Young's Double Slit Experiment Overview
📌 Thomas Young first demonstrated the interference of light using this experiment around 1801, challenging prior understanding.
💡 The original setup used a pinhole followed by two smaller pinholes, which resulted in faint and blurred interference fringes that were difficult to observe.
🔬 To improve visibility, the pinholes were replaced with narrow slits, and sunlight was replaced with a monochromatic light source (like a sodium lamp, typically having a wavelength $\lambda$ around $500 \text{ nm}$ to $1000 \text{ nm}$).
🌟 The resulting phenomenon, using two slits illuminated by coherent light, is known as the Double Slit Experiment, producing an alternating pattern of bright and dark fringes on a screen.

Experimental Setup and Coherence
📏 The arrangement involves a primary light source, a single slit (acting as a secondary source producing cylindrical wavefronts), and two narrow slits ($S_1$ and $S_2$) separated by a small distance ($d$).
🕯️ Coherence is crucial: sources must share the same frequency and phase relationship, which is achieved in this setup because light passing through $S_1$ and $S_2$ originates from the same primary wavefront.
🌊 Wavefronts emerging from $S_1$ and $S_2$ are cylindrical; when they overlap, they interfere constructively (bright fringes) or destructively (dark fringes).

Path Difference Calculation
📐 The path difference ($\Delta P$) between the waves reaching an arbitrary point $P$ on the screen at a distance $D$ from the slits (where $D \gg d$) is derived using geometric construction involving the distances $y$ (from the center $O$ to $P$) and $d$.
📐 The derived expression for the path difference is $\Delta P = S_2P - S_1P \approx \frac{xd}{D}$, where $x$ is the distance of point $P$ from the center $O$.
📐 The final path difference calculation simplification, based on the identity $S_2P^2 - S_1P^2 = 2x\frac{d}{2} = xd$, results in the path difference being $\Delta P = \frac{xd}{D}$.

Conditions for Bright and Dark Fringes
🌟 Bright Fringes (Constructive Interference): Occur when the path difference is an integral multiple of the wavelength: $xd/D = m\lambda$, where $m = 0, \pm 1, \pm 2, \dots$.
⚫ Dark Fringes (Destructive Interference): Occur when the path difference is a half-integral multiple of the wavelength: $xd/D = (m + 1/2)\lambda$, where $m = 0, \pm 1, \pm 2, \dots$.
📏 The Fringe Width ($\beta$), the separation between two consecutive bright or dark fringes, is calculated as $\beta = \frac{\lambda D}{d}$.

Key Points & Insights
➡️ To achieve observable interference, the light source must be monochromatic and the slits must act as coherent secondary sources originating from a single primary source.
➡️ The position of the $m^{th}$ bright fringe from the center is given by $x_m = \frac{m\lambda D}{d}$.
➡️ The experiment confirms the wave nature of light by demonstrating interference patterns resulting from the superposition of light waves.

📸 Video summarized with SummaryTube.com on Dec 31, 2025, 16:48 UTC