"true or false. in a phase contrast microscope, the phase plate causes undiffracted light waves to undergo a phase shift by 1/2 wavelength."

False

In a phase contrast microscope, the phase plate causes diffracted light waves to undergo a phase shift by 1/2 wavelength.

Explanation:

Phase contrast microscope is dependent on the difference in refracting index from object to surrounding. Light beam passes through the organism and it gets refracted through it. The phase change is difference in the refracting index of the two medium through which light passes.

Thus, the diffracted beam of light causes the phase shift by 1/2 wavelength.

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.