieces of pyrexglass in the beaker, youcannot see it in fact, there was a pyrexbeaker containing oil alreadysitting in the beaker!! didn'tsee it LOL- Total Internal Reflectiono Roughly 10% of light is lost in areflectiono If light is moving from high index ofrefraction to low index of refraction,the light refracts away. A little tiny bitis reflected in.o If theta is increased (image b), thenget a greater refraction. There is a point where the refracted angle would be 90 degrees (image c) What does that mean? Even though the boundary is transparent, the light does not actually get out. The corresponding theta to a refracted 90 angle is the CRITICAL ANGLE At an angle greater than the critical angle, 100% of the light gets reflected back inside. Zero light loss!! Light cannot get out and is trapped inside themedium. This is essentially what a fiber optic cable is - it is plastic shielded by another piece of plastic. The inside has a higher index of refraction than the outside shield. Light shone at angle greater than critical angle, light will stay inside and propagate along with zero light loss and at the speed of light. SO you can send a lot of information very quickly. Fiber optic cables used to see inside people's bodies- The critical angle is the angle at which total internal reflection just occurso Image 2 in notebook: the critical angle for water-air is Formula: θwater = sin-1
View Full Document
Unlocking...