What line of longitude forms the other side of the prime meridian

The meridian directly opposite the Prime one ... roughly down the middle of the Pacific Ocean ... is the meridian of 180° longitude, both East and West.

Technically, it should also be the International Date Line, but it's not.  The International Date Line has been defined with several kinked segments in it, in order to avoid having it cut through the few islands that lie along 180°.  

If it were allowed to do that, there would be nations that would have two different calendar dates on the same island.  Kids who live on one side and go to school on the other side would constantly be either cutting school or showing up on Saturday, and people would be missing their dental appointments on the other side.  Much easier just to put a kink in the International Date Line so that whole country is on the same calendar date.

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.