If two substances react to form one product , a reaction has occurred

a - double displacement
b - single displacement
c - combustion
d - synthesis

If two substances react to form one product , a synthesis reaction has occurred

Explanation:

The reaction in which multiple substances react to form a single product then it is called a synthesis reaction.

The synthesis reaction in general form is given by:

where maybe any compound or elements and is the compound product formed by the reactants.

An example of this form of reaction is:

Formation of water

Here 2 moles of hydrogen reacts with 1 mole of oxygen to give 2 moles of water.

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.

more than, less

less than, more