¶ … Properties of Light Light is one of the most basic physical phenomena. It is observed by most people on a daily basis, and even people who lack formal understanding of the properties of light have some understanding of its properties. For example, most people have seen mirrors, rainbows, and know that glasses can improve vision, though they may not understand that reflection of light explains how mirrors work, that dispersion explains the formation of a rainbow, or that refraction of light is used to form optic lenses. This paper will describe the various properties of light and explain how it acts in various mediums. It will discuss: the nature of light, which is a particular but behaves like a wave; color; velocity; refractive index; reflection; refraction; dispersion; total internal reflection; diffraction; and interference. Taken together, these various properties help explain how light functions.

Light is a form of electromagnetic radiation. As its name implies, electromagnetic radiation is a form of energy with both electric and magnetic field components. Those components are in a fixed ratio of intensity to one another; as the electric field increases, the magnetic field also increases. These fields oscillate perpendicularly to one another and also perpendicular to the direction of energy. This causes the light to seem to move in a wave-like pattern. While many people think of light as only that light in the visible spectrum, light actually includes visible light, ultraviolet light, and infrared radiation. However, while light includes visible light and light outside of the visible spectrum, it is important to realize that not all electromagnetic radiation is light; electromagnetic radiation also includes "gamma rays, x-rays, microwaves and radio waves" (Trevor-Jones).

Light is sometimes described as a particle and sometimes described as a wave, and it exhibits properties of both. Light is not actually a particle, but it travels in packets known as photons. "A photon is defined as a quantum of electromagnetic radiation" (Trevor-Jones). Photons travel basically in straight lines. However, light is also referred to as a wave. While light is not actually a wave, the oscillation between the electric and magnetic fields causes it to have wave-like characteristics, particularly in the way that it moves through various forms of media. Waves are characterized by frequency and wavelength, which have an inverse relationship; the larger the frequency, the smaller the wavelength. Light is characterized in terms of wavelength and frequencies. Moreover, the energy of the photons are related to frequency and wavelength; the higher the frequency of light, the shorter its wavelength and greater its energy.

One of the most easily identifiable properties of light is its color. Light's color is related to the energy of the photons, and thus to its wavelength and frequency. Photoreceptors, which are photosensitive cells in the eyes, respond to particular ranges of energy and perceive them as a color. "If only a single wavelength or limited range of wavelengths are present and enter our eyes, they are interpreted as a certain color... If all wavelengths of visible light are present, our eyes interpret this as white light" (Nelson). Color is generally referred to in terms of wavelength. Blue light is around 400 nm, and red light is around 700 nm. From 100nm to 380 nm is the ultraviolet spectrum. Visible light ranges from 400 nm and 700 nm. Violet light has a wavelength of about 400 nm. Indigo light has a wavelength of about 445 nm. Blue light has a wavelength of about 475 nm. Green light has a wavelength of about 510 nm. Yellow light has a wavelength of about 570 nm. Orange light has a wavelength of about 590nm. Red light has a wavelength of about 650nm. Infrared radiation, which humans perceive as heat, is from 750nm to 2500nm. Therefore, light takes up only a small part of the electromagnetic spectrum, and visible light an even smaller part of that spectrum.

One of the other known properties about light is its velocity or speed. The equation E = h? = hC/?

expresses the relationship between energy (E), frequency (?), and wavelength (?). Planck's constant, which is represented by h and is 6.62517 x 10-27 erg.sec and C. is the velocity of light 2.99793 x 1010 cm/sec. This velocity of light refers to light's speed in a vacuum and is light's highest possible speed. The speed of light can be slower, but not faster, than C. The speed of light is the frequency multiplied by the wavelength. This is critical...

Thus, if the velocity, C, is reduced on passage through a substance, the wavelength, ?, must also decrease (Nelson). The speed of light has critical applications in much of physics, particularly because it is seen as the defining upper limit of speed and is critical in equations such as the calculation of energy. Though a thorough explanation of those relationships is beyond the scope of this paper, it is important to note the role that light plays within the broader overall context of physics and the natural universe.
The speed of light is the upper limit of how fast light can move, but light does not move that quickly outside of the vacuum of space. Instead, its speed is hindered by the media through which it moves. It is important to keep in mind that even air, which is often thought of as "nothing" is a media and impacts the movement of light. Moreover, how light travels through a substance is determined by the refractive index (n), of that substance. The refractive index is the ratio of the speed of light in a vacuum (C) to the speed of light in that substance Cm (Nelson). The refractive index is always greater than 1.0 because the speed of light in a substance will never be greater than the speed in a vacuum. Generally, the greater the density of the substance, the higher the refractive index. This impacts two other properties of light, interference and refraction. Furthermore, the wavelength of light can impact a substance's refractive index, since "different wavelengths are interfered with to different extents by the atoms that make up the material (Nelson).

One of the better-known properties of light is reflection. Light reflects off of surfaces. A mirror is the best example of reflection, as it causes light to bounce-back directly to the observer, though reflection was observed before mirrors, since bodies of water also have reflective properties. Reflection refers to what occurs when light changes direction after bouncing off of a reflective surface, such as a mirror. When the reflecting surface is smooth, reflection is governed by three rules or laws. The first rule is that the incident ray, the reflected ray, and the normal to the reflection surface are all in the same plane. The second rule is that the angle that the incident ray makes with the normal is equal to the angle that the reflected ray makes with the normal. The third rule is that the reflected ray and the incident ray are on the opposite sides of the normal. This change of angles around the normal results in images appearing "flipped" in a mirror or other reflective surface.

Another property of light is refraction. "Refraction means that light bends when it passes from one medium to another. When light enters a denser medium from one that is less dense, it bends toward a line normal to the boundary between the two media" (Physics Planet). Refraction occurs when the light travels from one medium to another at an oblique angle. When the difference between the densities in the two different media is larger, the light bends more. One of the interesting aspects of refraction is that the wavelength of the light changes, but the frequency remains the same. In refraction, a ray that is moving along the normal will change speed but not direction. Refraction has many practical applications, including optic lenses and telescopes.

Refraction enables another property of light: dispersion. Dispersion "refers to the ability to break white light into its constituent colors" (Physics Planet). As noted above, white light actually contains all of the wavelengths of visible light. When this white light enters a prism, it is bent through refraction. However, the angle of dispersion differs for each wavelength of light in the spectrum. This results in the light coming out at different angles. As a result, when the light emerges, it is separated into its different constituent wavelengths, which are represented as colors. Any person who has observed a rainbow has observed dispersion; the water droplets in the air act as prisms, separating light into its different visible colors.

Of course, different boundaries and surfaces interact with light in different ways. What is interesting is that light can experience multiple properties at once. For example, total internal reflection combines refraction and reflection. It occurs when a ray of light hits a medium at a boundary angle that is greater than the critical angle needed for that medium. If the refractive…