Reflection and Refraction – Physics | Lecturio



we've discussed now how electromagnetic waves work so we're ready to enter into the new topic of optics how these electromagnetic waves this light behaves as it bounces around or goes from one medium to the next and we'll start this discussion on optics by discussing reflection and refraction of these two sort of joint properties in the overall topic of optics after we've begun with the reflection and refraction will talk about mirrors and lenses and then finally move to some optical instruments and how they use the principles that we'll discuss in order to do what they do whether they're zooming into small things or looking at very big very faraway things let's start with the definitions of reflection and refraction first we'll need to know that as light is moving through some medium we've already mentioned that the speed of light or the velocity of any wave through a medium only depends on that medium and we properties of that medium with that said we know that light isn't always travelling in a pure vacuum just out in space usually it's traveling in our context through something like air or water and through each of these different media the light's speed will actually be different as it interacts with all the particles in that medium we can define for each one of these mediums whether it's air or the water something called the index of refraction and that is the ratio of the normal speed of light in a vacuum sea to the speed of light in whatever medium it's traveling through and notice by this definition of the index of refraction n that for a higher end for a higher index of refraction we mean that the speed of light is slower in that medium so a more dense medium like water for example would have a higher index of refraction and a lower speed for light to travel through that medium secondly we should start talking about the refraction since we've introduced the index of refraction for when a beam enters from one medium into another so suppose we have a light beam like this one it's moving towards the air in this case starting in a vacuum when the incident beam hits the air hits this new medium it will Bend and we talk about the amount of light that goes into the new medium as the transmitted amount of light so this bending effect is part of what the light does but some of the light will also be reflected when it hits the boundary of going into a new medium and so we have an incident wave which is the light wave that's going in towards the medium and then part of that incident way that initial wave will vent into the medium and part of it will be reflected from the medium what we do to describe the incident and reflected or transmitted beams as it goes into this meeting its first we draw an axis this dotted green line here this dotted green – axis is normal – or perpendicular to the surface between the two mediums and then given this line and knowing the incident ray and the exiting ray the reflected ray we can define an angle of each ray with this horizontal line we call the incident angle the angle of incidence we calling this theta sub I for the incident angle and then we have a reflected angle or the angle of refraction of reflection theta sub R lastly we could also define a transmitted angle the angle at which the light that goes that goes into the new medium is transit transmitted into that medium and again because the light has bent into that medium we would expect that the incident angle as you can see here on its way in will be different from the transmitted angle because the light wave bends as it goes into this this new medium the amount of the reflection will decrease as we decrease the incident angle in other words when we send light more directly into a new medium less of that light will be reflected and more will be transmitted into that medium the incident angle and the reflected angle ignoring our right now the transmitted amount of light are always going to be the same and this sort of follows our intuition that if light hits a surface and bounces off it will bounce off at the exact same angle that it came in at

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