Light is directly responsible for how strong you are. If you haven't realized that by now, then I am honestly shocked; it is how its been in this game since Day 1.
Its why they put "recommended light level' on the raid, nightfall, and heroic.
Regardless of all that, here is the math that determines how well you do in the game when attacking any target
The math that determines how much damage you do, versus the enemies damage reduction of your target, is the same in PVE as in PVP.
For every 25 light you have above your target, and its weapons, you enjoy a %5 reduction in damage. This is in addition to another %5 auto added for being different light levels.
Example: Here is how it breaks down:
level 40 guardian, Light level 305, being attacked by a level whatever, light level 170 character
(305-170)/25*5%+5% = 32%
305 light level defender
subtract
170 for attacker
divided by 25
multiplied by 5
and then add in the last 5 for the auto-penalty
this equals 32% reduction in damage. Or if you prefer, if you hit for 100 points of damage, the defender only ends up taking 68 points.
NOTE: This does not take into account Critical hit bonuses. Only damage reduction per bullet hitting a target. Burst weapons are still reduced but each bullet quickly adds up.