• Tags: YDSE, medium, white-light
• Difficulty: Moderate

When YDSE is performed in a medium of refractive index n, the wavelength changes to λ' = λ/n while the frequency remains constant. Fringe width becomes β' = λ'D/d = λ$\frac{D}{nd}$ = β/n — the pattern compresses. With white light: the central fringe is white (all wavelengths have zero path difference there). Higher-order fringes are colored because different wavelengths have different fringe widths (β_red > β_violet). After about 4-5 orders, the pattern washes out due to overlapping of different colors. The first visible color away from center is violet (smallest β), and the outermost is red (largest β).