Cover one slit with thin glass.

In Young's double-slit experiment, a thin sheet of glass is placed over one of the two slits. The interference pattern:

Answer: shifts towards the covered slit; same fringe width.

A glass sheet of thickness t adds an extra optical path of (n − 1)t to the light from the covered slit. To keep the path difference zero, the central bright fringe must move to a point closer to that slit.

Δ x = \frac{(n - 1) t D}{d} (towards the covered slit)

β = \frac{λ D}{d} (unchanged)

The whole pattern shifts towards the covered slit, but the fringe width stays the same, because the wavelength in the air between the slits and the screen is unchanged. Narrower fringes come from changing the medium everywhere, not from covering one slit.

Original practice question · JEE Physics Decoded

What is Level 2?

One step past the obvious.

Every Level 2 starts from a quick question, Level 1, and changes one thing. Seeing what that change does is the skill JEE Main tests. Try it here, free and with no signup, then read the worked answer.

Practise more like this, free Continue practising All Level 2 questions

Free account · Students 15+ in India · Parent approval under 18

Get the Android app on Google Play → (opens in a new tab)

Keep going

Practise more like this, free.

Tests marked like the real exam, and a worked explanation for every question.

Start practising free

Cover one slit with thin glass.

One step past the obvious.

More Level 2 questions

Practise more like this, free.