# Doppler effect - Apparent change in frequency or wavelength?

• amenhotep
In summary, the Doppler effect is a phenomenon that causes a change in frequency or wavelength of a wave when there is relative motion between the source of the wave and the observer. This can be observed in cases where the source is moving towards or away from the observer, or when the observer is moving towards or away from the source. However, in case 3, where there is a change in wavelength as the wave enters a different medium, it is not considered an example of the Doppler effect because there is no relative motion between the source and the observer. The Doppler effect is only defined as the apparent change in frequency, not wavelength, and can be seen in phenomena such as red shift and blue shift.

#### amenhotep

Hello,
I've been thinking about what exactly is the doppler effect and I'm not sure I understand it because I thought it meant a change in frequency due to relative motion while at the same time its defined in terms of changing wavelength such as in red shift or blue shift.

Case 1
The ubiquitous example is that of a wave source moving toward or away from the an observer emitting waves in the process. If the observer is stationary and the source is moving toward him with speed v_s, since the speed of the wave, v, is determined by the medium, a reduction in the wavelength measured means an increment in frequency.
So in this case, the wave speed doesn't change while the frequency and wavelength of the wave received changes.

Case 2
Let the wave source be stationary and the observer move toward it. With respect to the observer, the wave fronts will appear to be hitting him with speed v+v_s. The wavelength measured is the same and so the frequency has to increase.
So in this case, the wave speed appears to change causing the apparent frequency to change while the wavelength is the same.

Case 3
Let a wave traveling in a vacuum enter another medium such glass. The frequency does not change but since the wave slows down, the wavelength measured in the glass medium is smaller.
So in this case, the wave speed decreases causing the wavelength to reduce while the frequency remaining constant.

I know that case 1 is an example of the doppler effect. Is case 2 an example of the doppler effect ? If it is so, why case 3 is not because I've never heard or read that its an example of doppler effect, but if we say that a star is undergoing a red shift is an example of the doppler effect, then obviously a change in wavelength inside the glass should be considered too. If not, why ? Is it because, doppler effect is defined only as the apparent change in frequency ? If that is the case then, red shift or blue shift are improper terms ?

Thanks

amenhotep said:
Is case 2 an example of the doppler effect ?
Yes.

Case 3 is not an example because nothing is moving relative to something else.
You can get the same with sound going from one medium to another.

## 1. What is the Doppler effect?

The Doppler effect is the apparent change in frequency or wavelength of a wave as perceived by an observer when the source of the wave is moving relative to the observer. This effect is commonly observed with sound waves, such as the change in pitch of a siren as an ambulance drives by.

## 2. What causes the Doppler effect?

The Doppler effect is caused by the relative motion between the source of the wave and the observer. When the source is moving towards the observer, the perceived frequency increases, and when the source is moving away, the perceived frequency decreases.

## 3. How is the Doppler effect used in everyday life?

The Doppler effect has many practical applications in everyday life. It is used in weather forecasting to track the movement of storms, in police radar guns to measure the speed of moving vehicles, and in medical imaging techniques such as ultrasound to create images of internal structures.

## 4. Can the Doppler effect be observed with all types of waves?

Yes, the Doppler effect can be observed with all types of waves, including sound, light, and water waves. However, the effect is most noticeable with sound waves because they have a lower frequency and travel at a slower speed than light waves.

## 5. How is the Doppler effect related to the redshift and blueshift of light from distant galaxies?

In astronomy, the Doppler effect is used to explain the redshift and blueshift of light from distant galaxies. When a galaxy is moving away from Earth, the light it emits is stretched, resulting in a longer wavelength and a shift towards the red end of the visible spectrum. Conversely, when a galaxy is moving towards Earth, the light is compressed, resulting in a shorter wavelength and a shift towards the blue end of the spectrum.