"if a 20 and a 40 ohm resistor are wired in parallel to a 20v source, what will be the current in the 40 ohm resistor? "

In parallel, neither resistor knows about the other one, and they have no effect on each other.

Each resistor is connected straight to the battery terminals. If the battery is hefty enough, it supplies current to each resistor as if the other one weren't there.

Current through a resistor = (voltage across it) / (its resistance).

Current = (20 V) / (40 ohms)

Current = 1/2 Ampere.

waves moving from a fast medium to a slow medium will experience a reflection as light travels more slowly than red light. total internal reflection occurs when the critical angle is.