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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Geometrical Optics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Geometrical Optics

A 1.00 m long pole stands half-dipped in a swimming pool where the...

A 1.00 m1.00 \text{ m} long pole stands half-dipped in a swimming pool where the water level is 50.0 cm50.0 \text{ cm} above the bed. The refractive index of the water is 1.331.33, and sunlight strikes at an angle of 45^{}^{{}^{\circ}} with the vertical. Determine the length of the pole's shadow on the bed.

Options

  1. A116.5 cm116.5 \text{ cm}
  2. B87.6 cm87.6 \text{ cm}
  3. C100.0 cm100.0 \text{ cm}
  4. D81.5 cm81.5 \text{ cm}

Correct answer

D. 81.5 cm81.5 \text{ cm}

Step-by-step solution

The total length of the pole is 100 cm100 \text{ cm}. Since it is half-dipped in the pool, the length of the pole above the water surface is 50 cm50 \text{ cm} and the length below the water surface is 50 cm50 \text{ cm}.

The sunlight strikes at an angle of 45^{}^{{}^{\circ}} with the vertical. The shadow cast by the portion of the pole above the water on the water surface has a length x1x_1 given by:

x_1 = 50 \tan 45^{}^{{}^{\circ}} = 50 \text{ cm}

The light ray from the tip of the pole strikes the water surface at an angle of incidence i = 45^{}^{{}^{\circ}}. Let the angle of refraction be rr. Applying Snell's law at the air-water interface:

sin⁡i=μsin⁡r\sin i = \mu \sin r

\sin 45^{}^{{}^{\circ}} = 1.33 \sin r

sin⁡r=11.332\sin r = \dfrac{1}{1.33\sqrt{2}}

Taking 2≈1.41\sqrt{2} \approx 1.41, we get:

sin⁡r≈11.33×1.41≈0.533\sin r \approx \dfrac{1}{1.33 \times 1.41} \approx 0.533

The tangent of the angle of refraction is:

tan⁡r=sin⁡r1−sin⁡2r≈0.5331−(0.533)2≈0.630\tan r = \dfrac{\sin r}{\sqrt{1 - \sin^2 r}} \approx \dfrac{0.533}{\sqrt{1 - (0.533)^2}} \approx 0.630

The refracted ray travels a vertical depth of 50 cm50 \text{ cm} to reach the bed of the pool. The horizontal distance x2x_2 covered by the refracted ray in the water is:

x2=50tan⁡r=50×0.630=31.5 cmx_2 = 50 \tan r = 50 \times 0.630 = 31.5 \text{ cm}

The total length of the pole's shadow on the bed is the sum of the horizontal distances:

L=x1+x2=50+31.5=81.5 cmL = x_1 + x_2 = 50 + 31.5 = 81.5 \text{ cm}

Answer: 81.5 cm81.5 \text{ cm}
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