(a) A beam of light consisting of two wavelengths $400\text{ nm}$ and $600\text{ nm}$ is used to illuminate a single slit of width $1\text{ mm}$. Find the least distance of the point from the central maximum where the dark fringes due to both wavelengths coincide on the screen placed $1.5\text{ m}$ from the slit. OR (b) In a Young's double-slit experimental set-up with slit separation $0.6\text{ mm}$ a beam of light consisting of two wavelengths $440\text{ nm}$ and $660\text{ nm}$ is used to obtain interference pattern on a screen kept $1.5\text{ m}$ in front of the slits. Find the least distance of the point from the central maximum where the bright fringes due to both the wavelengths coincide.

Year	Paper Code	Set	Position
CBSE 2026	Series 55-1	Set 1	Question 18
CBSE 2026	Series 55-1	Set 2	Question 17
CBSE 2026	Series 55-1	Set 3	Question 19

• Step 1: Stating the fundamental formula / law statement (+1 Mark)
• Step 2: Intermediate mathematical substitution or proof steps (+1 Mark)
• Step 3: Correct final value or conclusion with mandatory SI units (+1 Mark)