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# Using the laser and triangular block nd the critical

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Unformatted text preview: ritical angle. Verify this by sending a light ray through the prism and noting that at certain angles the ray is internally reﬂected so that it does not exit the side of the prism which is opposite the light source (all the light bends more than ninety degrees from its original direction of travel). Using the laser and triangular block, ﬁnd the critical angle for the acrylic block. Looking at the setup in Figure 2, you can measure the angle labelled α . Vary the incident beam (the laser) until no light is refracted out of the block. You should be able to measure α to within 1-2 degrees. Since we are using an equilateral triangle, we can use some simple geometry to determine the critical angle. α + β = 90◦ γ + θcrit = 90◦ (6) β + γ + 60◦ = 180◦ (7) (90◦ − α ) + (90◦ − θcrit ) + 60◦ = 180◦ (8) θcrit = 60◦ − α (9) Use Equation 4 and your critical angle to determine the index of refraction of the acrylic block. It should match your previous results. Does it? You can also see total internal reﬂection phenomenon directly by picking up the rectangular acrylic block and looking through the small...
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