A Level Physics Practice Exam

In the context of a spectrometer, when is a spectrum first observed?

1. At 30 degrees diffraction angle

2. At 50 degrees diffraction angle

3. At 0 degrees diffraction angle

4. At 90 degrees diffraction angle

The correct answer is: At 50 degrees diffraction angle

A spectrum is first observed in a spectrometer when light passes through a diffraction grating or prism, which separates the light into its component wavelengths. This occurs when the angle of diffraction allows the first distinct order of the spectrum to be visible. In most spectrometric setups, the initial appearance of a spectrum is typically associated with the first maxima of diffraction. This tends to happen at an angle where the sine of that angle corresponds to the wavelength of light being measured divided by the grating spacing. At a 0 degrees diffraction angle, light is not deviated, leading to the absence of a spectrum as all wavelengths overlap at a single point. At angles like 30 degrees and 90 degrees, while they are relevant in the context of diffraction orders, they may not correspond to the first observable spectrum. The choice indicating a 50 degrees diffraction angle aligns with the typical conditions under which the first observable spectrum is formed in many practical spectrometric setups, making it the correct choice.