Question

How will you verify the laws of refraction or how the refractive index of glass is determined in the laboratory?

Answer 1

Place a white sheet of paper on a drawing board. Put a rectangular slab of glass on this white sheet and trace out its boundary ABCD. At any point R on the edge AB, draw a normal NN’. Draw an incident ray PR at any angle say 30°. Fix two pins P₁ and P₂ vertically on the line PR and at a distance not less than 3 cm from each other. Looking through the other side of the block, fix two pins, P₃ and P₄ vertically on the white sheet, such that all the four pins P₁, P₂, P_3, and P₄ appear to be in a straight line as seen through the glass block. Mark the pinpoints with a fine pencil, remove the block and join the points P₃ and P₄ by the straight line RS.

PR represents the incident ray and RS represents the refracted ray. ∠PRN and ∠KRN’ are respectively angles of incidence and refraction.
With R as centre and convenient radius, draw a circle cutting the incident ray at Q and the refracted ray at Q’.

From Q and Q’, draw perpendiculars QT and QT’ on the normal NN’
Now sin i = `"QT"/"QR"`
and Sin r = `"Q’T’"/"Q’T’"`
μ = `Sin"i"/Sin"r"="QT"/"QR"."Q’R"/"Q’T’"="QT"/"Q’T’"`
Since QR = Q’R being the radius of the same circle.
Repeat this experiment for other angles of incidence, say 45° and 60°, and find μ.
It is found that for different values of i the ratio.
`sin"i"/Sin"r"` = constant
Thus, it is verified that,
(i) `sin"i"/Sin"r"` is a constant and is equal to _aμ_g.
(ii) The incident the refracted ray and normal line in the same plane.