if you want to understand total internal reflection then this is the video for you before we go further however it's importantly you have an understanding of critical angle if you're not sure what critical angle is please check out the early video which explains that and then come back to this one remember critical angle is the angle of incidence that gives rise to an angle of a fraction of 90 degrees and it only happens when we're moving from a more dense substance to a less dense substance for example from glass into air we've already got light here coming in at the critical angle with an angle of refraction of 90 degrees the question I want to ask now is what would happen if we increase this angle of incidence so that we go beyond the critical angle so what we've done here now is we've got another ray of light and the angle of incidence measure towards a normal line here the angle of incidence is bigger than the critical angle when the angle of incidence was the critical angle the light got refracted at 90 degrees now the angle of incidence is greater than the critical angle we do not get refraction anymore we get reflection so you'll notice there that it's line is not refracted but it is reflected and it stays inside the glass inside the more dense medium this is called total internal reflection total because all the light is reflected inside none of it escapes internal because the reflection takes place internally takes place inside the glass and reflection because well its reflection and not refraction now let's try a question in this diagram we have got a ray of light shining into a prism now we need to look at the angles very carefully this is 90 degrees this angle here is 45 degrees and this angle here is also 45 degrees that's very important now our job is to determine what happens to this ray of light well we can see it going down here it goes through the normal line in red now there's no refraction because it's a longer normal line and then it hits this surface here so what's it gonna do next as is so often the case in questions like this the first thing we need to do is to draw a normal line so I've drawn the normal line here in red and it's at 90 degrees to the surface as you can sink the angle of incidence is now here now if you remember for glass we said that the critical angle for glass was 42 degrees now in this case what is this angle of incidence well if this has gone through vertically here if this is 90 and this is 45 that means this angle of incidence is 45 degrees ie it is slightly over the critical angle which means we're not going to get refraction we're actually going to get total internal reflection the angle of incidence here is 45 degrees so the angle of reflection inside must also be 45 degrees which means the ray of light will move across horizontally like this until it strikes this surface and as we did earlier we draw a normal line here at 90 degrees to the surface and we look at what this angle is well if we think about the angles again if this is 90 if this is 45 if the ray of light is coming horizontally that means this angle of incidence is now 45 degrees which means it's bigger than the critical angle which means we don't get refraction we get total internal reflection again the angle of incidence here was 45 degrees which means the angle of reflection here is 45 degrees it's been totally internally reflected all the lights been reflected it's inside the glass and it makes its way up like this it goes through the normal line there now because it's a longer normal line it doesn't get refracted so it comes straight back out so what we've shown here is that light shone like this passes through this surface is totally internally reflected because the angle of incidence here is 45 which is bigger than the critical angle of 42 it's reflected over here again the angle of incidence is greater than 42 degrees it's 45 degrees which means we're going to get total internal reflection again and the ray of light comes back out like this and that's it for total internal reflection there are more videos on many other topics associated with GCSE physics please take a look at those and also consider subscribing