# A Level Physics Waves Help

Hi, there is this waves question I am stuck on. The mark scheme says you find the angle of incidence between the core cladding and this angle of incidence is the critical angle. What confuses me is that the question says the ray 'undergoes total internal reflection at the Angle A'. I'm confused at this because if angle of incidence is equal to the critical angle, then wouldn't the ray be refracted into boundary (angle of refraction is 90 degrees)? I thought total internal reflection only occurs if angle of incidence > critical angle?
Total internal refraction happens at the critical angle, but Angle A is not the critical angle. Complete internal reflection happens between the core and the cladding, meaning that the angle that I have drawn in blue is the critical angle (I have attached an image). If the Angle A is greater the angle between the core and cladding will be different, which is what the question is trying to explain. Hopefully this helps you finish the question. If not I could answer any more questions you have because I did this question recently.
The image isn't sending! The angle i drew in blue was between the core and the cladding
Original post by bertielarter
Total internal refraction happens at the critical angle, but Angle A is not the critical angle. Complete internal reflection happens between the core and the cladding, meaning that the angle that I have drawn in blue is the critical angle (I have attached an image). If the Angle A is greater the angle between the core and cladding will be different, which is what the question is trying to explain. Hopefully this helps you finish the question. If not I could answer any more questions you have because I did this question recently.
Ah sorry, my wording was bad - I know A isn't the critical angle. So if an angle is greater than A, it will decrease the angle of incidence at the core cladding hence why it says partial reflection cause it is likely to be refracted. But at A, angle of incidence = critical angle so angle of refraction is 90 degrees. And if angle is less than A, angle of incidence at core cladding boundary will be greater so TIR occurs?
(edited 1 month ago)
Sorry for misunderstanding. If the angle between the core and cladding is at the critical angle or greater TIR occurs. Partial internal refraction can also occur before the angle reaches critical, but there will be some refraction also. When angle A is increased, it will decrease the angle of incidence at the core cladding like you said, which will cause some of the light to be refracted.
Original post by bertielarter
Sorry for misunderstanding. If the angle between the core and cladding is at the critical angle or greater TIR occurs. Partial internal refraction can also occur before the angle reaches critical, but there will be some refraction also. When angle A is increased, it will decrease the angle of incidence at the core cladding like you said, which will cause some of the light to be refracted.
When reading online, it says if critical angle is equal to angle of incidence, it is refracted along the boundary?
(edited 1 month ago)
Sorry, I'm not giving great answers and I'm taking ages to reply.

Yeah you're right, I said that total internal reflection happens but actually a very small amount of light is also refracted along the boundary (which is why the angle of refraction is 90'), so I didn't give a proper explanation. At critical angle, almost all of the light is reflected and if the angle is even slightly greater there will be no light refracted at all. As the angle approaches the critical angle, the amount of light gets less and less, so the refracted light along the boundary will be very small but still there, this also means that partial reflection will happen before the critical angle as the amount of refraction gets smaller.

Hope this is a better explanation