Question

Radar waves are sent towards a moving airplane and the reflected waves are received. When the airplane is moving towards the radar, the wavelength of the wave

• A. decreases
• B. increases
• C. remains the same
• D. sometimes increases and sometimes decreases

Answer 1

The correct option is A

decreases

The explanation for the correct option:

In the case of option A,

1. As an airplane moves closer to the radar the apparent frequency of waves increases due to Doppler's effect.
2. As we know that wavelength and frequency are inversely proportional to each other.
3. Thus, the apparent wavelength of the wave decreases.

The explanation for the incorrect options:

In the case of option B,

1. As an airplane moves towards the radar, the apparent frequency of the reflected wave increases.
2. This way apparent wavelength will decrease after reflection, not increase.
3. So, incorrect option.

In the case of option C,

1. The wavelength cannot remain the same.
2. Instead due to Doppler's effect, it decreases.

In the case of Option D,

1. Sometimes increase or decrease is not possible because due to Doppler's effect apparent wavelength always decreases as the plane comes closer to the radar.
2. So, incorrect option.

Hence option (A) is the correct answer.