See now the thing is this: the angle between two refracting surfaces is called angle of the prism but in this diagram they had mentioned in my textbook that the angle of the prism is 90°. But by the R1+R2=A formula im not getting anything r1 is 0 r2 is 0


 How to know which are the refracting surfaces I'm so confused how light is coming out of the third surface I understand it's suffering total internal reflection but there are only two refracting surfaces in a prism...

Now if im wrong then here they have taken angle of the prism as 30° then only by the formula R1+R2= A R1=0 then r2=30

not total internal reflection

  • $\begingroup$ So you're asking why it's called a 90 degree prism rather than a 45 degree prism? $\endgroup$
    – M. Enns
    Feb 14 at 1:26
  • $\begingroup$ basically yes isn't the angle between the two refracting surfaces 45° $\endgroup$
    – sanya
    Feb 14 at 1:28
  • $\begingroup$ @M.Enna if not then explain this here we took A as 30 then only we got from that formula r1=0 r1+r2= A 0+r1=30. Im not able to post the pic of that diagram in this comment thing ill post it in my normal question $\endgroup$
    – sanya
    Feb 14 at 1:30

1 Answer 1


The way the diagram is drawn there isn't any refraction because the angle of incidence is 0° at the left and bottom surface. But, if the prism was rotated just slightly so that the angle of incidence on the left was a small, non-zero angle there would be refraction there and also on the bottom surface. These surfaces make a 90° with each other.


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