AS Daily A Level Physics question
In a ripple tank, a progressive water wave passes through a thin plastic mesh that reduces the wave’s displacement amplitude to 70% of its value just before the mesh. Two identical meshes are then placed in series. Assuming water depth and frequency remain the same, which option best estimates the intensity of the wave immediately after the second mesh, expressed as a percentage of the intensity before the first mesh?
Answer
The correct answer is D.
Correct: D — About 24% of the original. Each mesh leaves the amplitude at 0.70 of the incident value, so after two meshes the amplitude is 0.70 × 0.70 = 0.49 of the original, and the intensity scales with the square of amplitude, giving (0.49)^2 ≈ 0.24 (about 24%). A treats only one mesh’s effect on intensity, giving (0.70)^2 = 0.49, and ignores the second identical stage. B assumes intensity scales linearly with amplitude and also fails to compound the effect of two meshes. C incorrectly mixes one stage of intensity reduction (0.49) with a separate amplitude factor (0.70), instead of squaring the combined amplitude. D is correct because intensity depends on amplitude squared and the two identical attenuations compound before squaring: (0.70 × 0.70)^2 = 0.70^4 ≈ 0.24.