Interference of Light — Predicted 2026

AI-Predicted Question Angles for UPSC 2026

Based on trend analysis, current affairs, and recurring themes in Interference of Light.

Combined effect of medium change and thin film

high

NEET often tests the ability to combine multiple concepts. A question could involve calculating the new fringe width when the apparatus is immersed in a liquid, and then further asking about the shift caused by introducing a thin film in that liquid. This requires applying both $\lambda' = \lambda/\mu_{medium}$ and $\Delta y = \frac{D}{d}(\mu_{film} - \mu_{medium})t$, where $\mu_{film}$ is the refractive index of the film relative to the medium. This tests a deeper understanding of optical path difference in different environments.

Intensity distribution with unequal slit widths/amplitudes

medium

While most problems assume equal slit intensities ($I_1 = I_2$), questions might arise where $I_1 \neq I_2$. This leads to $I_{min} > 0$, meaning dark fringes are not perfectly dark. Students would need to apply the general intensity formula $I = I_1 + I_2 + 2\sqrt{I_1 I_2} \cos(\Delta\phi)$ or the amplitude ratio $\frac{I_{max}}{I_{min}} = \frac{(\sqrt{I_1} + \sqrt{I_2})^2}{(\sqrt{I_1} - \sqrt{I_2})^2}$. This tests a more nuanced understanding of intensity variation beyond the ideal case.

Angular fringe width and its independence from screen distance

low

While fringe width $\beta = \frac{\lambda D}{d}$ depends on D, the angular fringe width $\theta_\beta = \frac{\beta}{D} = \frac{\lambda}{d}$ is independent of the screen distance D. A question could ask about the angular separation of fringes, or how it changes if D is varied. This is a subtle point that can catch students off guard if they only focus on linear fringe width. It's a good test of conceptual clarity.