The Doppler Effect The Doppler Effect is the apparent change in the frequency of a wave due to the relative motion between the source of the wave and the observer. Consider a source S emitting a wave with crests 1, 2, 3 as shown. The distance between successive crests is the same; therefore the number of crests that pass point A in one second will correspond to the frequency of the wave. These waves will pass over an observer in equal intervals of time. This means that the wavelength and therefore the frequency will be the same. In this case the source is moving to the right while emitting the waves. The result is that: Ahead of the moving source, the crests are closer together than crests from the stationary source would be. This means that the wavelength is smaller and the frequency is greater. Behind the moving source, the crests are further apart than crests from the stationery source would be. This means the wavelengths are greater and therefore the frequency is less. Formula: f” = apparent frequency f = actual frequency c = of the wave u = speed of the moving source Remember that the sign below the line is minus if the source is moving towards the observer Is Towards’ – ‘Minus (MITS) The Doppler Effect Exam questions – Note that when explaining the effect the marking scheme looks for four separate points here: A series of circles. non-concentric Direction of motion of source and position of observer must be indicated. Reference to apparent change in wavelength. Reference to resulting apparent change in frequency. Practice re-arranging the variables in the formula to make first f, then c, and finally u the subject of the formula (with both the and the Each of these variations have been asked in different exam questions “-“ “+”). Note: The noise from a racing car as it approaches and then moves away from an observer is of the Doppler an example effect. But it is not an application!! Demonstration of the Doppler effect The Doppler effect for sound waves is dramatically demonstrated by swinging a ringing tuning fork around your head. Applications of the Doppler effect: Police speed traps Measuring the red shift of galaxies in astronomy* Using ultrasound to monitor the blood movement or heartbeat of a foetus. Weather forecasting. Leaving Cert Physics Syllabus
The Doppler Effect The Doppler Effect is the apparent change in the frequency of a wave due to the relative motion between the source of the wave and the observer. Consider a source S emitting a wave with crests 1, 2, 3 as shown. The distance between successive crests is the same; therefore the number of crests that pass point A in one second will correspond to the frequency of the wave. These waves will pass over an observer in equal intervals of time. This means that the wavelength and therefore the frequency will be the same. In this case the source is moving to the right while emitting the waves. The result is that: Ahead of the moving source, the crests are closer together than crests from the stationary source would be. This means that the wavelength is smaller and the frequency is greater. Behind the moving source, the crests are further apart than crests from the stationery source would be. This means the wavelengths are greater and therefore the frequency is less. Formula: f” = apparent frequency f = actual frequency c = of the wave u = speed of the moving source Remember that the sign below the line is minus if the source is moving towards the observer – ‘Minus Is Towards’ (MITS) The Doppler Effect – Exam questions Note that when explaining the effect the marking scheme looks for four separate points here: A series of non-concentric circles. Direction of motion of source and position of observer must be indicated. Reference to apparent change in wavelength. Reference to resulting apparent change in frequency. Practice re-arranging the variables in the formula to make first f, then c, and finally u the subject of the formula (with both the “-“ and the “+”). Each of these variations have been asked in different exam questions Note: The noise from a racing car as it approaches and then moves away from an observer is an example of the Doppler effect. But it is not an application!! Demonstration of the Doppler effect The Doppler effect for sound waves is dramatically demonstrated by swinging a ringing tuning fork around your head. Applications of the Doppler effect: Police speed traps Measuring the red shift of galaxies in astronomy* Using ultrasound to monitor the blood movement or heartbeat of a foetus. Weather forecasting. Leaving Cert Physics Syllabus
16.%20Waves.docx