A surface is bombarded by particles, each of mass small 'm', which have velocity normal to the surface. On average, n particles strike unit area of the surface each second
and rebound elastically. What is the pressure on the surface?

A. nmv
B. 2nmv
C. nmv²
D. 1/2nmv²

B. 2nmv

Explanation:

Pressure is force over area.

P = F / A

Force is mass times acceleration.

F = ma

Acceleration is change in velocity over change in time.

a = Δv / Δt

Therefore:

F = m Δv / Δt

P = m Δv / (A Δt)

The total mass is nm.

The change in velocity is Δv = v − (-v) = 2v.

A = 1 and Δt = 1.

Plugging in:

P = (nm) (2v) / (1 × 1)

P = 2nmv

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.