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# fibre optics watch

1. hi i have been given the following question but i am unsure about what i should do
1) calculate the effective speed of light in
a) a fibre optic glass of refractive index n=1.58
b) fibre optic cladding of refractive index n=1.56

could any1 please explain to my step by step how i would go about answering this question?

thanks
2. anyone?
3. Ok do you know any equations you can use involving a refractive index?
4. is this one n=sini/sinr
5. Does this sound familiar?

where is the refractive index from medium 1 (here it should be a vacuum) to medium 2 (the glass) and is the speed of light in medium 1 (the vacuum) divided by the speed of light in medium 2 (the glass)?
6. so do you know for q1 part a would the answer be 1.58=speed of light in air/ speed of light in optic glass?
1.58=3*10^8/?
?=3*10^8/1.58
8. i get this on calculator 189873417.7 would i round this to 1.89*10^8
9. i am unsure about question 1 part b
do i do the same as in q 1a or do i need to divide the refractive index by something first?
10. (Original post by panjabiflower)
i get this on calculator 189873417.7 would i round this to 1.89*10^8
if you're doing that, at least round it properly to 1.90*10^8
11. (Original post by alex-hs)
if you're doing that, at least round it properly to 1.90*10^8
oh yh stupid mistake - have you got any idea about what i do for question 1 part b?
12. (Original post by panjabiflower)
oh yh stupid mistake - have you got any idea about what i do for question 1 part b?
the same as you did for part a, but with the other value for the refractive index. Or am I missing something
13. In question 1b you do the same thing as in 1a - speed of light in air and in the new glass instead of the old one... Basically just replace 1.58 with 1.56...
14. Formula to use is:

v - velocity of light in medium
c - velocity of light in vacuum
n - refractive index of medium
15. so is answer for part b) 1.92*10^8
16. Yup!
17. does any1 know any equations to work out critical angles?
18. HINT: what's the angle of refraction at the critical angle?
19. is it 90 degrees
20. yup - so sin r = ?

EDIT: And then plug into n = sin i/sin r

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