Light exhibits wave nature, and one important consequence of this is diffraction — the bending and spreading of light when it passes through a narrow aperture. When a monochromatic source such as a He–Ne laser passes through a single narrow slit, a characteristic diffraction pattern is formed on a screen.

The He–Ne laser emits nearly monochromatic red light of wavelength approximately 632.8 nm, making it ideal for diffraction experiments because it produces sharp and well-defined patterns.

Let a parallel beam of monochromatic light of wavelength λ be incident normally upon a narrow slit AB of width d, where it gets diffracted as shown in Fig. 1. If a lens L is placed in the path of the diffracted beam, a real image of the diffraction pattern is formed on the screen SS′ in the focal plane of the lens.

Path Difference:

BD = AB sinθ = d sinθ    … (1)

The corresponding phase difference:

φ = (2π / λ) × path difference = (2π / λ) × d sinθ    … (2)

Now, consider the width AB of the slit divided into n equal parts. Each part forms an elementary source. The amplitude of vibration at P₀ due to the wave from each part will be the same, and the phase difference of waves from any two consecutive parts is:

e = (1/n) × (2π / λ) × d sinθ

Hence, the resultant amplitude at P₀ is given by:

A = [ a × sin(ne/2) ] / sin(e/2)    … (3)

Since the magnitude of intensity at any point in the focal plane of the lens is a function of a and θ, we obtain a series of maxima and minima.

Condition for a maximum (approximation):

• θ = 0°  or  d sinθ ≈ (k + ½) λ
• d — width of slit
• θ — angle of diffraction
• k — order of the maximum (0, 1, 2, 3, …)
• λ — wavelength

Condition for a minimum:

• θ > 0°,   d sinθ = kλ
• d — width of slit
• θ — angle of diffraction
• k — order of the minimum (1, 2, 3, …)
• λ — wavelength

Fraunhofer diffraction is shown in Fig. 2, which explains how light waves spread and form patterns (bright/dark fringes) when passing through a slit under conditions where the source and screen are effectively at infinite distances.