Visible Light

Visible Light is Electromagnetic radiation of those wavelengths to which the human eye responds, i.e. from just below 400 nanometers (4×10⁻⁷ m, 0.4microns, or 4000 angstroms) to just over 700 nm. Different wavelengths within this range correspond to different colors as perceived by the eye. In order of decreasing wavelength, the colors are red, orange, yellow, green, blue, indigo, and violet. Wavelengths shorter than visible are referred to as ultraviolet, wavelengths longer than red are infrared.

The speed of light, although quite fast, is not infinite. The speed of light in vacuum is expressed as c = 2.99 × 10⁸ m/s. Light travels in a vacuum at a fixed speed, and this speed is considered a universal constant. It is interesting to note that speed changes for light traveling through non-vacuum media such as air (0.03% slower) or glass (30.0% slower). For ordinary purposes, we may represent light in terms of its magnitude and direction. In vacuum, light travels in a straight line at a fixed speed, carrying energy from one place to another. Two key properties of light rays interacting with a medium are:

• It can be deflected upon passing through one medium to another (refraction).
• It can be bounced off a surface (reflection).

Light energy behaves like waves as it moves through space, or it can behave like a discrete particle with a discrete amount of energy (quantum) that can be absorbed and emitted. The particle-like nature of light is modeled with photons. A photon has no mass and no charge. It carries electromagnetic energy and interacts with other discrete particles (e.g., electrons, atoms, and molecules). A beam of light can be modeled as a stream of photons, each carrying a well-defined energy that is dependent upon the wavelength of the light. The particle model of light describes large-scale effects such as light passing through lenses or bouncing off mirrors.

A wavelike model has to be used to describe fine-scale effects such as interference and diffraction that occur when light passes through small openings or by sharp edges. The propagation of light or electromagnetic waves through space can be described in terms of a traveling wave motion. The wave transports energy, without moving mass, from one place to another at a speed independent of its intensity or wavelength. Light waves are complex. They are not one-dimensional waves but instead are composed of mutually perpendicular electric and magnetic fields with wave motion at right angles to both fields. The wave carries light energy with it. The amount of energy that flows in a second across a unit area perpendicular to the direction of travel is called the irradiance (flux density) of the wave.

Questions to Ponder

• If the speed of light varies when it moves from one medium to another, does that mean the wavelength also changes? If so, does that mean that a visible ray can be converted to a Microwave just by passing it through a medium?
• Why is the speed of light limited to the particular value? Why can’t light accelerate?