A certain type of film requires an exposure time of 0.013 s with an f /1.8 lens setting. Another type of film requires twice the light energy to produce the same level of exposure. What f - stop does the second type of film need with the 0.013 s exposure time? (Common f - stops are 1, 1.2, 1.4, 1.8, 2, 4, 8, 11)

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.