# Refractive index (1 Viewer)

##### Member
Hi how do you measure the refractive index of water using a semi circular plastic container?

#### idkkdi

##### Active Member
1. The circular shape is useless for measuring refraction. More straightforward to use a straight block.
2. Make sure the plastic isn’t too thick or else the light gets absorbed.
3. Darkroom, lightbox/beam of light, angle it at an angle to the normal, check the angle between the refracted ratio and the normal, compare the incident with the refracted.
N2/n1=sin1/sin2 snells law - you should have two angles. Look up Snells law if u don’t know what angles to take. Assume airs refractive index is the same as a vacuum, negligible difference between the two.

##### Member
I used filled it up with water and pointed the beam of light into the flat part and not the horizontal part is it correct?

also how do i measure the critical angle of glass using a rectangular prism

#### tito981

##### Member
For the critical angle you start the angle of incidence at 0 degrees to the normal (note the medium which the initial ray starts in has to have a higher refractive index than the medium that the light is travelling towards, you can do the opposite process for glass) and then you slowly make the angle larger till the refracted ray is parallel to the surface. Once you get near the critical angle just fiddle with the initial ray so that it is as low of an angle as possible.

##### Member
For the critical angle you start the angle of incidence at 0 degrees to the normal (note the medium which the initial ray starts in has to have a higher refractive index than the medium that the light is travelling towards, you can do the opposite process for glass) and then you slowly make the angle larger till the refracted ray is parallel to the surface. Once you get near the critical angle just fiddle with the initial ray so that it is as low of an angle as possible.
Thank you and sorry I’m a bit confused do you perhaps have a diagram that i could see?

#### tito981

##### Member
h ttps://www.livephysics.com/tools/optics-tools/find-critical-angle/#:~:text=The%20critical%20angle%20can%20be,angle%20equal%20to%2090%C2%B0.&text=For%20any%20angle%20of%20incidence,and%20part%20will%20be%20reflected.
try this site ^