Question

Difficulty: MediumWave Phenomena: Reflection, Refraction, Interference, Diffraction, and Polarization

A water wave traveling in deep water has a wavelength of 0.80 m0.80\text{ m} and a speed of 2.4 m/s2.4\text{ m/s}. Upon entering a shallow region, its speed drops to 1.8 m/s1.8\text{ m/s}. What are the frequency and wavelength of the wave in the shallow region?

  1. Frequency = 3.0 Hz3.0\text{ Hz}, Wavelength = 0.60 m0.60\text{ m}Answer
  2. B
    Frequency = 2.25 Hz2.25\text{ Hz}, Wavelength = 0.80 m0.80\text{ m}
  3. C
    Frequency = 2.25 Hz2.25\text{ Hz}, Wavelength = 0.60 m0.60\text{ m}
  4. D
    Frequency = 0.33 Hz0.33\text{ Hz}, Wavelength = 0.60 m0.60\text{ m}

Answer

Frequency = 3.0 Hz3.0\text{ Hz}, Wavelength = 0.60 m0.60\text{ m}
When a wave passes from deep to shallow water (refraction), its frequency remains unchanged because frequency is fixed by the source. Using v=fλv = f\lambda, the initial frequency is f=2.40.80=3.0 Hzf = \frac{2.4}{0.80} = 3.0\text{ Hz}. In shallow water, the new wavelength is λ=1.83.0=0.60 m\lambda' = \frac{1.8}{3.0} = 0.60\text{ m}.

Step-by-Step Solution

1
Calculate the frequency of the wave in deep water using the wave equation v=fλv = f \lambda.
f=vλ=2.4 m/s0.80 m=3.0 Hzf = \frac{v}{\lambda} = \frac{2.4\text{ m/s}}{0.80\text{ m}} = 3.0\text{ Hz}.
The frequency depends on the wave source and can be determined from the given initial speed and wavelength.
2
Apply the boundary condition for wave refraction.
The frequency in shallow water remains f=3.0 Hzf = 3.0\text{ Hz}.
When a wave travels from one medium to another, its frequency remains constant.
3
Calculate the new wavelength in shallow water using λ=vf\lambda' = \frac{v'}{f}.
λ=1.8 m/s3.0 Hz=0.60 m\lambda' = \frac{1.8\text{ m/s}}{3.0\text{ Hz}} = 0.60\text{ m}.
The wavelength changes proportionally with speed when frequency is constant.

Key Concept

Constancy of wave frequency during refraction across medium boundaries
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