# Doppler effect

#### Karol

1. Homework Statement
I read that when a body approaches a stationary one which emits radar waves, the wavelength λ2 of the returned waves is longer than λ1, the wavelength of the emitted ones, but I get the opposite. I suppose this is true for sound waves also.
See picture.
3. The Attempt at a Solution
Velocity of sound in the air: u
Velocity of approaching object: v
The frequency at which the waves from the stationary source hit the moving object:
$$f_1=\frac{u+v}{\lambda_1}$$
The wavelength of the reflected wave:
$$\lamda_2=\frac{u-v}{f_1}=\frac{(u-v)\lambda_1}{u+v}$$
Now:
$$\frac{u-v}{u+v}<1\rightarrow\lambda_1>\lambda_2$$

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#### Doc Al

Mentor
Your calculation looks OK. But why would you think that the reflected waves would have a longer wavelength? Where did you read that?