In what year did Christian Doppler discover the "Doppler Principle"?

• A. 1886
• B. 1842
• C. 1935
• D. 1972

Answer: (B)

The "Doppler Principle," formulated by Christian Doppler, was first introduced in the year 1842. This principle describes how the frequency of a wave changes for an observer moving relative to the source of the wave. Doppler's initial work focused on sound waves, but the principle has since been extended to electromagnetic waves, including light, explaining phenomena such as the redshift observed in astronomy.

Understanding this principle is crucial in various fields, including physics and engineering, particularly in applications related to speed measurement devices. The concept has significant importance in radar technology, used for measuring speed, as it relies on the frequency shift of waves to determine the velocity of an object relative to the observer. The other years listed do not correspond to the discovery of the Doppler Principle.