
Concept explainers
Light is incident on a prism as shown in Figure P38.31. The prism, an equilateral triangle, is made of plastic with an index of refraction of 1.46 for red light and 1.49 for blue light. Assume the apex angle of the prism is 60.00°.
- a. Sketch the approximate paths of the rays for red and blue light as they travel through and then exit the prism.
- b. Determine the measure of dispersion, the angle between the red and blue rays that exit the prism.
Figure P38.31
(a)

The sketch of the appropriate path of the red and blue light rays through the prism.
Answer to Problem 31PQ
The sketch of the appropriate path of the red and blue light rays through the prism is as shown below.
Explanation of Solution
The wavelength of the violet light is 410 nm and the wavelength of the red light is 680 nm.
The wavelength of light is inversely proportion to the refractive index of the material through which it is passing.
The wavelength of the violet light is lesser than the wavelength of the red light. Hence, the refractive index of the violet light is more as compared to the refractive index for the red light.
Therefore, violet light refracts more as compared to the red light as shown in the figure below.
Figure-(1)
(b)

The dispersion angle between the red and blue rays when exit the prism.
Answer to Problem 31PQ
The dispersion angle between the red and blue rays when exit the prism is 2.64°.
Explanation of Solution
Write the expression for Snell’s law.
n1sinθi=n2sinθt
Here, incidence angle is θi, refractive index of the first medium is n1, refraction angle is θt and the refractive index of the second medium is n2.
Further solve the above equation for θt.
sinθt=n1n2sinθiθt=sin−1(n1n2sinθi) (I)
The prism diagram on which light is incident at an angle 45° is as shown below.
Figure-(1)
Write the expression for dispersion angle between the red and blue rays when exit the prism.
ϕ=(θ't)b−(θ't)r (II)
Here, dispersion angle between the red and blue rays when exit the prism is ϕ, refracted angle at the exit of the prism for blue light is (θ't)b and refracted angle at the exit of the prism for red light is (θ't)r.
Conclusion:
Case (i): For blue light.
Substitute 1 for n1, 1.49 for n2 and 45.0° for θi in equation (I) to find (θt)b
(θt)b=sin−1(11.49sin45°)=sin−1(0.474)=28.33°
Solve for α.
α=90°−(θt)b=90°−28.33°=61.67°
Solve for β.
β=180°−60°−α=180°−60°−61.67°=58.33°
Solve for θ'i.
θ'i=90°−β=90°−58.33°=31.67°
Substitute 31.67° for θ'i, 1.49 for n1 and 1 for n2 in equation (I) to find (θ'j)b.
(θ't)b=sin−1(1.491sin31.67°)=sin−1(0.782)=51.47°
Case (ii): For red light.
Substitute 1 for n1, 1.46 for n2 and 45.0° for θi in equation (I) to find (θt)b
(θt)r=sin−1(11.46sin45°)=sin−1(0.484)=28.96°
Solve for α.
α=90°−(θt)b=90°−28.96°=61.04°
Solve for β.
β=180°−60°−α=180°−60°−61.04°=58.96°
Solve for θ'i.
θ'i=90°−β=90°−58.69°=31.04°
Substitute 31.04° for θ'i, 1.49 for n1 and 1 for n2 in equation (I) to find (θ'j)r.
(θ'j)r=sin−1(1.461sin31.04°)=sin−1(0.752)=48.83°
Substitute 51.47° for (θ't)b and 48.83° for (θ'j)r in equation (II) to find ϕ.
ϕ=51.47°−48.83°=2.64°
Therefore, the dispersion angle between the red and blue rays when exit the prism is 2.64°.
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Chapter 38 Solutions
Physics for Scientists and Engineers: Foundations and Connections, Advance Edition, Volume 2
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