13 Jul 2026

Wavelength of Laser Light by Plane Diffraction Grating

practical pg-i laser diffraction-grating wavelength

Aim

To determine the wavelength of a laser using a plane transmission diffraction grating.

Apparatus

Laser, plane grating, screen, metre scale, grating holder, and optical bench.

Experimental arrangement

The laser beam is incident normally on the grating and the positions of the symmetric diffraction orders are measured on the screen.

Theory

A grating has a large number of equally spaced transparent slits. Waves from successive slits interfere constructively when the path difference is an integral multiple of the wavelength. For normal incidence,

\[d\sin\theta=n\lambda,\]

where $d$ is the grating element, $n$ is the order, and $\theta$ is the diffraction angle. If the distance from grating to screen is $D$ and the distance of the $n$th order from the central maximum is $x$, then $\tan\theta=x/D$.

Observations

Order $n$	Left distance (cm)	Right distance (cm)	Mean $x$ (cm)
1	18.4	18.6	18.5
2	38.5	38.7	38.6
3	64.1	64.3	64.2

With $d=1/600000$ m and $D=100$ cm, the mean wavelength is approximately $650$ nm.

Result

The wavelength of the laser is approximately $\boxed{650\,\text{nm}}$.

Viva Questions

1. Why are readings taken on both sides? To reduce error in locating the central maximum and diffraction orders.
2. What is grating element? The sum of the slit width and opaque spacing.
3. Why does the central maximum have order zero? Its path difference is zero.

Maxima Code

Download the PG-I optics calculation file.