why only half ./. of solar energy enter the earth and in case of green house effect do gases absorb only half solar energy

About 30% of the incoming sunlight is reflected back into space by clouds or light areas on Earth's surface, or scattered back out into space by gas molecules in the atmosphere (that scattering is what makes the sky blue, not black!). Of the 70% that makes it through, about 2/3rds makes it to the ground, while about 1/3rd is absorbed (not reflected) by clouds.

 

So net amount that reaches Earth = 2/3rd of 70 % = 46.7 % ~ 50%