Refraction in Gases - By MITK12Videos
Transcript
| 00:08 | Have you ever wondered how light travels through different materials | |
| 00:10 | to our eyes When you look through the space the | |
| 00:14 | light beds ? And so the image you see is | |
| 00:15 | distorted . This phenomenon is called refraction . It's all | |
| 00:20 | around us every day , like when you read to | |
| 00:22 | your glasses or when you look at something through a | |
| 00:25 | glass of water , as you can see , the | |
| 00:27 | pen appears to bend as it enters the water . | |
| 00:30 | Refraction is a slowing down and bending of light as | |
| 00:33 | it moves through different transparent substances . Let's take a | |
| 00:37 | look at what happens to light lays as they crossed | |
| 00:39 | the border between two different materials . Well , say | |
| 00:42 | for now that the material on top of air and | |
| 00:45 | the material on the bottom is glass . You can | |
| 00:47 | see that the lightwaves travelling in the glass are moving | |
| 00:50 | slower than those in the air . The material property | |
| 00:52 | that causes this difference is the index of refraction . | |
| 00:56 | We define this as a speed of light in vacuum | |
| 00:59 | , divided by the speed of light in the material | |
| 01:02 | . The more the light interacts with the material , | |
| 01:04 | the slower travels and the higher the index of refraction | |
| 01:08 | . For example , the index of refraction of glass | |
| 01:10 | is about 1.5 , Meaning light travels 1.5 times slower | |
| 01:15 | in gloss than in vacuum . As light crosses the | |
| 01:18 | boundary between two materials , the direction also changes the | |
| 01:22 | portion of the wavefront that enters first slows down , | |
| 01:24 | causing the raid to bend in that direction . Essentially | |
| 01:28 | Light X as if it wants to go to the | |
| 01:30 | area of higher index or to take the quickest path | |
| 01:33 | between two points take the phenomenon of a mirage . | |
| 01:38 | This is when you see something on the horizon that | |
| 01:40 | isn't actually there because the light has traveled from beyond | |
| 01:43 | the horizon to meet your eye In a mirage , | |
| 01:46 | the air close to the ground heats up and expands | |
| 01:49 | , reducing the index of refraction . The light waves | |
| 01:52 | passing close to the ground , bent upwards and look | |
| 01:55 | like reflections in a lake . We can make a | |
| 01:57 | similar situation in water by adding a layer of brine | |
| 02:00 | in the bottom of the tank . For this experiment | |
| 02:03 | will need water salt , a tank and a laser | |
| 02:10 | . The index of refraction of the bride is higher | |
| 02:13 | than that of the freshwater and act in the same | |
| 02:16 | way as the cold air rushing . We're using a | |
| 02:29 | funnel to put the salt solution at the bottom of | |
| 02:31 | the tank every mm we've let the tanks sit for | |
| 02:43 | a while to allow the fresh water in the salty | |
| 02:45 | water to mix a little . You can see that | |
| 02:47 | the concentration of salt changes across the boundary and so | |
| 02:51 | does the index of refraction . The more salt there | |
| 02:54 | is , the harder it is for the light to | |
| 02:55 | move forward and thus the higher the refractive index . | |
| 02:59 | Now , let's see what happens when we turn off | |
| 03:01 | the light and turn on the laser as you go | |
| 03:04 | deeper into the tank , the light bends and we've | |
| 03:07 | created a kitchen sized , watery mirage . You might | |
| 03:10 | notice that the light in this case is bending down | |
| 03:13 | opposite to what you would expect in a mirage . | |
| 03:16 | This is because the high refractive index material is on | |
| 03:19 | the bottom of the tank . Now this is pretty | |
| 03:23 | obvious and things like magnifying lenses and water , but | |
| 03:26 | does light bending gases , like the air around us | |
| 03:29 | ? The short answer is yes . The long answer | |
| 03:32 | is yes , but only if something in the gas | |
| 03:34 | changes like its temperature or its composition in nature . | |
| 03:38 | On the seashore , it is called a fata morgana | |
| 03:41 | or inverted mirage and can look pretty impressive . Its | |
| 03:45 | origin lies in the humidity and temperature of air above | |
| 03:49 | the water . The cold air is denser close to | |
| 03:52 | the sea level , then high above it . This | |
| 03:55 | creates a density gradient and thus a change of refractive | |
| 03:58 | index . Just like in our experiment , the light | |
| 04:01 | coming from the boat is curved by the air layers | |
| 04:04 | but our eyes think that the light travels in straight | |
| 04:06 | lines so it looks like we're seeing a flying boat | |
| 04:12 | . All of these examples of light bending are caused | |
| 04:15 | by the same thing . Refraction in the lab we | |
| 04:18 | can make foot long mirages but in the wild the | |
| 04:21 | length scale is more like several miles . So next | |
| 04:25 | time you get lost in the desert or see a | |
| 04:26 | flying ship . Don't trust your eyes . Trust refraction | |
| 04:36 | . Yeah . Yeah . |
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