a positive rod is placed to the left of sphere a, and the spheres are separated. which best describes the spheres? a) sphere a is negative and sphere b is positive because protons from sphere a are attracted to the rod and move to sphere b. b) sphere a is negative and sphere b is positive because electrons from sphere b are attracted to the rod and move to sphere a. c) sphere a is positive and sphere b is negative because electrons from the rod move to sphere b and protons then move to sphere a. d) sphere a is positive and sphere b is negative because protons move from sphere b to the rod and also to sphere a.

B - Sphere A is negative and sphere B is positive because electrons from sphere B are attracted to the rod and move to sphere A.

Explanation:

The rod is positively charged which means that it will repel positive charges (protons) and the rod will attract negative charges (electrons)

If you look at all the options that are given each one of them has a flaw, lets look at them one by one for better understanding of the concept:  

(A) Sphere A is negative and sphere B is positive because protons from sphere A are attracted to the rod and move to sphere B.  

      -Incorrect because protons which carry a positive charge CANNOT be    attracted by the rod because as by the diagram the rod is also positively charged  

(C) Sphere A is positive and sphere B is negative because electrons from the rod move to sphere B and protons then move to sphere A.’

Incorrect because sphere B has protons and not electrons as we can see it in the diagram given below.

(D) Sphere A is positive and sphere B is negative because protons move from sphere B to the rod and also to sphere A.

Incorrect because protons cannot move from sphere B to the rod since the rod is positively charged and like charges always repel.

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.