There could be a lot of reasons. To help break down food for the plants, kill off bad and harmful things that might damage the plant.
The slowing down of the ray of light also causes the light to change direction
Through looking at layers of rock you can tell what kind of things transpired. If you find Fish fossils you can assume that at one point in time, there may have been a body of water of some sort. if you find certain plants, you can deduce that there was a forest or a grassy pasture. Or if there are variations in the layer you may be able to tell what kind of soil was there at one point in time, or maybe if there is volcanic rock, you could rightly assume there was a volcanic eruption at some point.
Explanation:
In acid – base chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. Basic salts contain the conjugate base of a weak acid, so when they dissolve in water, they react with water to yield a solution with pH greater than 7.0.
The Hardy-Weinberg equation is as follows:


Where:
(convert all % to decimals)
p= homozygous dominant
q= homozygous recessive
pq= heterozygous
While you did not specify whether the 0.2 frequency was for dominant or recessive, we can still figure out the answer.
Using the 1st equation, we can solve for the other dominant/recessive frequency:
1-0.2=0.8
Meaning that:
p= 0.8 & q=0.2
If the heterozygouz frequency is 2pq, then it becomes a simple "plug & chug" sort of approach.
2(0.8)(0.2)= 2(0.16)= 0.32
So, the heterozygous frequency would be:
0.32
Hope this helps!