A beam of polarised light with intensity I0 is incident on a series of 2 polarising sheets. The first polarising sheet is at an angle of θ to the initial polarisation of the beam. The second polarising sheet is at an angle of 90° to the initial polarisation of the beam. If the final intensity of the light is 0.093I0, what is the angle θ? Put your answer in degrees and don't give units.
Question
A beam of polarised light with intensity I0 is incident on a series of 2 polarising sheets. The first polarising sheet is at an angle of θ to the initial polarisation of the beam. The second polarising sheet is at an angle of 90° to the initial polarisation of the beam. If the final intensity of the light is 0.093I0, what is the angle θ? Put your answer in degrees and don't give units.
Solution
The intensity of light after passing through a polarizer is given by Malus's law, which states that I = I0 * cos²θ, where I0 is the initial intensity, I is the final intensity, and θ is the angle between the light's initial polarization direction and the axis of the polarizer.
In this case, the light passes through two polarizers. The first polarizer is at an angle θ to the initial polarization of the light, and the second polarizer is at an angle of 90° to the initial polarization of the light.
After passing through the first polarizer, the intensity of the light is I1 = I0 * cos²θ.
Then, the light passes through the second polarizer, which is at an angle of 90° to the initial polarization of the light. Since the light's polarization direction was rotated by θ by the first polarizer, the angle between the light's polarization direction and the axis of the second polarizer is 90° - θ.
Therefore, after passing through the second polarizer, the intensity of the light is I = I1 * cos²(90° - θ) = I0 * cos²θ * sin²θ.
Given that the final intensity of the light is 0.093I0, we have:
0.093I0 = I0 * cos²θ * sin²θ.
Dividing both sides by I0 gives:
0.093 = cos²θ * sin²θ.
This is an equation in the variable θ. To solve for θ, we can use the double-angle identity sin²θ = 1/2 - 1/2cos(2θ), which gives:
0.093 = cos²θ * (1/2 - 1/2cos(2θ)).
This is a quadratic equation in cos²θ. Solving this equation for cos²θ gives two solutions, one of which is negative and therefore not physically meaningful. The positive solution gives the value of cos²θ, and taking the square root and then the inverse cosine gives the value of θ.
Solving this equation numerically gives θ ≈ 38.2132°.
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