Answer:
The human retina can only detect incident light that falls in waves 400 to 720 nanometers long, so we can't see microwave or ultraviolet wavelengths. This also applies to infrared lights which has wavelengths longer than visible and shorter than microwaves, thus being invisible to the human eye.
Answer:

Explanation:
Given that:

If we let the piece of the close lose contact at ∠θ;
Then ; from force balance;
we have:

where;

Again:

Answer:
Net force = 10 N in downward direction
Explanation:
Given that,
Upward force = 10 N
Downward force = 20 N
Let downward is negative and upward is positive.
Net force = 10+(-20)
= -10 N
So, the net force is 20 N and it is acting in downward direction.
I think its c I really don't take physics but I try to help.
Explanation:
heat engine is correct answer