What is the primary difference between single-slit diffraction and double-slit interference?

The primary difference lies in the source of interference and the resulting pattern. In single-slit diffraction, interference occurs between secondary wavelets originating from different points *within the same single slit*. This produces a central bright maximum that is significantly wider and brighter than the secondary maxima, which rapidly decrease in intensity. In double-slit interference, interference occurs between waves originating from *two distinct, coherent slits*. This typically produces equally spaced bright fringes of nearly uniform intensity (assuming the slits are very narrow), modulated by a diffraction envelope.

Why is the central maximum in single-slit diffraction so much wider and brighter than the secondary maxima?

The central maximum occurs at $ heta = 0$, where all secondary wavelets from the entire slit arrive in phase, leading to maximum constructive interference. Its width is defined by the first minima on either side ($a sin heta = pm lambda$). For secondary maxima, constructive interference is only partial, occurring when the path difference between the extreme ends is an odd multiple of $lambda/2$. The effective number of wavelets contributing constructively is much smaller, leading to significantly lower intensity and narrower width compared to the central maximum.

How does changing the slit width affect the diffraction pattern?

The width of the central maximum is inversely proportional to the slit width ($W = 2lambda D/a$). If the slit width '$a$' is increased, the central maximum becomes narrower and more intense. Conversely, if the slit width is decreased, the central maximum becomes wider and less intense. If the slit becomes very wide compared to the wavelength, diffraction effects become negligible, and the light essentially casts a sharp image of the slit.

What happens to the diffraction pattern if monochromatic light is replaced by white light?

If monochromatic light is replaced by white light, the diffraction pattern will consist of a central white maximum. This is because all wavelengths (colors) of white light have their central maxima at $ heta = 0$. However, the secondary maxima will be colored. Since the angular width of the maxima depends on wavelength ($ heta propto lambda$), blue light (shorter wavelength) will have narrower fringes closer to the center, while red light (longer wavelength) will have wider fringes further away. This results in a spectrum of colors in the secondary maxima, with violet closer to the central maximum and red further away.

What is the significance of the condition $a sin heta = nlambda$?

The condition $a sin heta = nlambda$ is the fundamental equation for locating the dark fringes (minima) in a single-slit diffraction pattern. It states that destructive interference occurs when the path difference between the wavelets from the extreme edges of the slit is an integer multiple of the wavelength. This condition is derived by conceptually dividing the slit into segments where wavelets from corresponding points cancel each other out, leading to zero intensity at these specific angles.

Single Slit Diffraction

Physics

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Single slit diffraction is a phenomenon where light, upon passing through a narrow opening (a slit) whose width is comparable to its wavelength, spreads out and produces a characteristic pattern of bright and dark fringes on a screen. This pattern is not merely a sharp image of the slit but rather a central bright band flanked by progressively weaker and narrower bright bands, separated by dark re…

Quick Summary

Single-slit diffraction is the spreading of light waves as they pass through a narrow opening, resulting in a characteristic pattern of bright and dark fringes on a screen. This phenomenon is a direct consequence of the wave nature of light and Huygens' principle, where every point in the slit acts as a source of secondary wavelets that interfere.

The pattern consists of a very wide and intense central bright maximum, flanked by progressively weaker and narrower secondary bright maxima, separated by dark minima. The conditions for these dark minima are given by $a sin \theta = nlambda$ , where ' $a$ ' is the slit width, ' $heta$ ' is the angle of the minimum, ' $lambda$ ' is the wavelength, and ' $n$ ' is an integer ( $pm 1, pm 2, dots$ ).

The linear width of the central maximum on a screen at distance $D$ is $W = \frac{2lambda D}{a}$ . Key relationships include: wider slits produce narrower central maxima, and longer wavelengths produce wider central maxima.

This phenomenon is crucial for understanding the resolution limits of optical instruments.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Condition for Minima

The dark fringes in a single-slit diffraction pattern occur at specific angles where destructive interference…

Linear Width of Central Maximum

The central maximum is the brightest and widest part of the diffraction pattern. Its linear width on a screen…

Effect of Medium on Diffraction Pattern

When the entire single-slit diffraction apparatus (slit, light source, and screen) is immersed in a medium…

Diffraction: — Bending of waves around obstacles/apertures.

Single Slit Minima: —

a sin \theta = nlambda

, where

n = pm 1, pm 2, dots

Angular Width of Central Max: —

Delta \theta = \frac{2lambda}{a}

(in radians)

Linear Width of Central Max: —

W = \frac{2lambda D}{a}

Central Max: — Brightest, widest (twice width of secondary maxima).

Secondary Maxima: — Weaker, narrower, intensity decreases with order.

Effect of Medium: —

lambda' = lambda/mu

, so

W' = W/mu

Proportionalities: —

W propto lambda

W propto D

W propto 1/a

For Single Slit Minima: 'A Sinful Noodle Lambda' (a sin $\theta$ = n $\lambda$ ). For Central Max Width: '2 Large Donuts, please, A-side' (2 $\lambda$ D/a).