A Level Physics Practice Exam

Given a grating with 1480 lines per mm and angle 51 degrees, what is the first step to find the wavelength?

1. Calculate d using d = 1 / (number of lines / mm)

2. Identify n as 2

3. Use the angle in radians

4. Measure the angle of incidence

The correct answer is: Calculate d using d = 1 / (number of lines / mm)

To find the wavelength using a diffraction grating, the first step is to calculate the grating spacing, denoted as "d". The grating spacing is the distance between adjacent lines on the grating and is essential for using the diffraction equation. The formula for calculating "d" is indeed \( d = \frac{1}{\text{number of lines per mm}} \). In this case, with 1480 lines per mm, you would calculate "d" as follows: \[ d = \frac{1 \text{ mm}}{1480} = 6.76 \times 10^{-4} \text{ mm} = 6.76 \times 10^{-7} \text{ m} \] By calculating this value, you prepare to apply it in the diffraction equation: \[ d \sin(\theta) = n \lambda \] where \( n \) is the order of diffraction (typically starting with \( n = 1 \)), \( \theta \) is the angle of diffraction, and \( \lambda \) is the wavelength. Calculating "d" is the foundational step before proceeding with the angle and wavelength calculation, making this the correct approach to begin the