How many electrons does each oxygen atom gain during the course of this reaction?

The chemical reaction is the oxidation of calcium metal by molecular oxygen. During the reaction, calcium metal is oxidized, that is, loses electrons and oxygen is reduced, that is gains electrons.
During the reaction, the calcium atom donate two electrons in order to become stable and the oxygen atom accepts these two electrons. Therefore, during the reaction, EACH OXYGEN ATOM GAINS TWO ELECTRONS.

the line Spectra of hydrogen lies between the ultra-violet, visible light and infra-red of the electro magnetic spectrum

Explanation:

Electromagnetic radiation spans an wide range of wavelengths and frequencies. This range is called the electromagnetic spectrum. The electromagnetic spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency. The 7 regions includes; radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma rays.

lower-energy radiation, such as radio waves, is expressed as frequency while microwaves, infrared, visible and UV light are usually expressed as wavelength and finally, higher-energy radiation such as X-rays and gamma rays, is expressed in terms of energy per photon.

Therefore, hydrogen lies between the ultra-violet, visible light and infra-red region of the electro magnetic spectrum.