Here is a widely publicized, true story about how failing to convert units resulted in a huge loss. in 1998, the mars climate orbiter probe, instead of entering orbit, crashed into the surface of mars. the resulting inquiry revealed that nasa navigators had been making minor course corrections in si units, whereas the software written by the probe's makers implicitly used british units. in the united states, most scientists use si units, whereas most engineers use the british (or imperial) system of units. (interestingly, british units are not used in the united kingdom.) for these two groups to be able to communicate with each other, unit conversions are necessary. the unit of force in the si system is the newton, n, which is defined in terms of basic si units as 1n=1(kg⋅m)/s2. the unit of force in the british system is the pound, lb, which is defined as 1lb=1(slug⋅ft)/s2, where the slug is the unit of mass.

close lines strong magnet less close weaker

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.