The observed frequency refers to the frequency at which an observer perceives a sound wave or light wave due to relative motion between the source and observer.
Imagine you are standing on a sidewalk while cars pass by. As each car approaches you, its engine sounds higher-pitched (higher frequency), but as it moves away from you, its engine sounds lower-pitched (lower frequency). This change in pitch is similar to how observed frequency changes due to relative motion.
Doppler Effect: The Doppler effect describes how the perceived frequency of waves changes when there is relative motion between the source of waves and an observer.
Redshift/Blueshift: Redshift refers to an increase in wavelength (and decrease in observed frequency) of light waves due to objects moving away from us. Blueshift refers to a decrease in wavelength (and increase in observed frequency) of light waves due to objects moving towards us.
Sonic Boom: A sonic boom occurs when an object moves faster than the speed of sound, creating shockwaves that produce a loud noise.
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