A Level Physics Practice Exam

When the angle of diffraction θ reaches about 50°, what order of spectrum is observed?

1. Second order spectrum

2. First order beam

3. Third order spectrum

4. Zero order spectrum

The correct answer is: First order beam

To determine the order of spectrum associated with a diffraction angle of about 50°, it is essential to understand how diffraction patterns are formed, particularly through the use of a grating equation. The grating equation is given by: \[ d \sin \theta = n \lambda \] where: - \( d \) is the distance between the grating lines (grating spacing), - \( \theta \) is the angle of diffraction, - \( n \) is the order of the spectrum, and - \( \lambda \) is the wavelength of the light being diffracted. As the angle of diffraction increases, higher-order spectra can be observed. The first-order spectrum (where n=1) occurs when the path difference between adjacent waves is equal to one wavelength. This corresponds to a relatively small angle. For larger angles, such as 50°, we typically check the next integral values of n to see what can physically align under the given diffraction circumstances. At an angle of 50°, while it is indeed a significant angle, it is plausible for light to be diffracted in the first order particularly when the wavelength and grating spacing correspond well. The first-order spectrum would be expected to still be quite pronounced when \(