Plate, Film, and Wedge Fringe Shift
A normally traversed plate of thickness t and refractive index mu replacing air adds optical path (mu-1)t. In YDSE this shifts the central fringe by D(mu-1)t/d, or by (mu-1)t/lambda fringe widths.
Why this shows up in the exam
Measuring sheet thickness · Refractometry · Interferometer phase control
Learn the idea
Extra optical path in one arm shifts the whole interference pattern without changing its basic spacing. Putting glass before one slit delays that wave. The screen point that used to balance the paths must move until geometry supplies the opposite delay.
🧠 Memory hook: Glass adds (mu minus one) times thickness.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Delta_plate = (mu - 1)t — extra optical path relative to air
- N_shift = (mu - 1)t/lambda — shift measured in fringe widths
- y_shift = D(mu - 1)t/d — linear shift of the central fringe
How to approach it
- 1Compute each arm's added optical path
- 2Subtract arm delays with a sign
- 3Report magnitude only if direction is not requested
Common slip-ups that cost marks
- •Using mu t instead of (mu-1)t
- •Changing fringe width when only one uniform plate is inserted
- •Losing the shift direction sign
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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