Pls help me w this, I've been doing this since 5 minutes ago

t=0.0625s

Explanation:

F=number of swings/time taken

DATA

Frequency=4.0Hz

number of swings from Q to R

=1/4

time taken=?

Frequency=number of swings/time taken

make t the subject of the formula

t=n/f

substitute the given date

t=0.25/4.0

t=0.0625s

option A is collect

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.

3. dark matter

empty space could also be a simple vacuum