Refraction of Light

Laws of Refraction

• The incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in one plane.
• For any two given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

The second law is called Snell's law after the scientist Willebrod Snellius who first formulated it

Where is the refractive index of the second medium with respect to the first medium.

• Place a rectangular glass slab on the white sheet of paper fixed on a drawing board.
• Trace the boundary ABCD of the glass slab.
• Remove the glass slab and draw a normal N₁N₂ at O.
• Draw a straight line IO inclined at an angle say 30^o with the normal. IO is the incident ray.
• Fix two pins P and Q on the incident ray IO.
• Place the glass slab within its boundary ABCD.
• Looking from the other side of the glass slab fix two other pins R and S such that P, Q, R and S appear to lie on the same straight line.

  

• Remove the glass slab and the pins. Mark the pin points P, Q, R and S.
• Join the pins R and S and produce the line on both sides. The ray O¹E is the emergent ray.
• Join OO¹. It is the refracted ray.
• The incident ray, the refracted ray and the normal are all lying in the same plane.
• This proves the first law of refraction.
• Let us now prove the second law of refraction
• With O as centre, draw a circle of a convenient radius 'R' in such a way that it cuts the incident and the refracted rays at F and G respectively.
• From F and G draw perpendiculars to the normal N₁N₂.
• D FHO and D GKO are right-angled triangles.

But OG = OF =R

• Measure the length of FH and GK and record them in the observation table.
• Repeat the experiment for different values of angle of incidence.
• Find the value of for different values of i.
• In each case it is found that the ratio is the same, that is, the ratio of is a constant.
• This verifies the second law of refraction or Snell's law.