Electromagnetic Spectrum: Visible and Invisible Light

Light Dispersion and Prism Refraction

Light undergoes decomposition or dispersion when passing through a glass prism due to differences in how different wavelengths of light refract. As polychromatic light enters and exits a medium with a higher optical density such as a prism, each component wavelength travels at a different speed within the glass. This variation in speed causes each constituent wavelength to refract or bend at a distinct angle. Shorter wavelengths bend more significantly because they experience a higher refractive index, whereas longer wavelengths bend to a lesser degree. Consequently, as the beam exits the prism, it splits and spreads out into its constituent spectral colors.

The Visible Spectrum and Continuous Light

The electromagnetic spectrum is categorized into two primary divisions: the visible spectrum and the invisible spectrum. The visible spectrum represents the range of ordinary light that can be detected and distinguished by the human eye. This region occupies a very narrow band of wavelengths within the broader electromagnetic spectrum, spanning precisely between $350\,\text{nm}$ and $800\,\text{nm}$.

Within the visible spectrum, light appears as an unbroken band of consecutive colored rays, where each color corresponds to a specific wavelength and frequency. Because there are no distinct boundaries or visible separations between the adjacent colors, this band is formally termed a continuous spectrum. The colors transition smoothly from one wavelength to another without interruption. A primary real-world example of a continuous visible spectrum is a rainbow appearing in the sky. This phenomenon occurs when suspended raindrops act as microscopic prisms, scattering and refracting sunlight into its full visible spectral array.

The Invisible Spectrum and Radiation Applications

The invisible spectrum includes all regions of electromagnetic radiation featuring wavelengths outside the human visual perception range of $350\,\text{nm}$ to $800\,\text{nm}$. Human eyes cannot visually perceive radiation in this spectrum, but these wavelengths are utilized in numerous modern technological and medical applications. The invisible spectrum consists of two distinct domains situated on either side of the visible light boundaries.

Electromagnetic waves with wavelengths exceeding $800\,\text{nm}$ are positioned beyond the red end of the visible spectrum. This low-frequency, long-wavelength region includes radio waves and television waves, which are fundamental to signal transmission and communication infrastructure. On the opposite side of the spectrum, electromagnetic waves with wavelengths shorter than $350\,\text{nm}$ are situated beyond the violet end of the visible spectrum. This high-frequency, short-wavelength region includes X-rays. Due to their high energy, X-rays penetrate soft tissues and are used extensively by physicians for medical imaging, such as taking detailed radiograms of internal body structures like bones.