If I walk 15 minutes miles north and then 20 miles South: What’s my distance?

What’s my displacement ?

Actually Welcome to the Concept of the Kinematics.

1.) Distance travelled = 15+20 = 35 miles.

Ai the total distance covered is 35 miles.

2.) Displacement is the shortest distance from the initial point to the final point.

here it's ==> 20-15 = 5 miles

hence, displacement = 5 miles

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.