Asmall metal ball hangs from the ceiling by an insulating thread. the ball is attracted to a positively charged rod held nearthe ball. the charge of the ball must be (a) positive.(b) negative.(c) neutral.(d) positive or neutral.(e) negative or neutral.

(b) negative

Explanation:

As objects with opposite charges attract. And we know that the rod held near the ball has positive charge. Then the ball must be negatively charged. Otherwise positive charges would push them further and neutral charges would neither push or pull.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.

at right angles to each other