The siren of a fire engine that is driving northward at 31.0 m/s emits a sound of frequency 2020 Hz. A truck in front of this fire engine is moving northward at 19.0 m/s. a) What is the frequency of the siren's sound that the fire engine's driver hears reflected from the back of the truck?
b) What wavelength would this driver measure for these reflected sound waves?

A. Using

Fl= ( v+vl/v+vz)fz

= (340+19/340+31) x 2020

= 1954.7Hz

Then to find the frequency of sound when reflected from the truck such that the driver becomes the listener

we use

F"= ( v+vz/v+vl) fz

= 340+31/340+19 x 2020

2087.5Hz

B to find the wavelength of sound we use

Wavelength= V+vl/ F"

= 340+31/2087.5= 0.18m

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.