Light: is it made of Waves...

Isaac Newton discovered in 1672 that light could be split into many colors by a prism, and used this experimental concept to analyze light, this pnenomenon is called dispersion (dispersion ). The colors produced by light passing through a prism are arranged in a precise array or spectrum from red through orange, yellow, green, blue, indigo and into violet. The students' memory trick is to recall the name "Roy G. Biv" where each letter represents a color. The order of colors is constant, and each color has a unique signature identifying its location in the spectrum. The signature of color is the wavelength of light.

The electromagnetic spectrum, which encompasses the visible region of light, extends from gamma rays with wave lengths of one hundredth of a nanometer to radio waves with wave lengths of one meter or greater.

Somewhat less than 100 years after Newton's discoveries, James Clerk Maxwell showed that light was a form of electromagnetic radiation. This radiation contains radio waves, visible light and X-rays. The Figure shows electromagnetic radiation as a spectrum of radiation extending beyond the visible radiation to include at one end radio waves and at the other end gamma rays. The visible light region occupies a very small portion of the electromagnetic spectrum. The light emitted by the sun falls within the visible region and extends beyond the red (into the infrared) and the ultraviolet (UV) with a maximum intensity in the yellow.
When we consider light as an electromagnetic wave, a color's spectral signature may be identified by noting its wavelength. We sense the waves as color, violet being the shortest wavelength and red the longest. Visible light is the range of wavelengths within the electromagnetic spectrum that the eye responds to. Although radiation of longer or shorter wavelengths are present, the human eye is not capable of responding to it.

Three typical waves of visible light are shown in the figure on the left. The wavelength is the distance from one wave crest to the next, and is represented by the Greek letter lambda, l. Violet light has a wavelength of 410 nm (nm, nanometer ).

Infrared and radio waves are at the long wavelength side while ultraviolet (UV), x-rays and gamma rays lie at the short wavelength side of the electromagnetic spectrum. Radiation with wavelengths shorter than 400 nm cannot be sensed by the eye. Light with wavelength longer than 700 nanometers is also invisible.

...or is it made of particles?

We can describe light as electromagnetic waves with color identified by its wavelength, but we can also consider light as a stream of minute packets of energy-photons - which create a pulsating electromagnetic disturbance. A single photon of one color differs from a photon of another color only by its energy.The intensity or brightness of the light is defined by the flux, or number of photons passing through a unit area in a unit time; i.e., number of photons per cm2 per sec.
Visible light extends from 400 nm to 700 nm (wavelength) and is composed of photons in the energy range going from 1.8 to 3.1 eV (eV, electron Volt ). As the energy of the light increases, the wavelength decreases.

The Color of Objects

Let's try to consider the color of an object illuminated by white light. Color is produced by the absorption of selected wavelengths of light by an object. Objects can be thought of as absorbing all colors except the colors of their appearance which are reflected as illustrated in figure on the left . A blue object illuminated by white light absorbs most of the wavelengths except those corresponding to blue light. These blue wavelengths are reflected by the object.