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If a coin at the bottom of a lake appears to be at a depth of 6 m, then the gauge pressure on the coin in pascal is (μ of water is 4/3)

  • A.\(8.2 \times 10^{3}\)
  • B.\(78.4 \times 10^{3}\)
  • C.\(38.2 \times 10^{3}\)
  • D.\(4.5 \times 10^{4}\)
  • E.\(29.4 \times 10^{3}\)

Answer: B

The gauge pressure is \(78.4 \times 10^{3}\) Pa.

The apparent depth is the real depth divided by the refractive index, so the real depth is \(6 \times \frac{4}{3} = 8\) m.

\[ \begin{aligned} p &= h\rho g = 8 \times 1000 \times 9.8 \\ &= 78.4 \times 10^{3}\ \text{Pa} \end{aligned} \]

Using the apparent depth 6 m gives \(58.8 \times 10^{3}\) Pa, and dividing by \(\frac{4}{3}\) instead of multiplying gives \(4.5\) m and about \(44 \times 10^{3}\) Pa.

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