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Question
How will you verify the laws of refraction or how the refractive index of glass is determined in the laboratory?
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Solution
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, P3, 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 aμg.
(ii) The incident the refracted ray and normal line in the same plane.
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