Young's Double Slit Experiment - Physics Class 12

Jun 20 2026

Aim: To demonstrate the interference of light and determine the wavelength of monochromatic light using Young's Double Slit setup.

Apparatus Required

A monochromatic light source (e.g., Sodium lamp or a Laser)
Two narrow, closely spaced slits (usually etched on a glass plate)
A screen or a traveling microscope with an eyepiece
Optical bench with uprights
Measuring scale

Theory & Principle

Young's Double Slit Experiment demonstrates the principle of superposition of light waves. When monochromatic light passes through two closely spaced narrow slits ($S_1$ and $S_2$), they act as coherent sources of light.

The waves emanating from these two slits interfere with each other. On a screen placed at a distance, a pattern of alternate bright and dark bands (fringes) is formed.

Constructive Interference (Bright Fringes): Occurs when the path difference is an integral multiple of the wavelength ($\Delta x = n\lambda$).

Destructive Interference (Dark Fringes): Occurs when the path difference is an odd multiple of half the wavelength ($\Delta x = (2n-1)\lambda/2$).

Fringe Width ($\beta$): The distance between two consecutive bright or dark fringes.
Formula: $\beta = \frac{\lambda D}{d}$
Where:
$\lambda$ = wavelength of light
$D$ = distance between the slits and the screen
$d$ = distance between the two slits

Procedure

Mount the light source, the double slits, and the screen (or eyepiece) on the optical bench.
Align them so they are at the same height and in a straight horizontal line.
Turn on the monochromatic light source and allow it to illuminate the slits.
Look through the eyepiece or observe the screen. Adjust the distance between the slits and the screen until a clear, sharp interference pattern of bright and dark fringes is visible.
Measure the distance between the slits ($d$) using a traveling microscope if it is not already provided.
Measure the distance from the slits to the screen ($D$) using the scale on the optical bench.
Using the crosswire of the eyepiece, measure the position of the $1^{st}$ bright fringe on one side of the central maximum, then move to the $n^{th}$ bright fringe and measure its position.
Calculate the average fringe width $\beta$.
Using the formula $\lambda = \frac{\beta d}{D}$, calculate the wavelength of the light.

Observation Table

S.No.	Distance between slits & screen $D$ (cm)	No. of fringes measured ($n$)	Width of $n$ fringes (cm)	Fringe width $\beta$ (cm)
1
2
3

Result

The interference pattern consisting of alternate bright and dark fringes was observed, confirming the wave nature of light. The wavelength of the given monochromatic light is calculated to be _______ $\AA$.

Precautions

The two slits must be very narrow and close to each other to obtain a widely spaced, observable fringe pattern.
The source must be strictly monochromatic to avoid overlapping of different fringe patterns.
The slits and the light source must be parallel to each other.
The experiment should be performed in a dark room for clear visibility of the fringes.

Viva Questions & Answers

Q1: What are coherent sources?

Coherent sources are sources of light that emit waves of the same frequency and maintain a constant phase difference with respect to each other over time.

Q2: Why do we use a single source to illuminate the two slits?

Independent light sources cannot be coherent because light is emitted by independent atoms in random, rapid bursts. By deriving the two slits from a single primary source, coherence is guaranteed.

Q3: What happens to the fringe width if the entire apparatus is immersed in water?

The wavelength of light decreases in a denser medium like water ($\lambda_{water} = \lambda_{air} / \mu$). Since fringe width $\beta$ is directly proportional to wavelength, the fringe width will decrease.