Spectra

When light is absorbed by atoms to transition them from lower to higher energy states, certain wavelengths are removed from the spectrum. This results in a line absorption spectrum, characterized by sharp dark lines against a continuous bright background. The Fraunhofer lines observed when sunlight is dispersed are an example.

When light is absorbed by molecules, exciting them from lower to higher energy states, the missing wavelengths are grouped into bunches. This results in a band absorption spectrum, which consists of separate dark bands on a continuous bright background. This type of spectrum is observed when light passes through hydrogen gas at moderate temperatures or certain solutions.

Light that causes visual sensation lies between wavelengths of approximately 380 nm (violet) to 780 nm (red). Radiation with wavelengths shorter than the visible region (less than ~380 nm) is called ultraviolet radiation. Radiation with wavelengths longer than the visible region (greater than ~780 nm) is called infrared radiation. Ultraviolet range is roughly 15 nm to 380 nm, and the infrared range is roughly 780 nm to 40000 nm. Beyond ultraviolet are X-rays and gamma-rays; above infrared are radiowaves.

A spectrometer is an instrument used to produce and study pure spectra, primarily in the visible region. It consists of three main parts: Collimator: A cylindrical tube with an achromatic converging lens at one end and a linear slit at the other. It makes the incident light parallel before it falls on the dispersing element. Prism Table: A horizontal platform that can rotate and whose height is adjustable. The dispersing element (prism, grating, etc.) is placed on the prism table. Telescope: An astronomical telescope used to view the dispersed light. It consists of an objective lens and an eyepiece.