I may be wrong but parallel shift is the final light beam path subtracted by the original light beam path the light would of travelled without the glass plate, so that you end up with a difference from the original beam if you trace your light beam straight through the glass plate. You should see that the 2 beams are parallel. Then use trigonometric properties and solve for the shift.
Draw the diagram that IrAlien is suggesting. Draw the plate and the incident beam. Draw the angular shift upon entering the plate. Extend that beam through the plate until you hit the glass-->air boundary, then draw the angular shift there and draw the final air beam out a ways. Then notice that the beam on the other side of the glass plate has the same direction as the incident beam, but is just shifted to the side a bit. That's when you use trig to figure out how far it is shifted. The thicker the glass, the more offset the shift is.
the full question is -Given is a parallel glass plate with a thickness of 30 mm and a refractive
index n = 1.65. A light beam hits the surface under an angle of β = 45°.
(a) What is the parallel shift of the light beam? (b) What changes when
the experiment is performed in water??
if i use snell's law to calculate for part a) then what should i do in part b) ??
because i assume part a) is between water and refractive index n = 1.65
and also may i know what should i do with the thickness given??
when the ray of light is passes through a parallel plate the emergent ray (after two refractions) is parallel to the incident ray.(apply snell's law on both surfaces). The perpendicular distance between the incident ray and the emergent ray is called parallel shift. First of all draw a ray digram for air and glass, produce the incident ray and use geometry and trigonometry to find perpendicular distance between the rays in terms of the thickness given.
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