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# The relativistic doppler effect for light.pptx

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# The relativistic doppler effect for light.pptx

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### The relativistic doppler effect for light.pptx

1. 1. RELATIVISTIC DOPPLER ELECTED FOR LIGHT @Asan Fizika @fizika_physics
2. 2. What do you observe when you throw a pebble into a body of water? (apart from the pebble sinking). Doppler effect explained why we perceive a change in frequency when the wave source approaches or retreats from us... Doppler effect is the change in frequency of a wave for an observer moving relative to its source. The observer observes an upward shift in frequency when the wave source is approaching and a downward shift in frequency when the wave source is retreating. Doppler effect applies to all waves including; Sound waves, Light waves, Water waves. Doppler effect originated in 1842 by an Austrian physicist “Christian Doppler”. He proposed doppler effect in his article “Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels”. Translated as “On the colored light of the binary stars and some other stars of the heavens”.
3. 3. The Relativistic Doppler Effect Suppose an observer in S sees light from a source in S′ moving away at velocity v. The wavelength of the light could be measured within S′ — for example, by using a mirror to set up standing waves and measuring the distance between nodes. These distances are proper lengths with S′ as their rest frame, and change by a factor 1 − 𝑣2/𝑐2 when measured in the observer’s frame S, where the ruler measuring the wavelength in S′ is seen as moving.
4. 4. (a) When a light wave is emitted by a source fixed in the moving inertial frame S', the observer in S sees the wavelength measured in S'. to be shorter by a factor 𝟏 − 𝒗𝟐/𝒄𝟐. (b) Because the observer sees the source moving away within S, the wave pattern reaching the observer in S is also stretched by the factor (cΔt+vΔt)/(cΔt)=1+v/c
5. 5. If the source were stationary in S, the observer would see a length cΔt of the wave pattern in time Δt. But because of the motion of S' relative to S, considered solely within S, the observer sees the wave pattern, and therefore the wavelength, stretched out by a factor of as illustrated in (b).
6. 6. where 𝝀𝒔𝒓𝒄 is the wavelength of the light seen by the source in S' and 𝝀𝒐𝒃𝒔 is the wavelength that the observer detects within S.
7. 7. Red Shifts and Blue Shifts The observed wavelength 𝝀𝒐𝒃𝒔 of electromagnetic radiation is longer (called a “red shift”) than that emitted by the source when the source moves away from the observer. Similarly, the wavelength is shorter (called a “blue shift”) when the source moves toward the observer. The amount of change is determined by
8. 8. where 𝝀𝒔 is the wavelength in the frame of reference of the source, and v is the relative velocity of the two frames S and S′. The velocity v is positive for motion away from an observer and negative for motion toward an observer. In terms of source frequency and observed frequency, this equation can be written as:
9. 9. Example: Calculating a Doppler Shift Suppose a galaxy is moving away from Earth at a speed 0.825c. It emits radio waves with a wavelength of 0.525 m. What wavelength would we detect on Earth? Strategy: Because the galaxy is moving at a relativistic speed, we must determine the Doppler shift of the radio waves using the relativistic Doppler shift instead of the classical Doppler shift.
10. 10. Application of Doppler effect Doppler effect has found its use in several other area which includes; Astronomy, Vibration measurement, To Sense Gesture (computer base) Audio Velocity profile measurement etc.
11. 11. In the 1600 years or so since Doppler first described the wave phenomenon that would cement his place in history, several practical applications of the Doppler effect have emerged to serve society. In all of these applications, the same basic thing is happening. One application of Doppler effect found in nature, occurs in bats hunting for their prey. Bats navigates it’s flight by emitting whistles and listening for the echoes. When chasing prey, its brain detects a change in pitch between the emitted whistle, and the echo it receives. This tells the bat the speed of its target, and the bat adjusts its own speed accordingly. The Doppler effect is used in some types of radar, to measure the speed of detected objects. For example a police officer uses radar guns to check for speeding vehicles. The radar gun emits waves at a particular frequency, which when strikes the vehicles bounce back toward the gun. The radar gun then measure the frequency of the returning waves, then eventually determine the speed.