You carried out an experiment to measure the wavelength of a monochromatic light source using a diffraction grating - Leaving Cert Physics - Question 2 - 2007

A suitable source of monochromatic light can be a sodium lamp or a laser.

During the experiment, the following measurements were taken:

• Correct angle to determine the diffraction pattern.
• Distance from the grating to the screen (let's refer to this as 'L').
• The distance between diffraction fringes (let's denote this as 'x').
• Adjustments made to the spectrometer to ensure the correct angle.

The distance between each line on the diffraction grating (d) can be calculated using the formula:

$d = \frac{1}{\text{number of lines per meter}} = \frac{1}{600000} = 1.66667 \times 10^{-6} \text{ (m)}$

The wavelength (λ) of the light can be determined using the formula:

$d \sin(\theta) = n \lambda$

Where:

• d is the distance between lines on the grating.
• \theta is the angle of the first order maximum.
• n is the order of the diffraction (typically n = 1 for the first maximum).

This can be rearranged to find: $\lambda = \frac{d \sin(\theta)}{n}$

One precaution taken to ensure an accurate result is: properly adjusting the spectrometer to eliminate any parallax errors, especially when reading angles. Additionally, ensuring the screen is positioned at a proper distance from the grating helps to prevent dim spots in the diffraction pattern, which could lead to inaccurate readings.