Wave intensity

Wave Intensity:

•Waves transfer energy, therefore we can define a quantity which is a measure of the power (energy per unit time) carried by a wave.

•Intensity is the energy per unit time which is transferred by a wave through a plane of unit area perpendicular to wave’s motion.

Wave intensity

Wave intensity

Spherical Waves (a special case):

Many sources of wave energy, like sound, light etc., emit waves in all directions, so these waves travel in spherical wavefronts.

All the points on a wavefront have the same “phase”, have the same displacement (disturbance).

Wave intensity

Wave intensity

Therefore, for spherical waves, as r increases, the energy of the source is spread over larger area and so the intensity decreases.

24PIrπ

=

Wave intensity

Wave intensity

Wave intensity

Wave intensity

Problem:

At a distance 3.0 m from a sound source, the intensity is measured to be 80. dB.

a) What is the intensity in W/m2?

b) What is the power of the source

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Doppler

Problem:

A fire truck siren at rest wails at 1400 Hz. Standing on the roadside as the truck approaches, you hear it at 1600 Hz. How fast is the truck going?

Assume that the speed of sound in air is 340 m/s.

Answer: 42 m/s

Doppler

Doppler shift also happen in electromagnetic waves.

Many applications including:

•In astronomy Doppler shift is use to determine the speed of distant galaxies moving away or towards us.

•Radar detectors use the Doppler shift in electromagnetic waves to determine how fast cars are travelling.

Radar frequency shift formula

'

'''

''

frequency observed by the moving car

frequency of the reflected wave as observed by the poli

(1 )

(1 )

(1 )

( )

c

1

e

uf fc

ff uc

ucf f uc

= + ⇐

= ⇐−

+=

−⇒