a) What must the phase difference between the reference light and the light that
ID: 1430024 • Letter: A
Question
a) What must the phase difference between the reference light and the light that transversed the sample at A?
b) Use the idea that a light wave of wavelength (gamma) that has traveled a distance d through a material of refractive index n accumulates a phase of 2(pi)(n)(d)/(gamma), to write expressions for the phase acquired by light transversing the sample at A and B.
c)Determine the phase difference between the reference light and the light that transversed the sample at B, and use this to show that the bacteria will appear bright in the image.
C. A variation on a standard microscope is needed to view a sample that is transparent in visible light. In an interference microscope, the illumination is by coherent light. The light transmitted through the sample is made to interfere with a reference beam before reaching the eyepiece of the instrument, where the image is viewed. An interference microscope (with illumination at a wavelength of 633 nm) is used to look at bacteria characterised by thickness, 1 = 15.0 m and refractive index n-1.35, in water of refractive index n.-1.33. A schematic diagram of the sample in cross-section is showrn. reference beam is adjusted so that destruective interference occurs in the image at A, and overthe entire background of the image of the sample (where light travelled through water and glass only). glass coverslip water bacterium bacterium glass microscope slideExplanation / Answer
a) phase difference between the reference light = 2 * 3.14 * 15 * 10-6*1.33/633*10-9
= 197.92
b) phase acquired by light = 2 * 3.14 * 1.33 * 15 * 10-6*633*10-9/3*108
= 2.643* 10-19
c) phase difference between the reference light = 2 * 3.14 * 15 * 10-6*1.35/633*10-9
= 200.9
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