When determining the refractive index of a prism, sodium light is often used instead of white light because sodium light has a specific and well-defined wavelength. This allows for more precise and consistent measurements compared to white light, which consists of a broad range of wavelengths.
The refractive index of a material is defined as the ratio of the speed of light in a vacuum to the speed of light in that material. However, the speed of light in a medium is dependent on the wavelength of light. Different wavelengths of light can experience different amounts of refraction as they pass through a material, leading to dispersion.
White light contains a combination of various wavelengths spanning the visible spectrum. When white light passes through a prism, each wavelength is refracted at a slightly different angle due to dispersion, resulting in the separation of colors and the formation of a spectrum.
In contrast, sodium light specifically refers to light with a wavelength of approximately 589.3 nanometers. By using sodium light, which has a single, well-defined wavelength, the effects of dispersion are minimized. This allows for more accurate measurements of the angle of deviation and simplifies the calculation of the refractive index using Snell's law.
By using a monochromatic light source like sodium light, the refractive index obtained for the prism is more reliable and representative of the material's behavior at a specific wavelength. However, it's worth noting that different materials can exhibit slightly different refractive indices for different wavelengths, which is why the refractive index is often given as a function of wavelength in a dispersion curve.