112 – Partial Refracion and Total Internal Reflection112–部分的折射与全反射

1、sometimes when you look out a window, you see what is outside as well as your own reflectionthis is because some light reflects and some light refracts at a surface between two media that have different indices of refractionthis phenomenon is called partial reflection partial reflection and refracti

2、onand refractionboth reflection and refraction occur, but not equallythe amount of each depends on the anglein this case, more light is refracted than reflected, as shown by the thickness of the raysexample: sun shining on water if the angle of incidence is nearly zero, (i.e., sun is directly overhe

3、ad) most of the light penetrates the surface and very little is reflected (see fig. a) as the angle of incidence increases (i.e., at sunset), more light is reflected at the surface and less light penetrates the surface to be refracted (see fig. b)rearview mirrors in most cars have a lever that allow

4、s the driver to choose how much light from behind the car will reach their eyes i.e., “daytime” and “nighttime” positionsthe mirrors are not actually flat, but wedge-shaped and silvered on the backlight coming from behind the car hits the mirror at a very small angle of incidence, so most of the lig

5、ht is refracted and reaches the silvered back of the mirror, where it is reflected to the drivers eyes“daytime” setting: the light that has reflected off the back of the mirror is directed to the drivers eyes allows a clear view of the traffic behind the car at night, this also reflects the headligh

6、ts of traffic directly into the drivers eyes“nighttime” setting: at this angle, most of the light penetrates the mirror glass and is refracted as before however in this case, only a small amount of reflected light is directed to the drivers eyes most of the light penetrates the mirror, refracts, hit

7、s the silvered back of the mirror, and is reflected away from the drivers eyes allows the driver to see the headlights, but at a lower intensityscuba divers can only see objects on the surface in an area directly above them (see picture on pg. 457)the light coming from directly above the diver will

8、penetrate the surface of the water, refract, and be visible to him/heras the angle of incidence increases, more of the light reflects off the water, and less reaches the diverfrom below the surface of the water, it looks like light is coming in through a large holeif you are standing in a clear lake

9、, it is easy to see stones on the bottom that are near you, but impossible to see ones that are farther awayfor you to see an object underwater, light must hit the object, reflect off it, and travel to your eyesbecause the incident rays are going from water to air, the refracted rays bend away from

10、the normalas the angle of incidence increases, the angle of refractionincreases more rapidlyas the angle of incidence continues to increase, the angle of refraction will eventually reach 90at this angle of incidence, the refracted ray lies along the boundary between the two mediano light passes into

11、 the second medium (air in this case)the angle of incidence that produces a refracted ray at an angle of 90 from the normal is called the critical anglecritical angle, cwhen the angle of incidence is larger than the critical angle, the angle of refraction cannot get any larger because the refracted

12、ray would no longer be in the second mediumso, at angles of incidence greater than the critical angle, no refraction occurs all the light is reflected back into the first mediumthis phenomenon is called total internal total internal reflection.reflection.a glass prismglass prism can change the direc

13、tion of light by creating the conditions for total internal reflectionthe critical angle between glass and air is less than 45, so letting light hit an inner surface at exactly 45 will be totally reflected inside the glasswhen light enters to the short side of the prism, the angle of incidence is ze

14、ro no refraction at surfaceat the long side of the prism, the angle of incidence is 45, so the angle of reflection is 45total change in direction of the light is 90when light enters the long side of the prism at any angle, the reflected light is reflected by 180, or directly back in the direction th

15、at it came fromwhen the angle of incidence into the prism is not 0, the light will be refractedafter the light has reflected off both inner short sides and then leaves the prism, it will refract at the same anglebinoculars the direction of light is reflected twice in binoculars by prisms to make the

16、 path of the light longer need to extend the path due to the distance to the focal point of the lenses used (more on that later in this unit!)retroreflectors look like small plastic prisms e.g., bicycle reflectors changes direction of incoming light by 180 regardless of the direction that light from

17、 headlights hits the reflectors, the light is always reflected directly back to the carfibre optics fibre optics have revolutionized all forms of communication, including the internet allows information to be sent as pulses of light instead of pulses of electricity optical fibres are made of a glass

18、 core which is surrounded by an optical cladding the cladding is a covering made of a different kind of glass than the fibre insidewhen light enters the end of the fibre in a direction that is almost parallel to the fibre, it hits the boundary between the core and the cladding at an angle that is larger than the critical angleeven when the fibre is bent, the light is totally internally reflected along the entir