All Physics help!!

[Louise29]

Good day, I have a question here: to find refractive index, we use sin i/sin r. But there are cases when we have to switch to sin r/sin i. I would like to ask, when do we switch?

 

[Michael A]

it depends on from where does the ray comes if for example from air to a block then sin i/sinr, from block to air sinr/sini

 

[Louise29]

it depends on from where does the ray comes if for example from air to a block then sin i/sinr, from block to air sinr/sini

Thank you!!