When a central dark fringe is observed in reflection in a circular interference pattern, waves reflected from the upper and lower surfaces of the medium must have a phase difference, in radians, of

Explanation:

Let the first wave is :

And another wave is :

is phase difference between waves

Let y is the resultant of these two waves. So,

The waves reflected from the upper and lower surfaces of the medium, it means that the resultant to be zero. So,

So, the phase difference between the two waves is .

i think if it were me i whould divide 300 dide 15

step-by-step explanation:

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.