The Nature of Light

Many photographers know little about the theories of light. While they produce beautiful photographs, knowledge of light will improve technique.  One aspect on the scientific side of photography is concerned with anticipating the behavior of light.

The most recent information supports both the electromagnetic (wave motion) and the quantum theory.  Photographers are generally concerned with mundane applications of light, so the wave motion theory more clearly describes the characteristics of light.

Wave motion may be easily visualized by considering the ripple effect caused by dropping a stone into a pool of water. While the light travels, it vibrates in all directions from it’s normal resting position. The maximum vibration, or displacement, from the normal resting position is known as “amplitude.” The wave-length is the distance from any convenient point on the wave to the next same relative position on the wave.

The wave phenomena described exist within most electric or magnetic fields. These are collectively known as the electromagnetic spectrum. Note that the kind of wave is determined by its wavelength.  The wave phenomena described exist within most electric or magnetic fields. Within this electromagnetic spectrum, a small amount of these waves are visible to the human eye as colors. This small group of waves is known as the “visible spectrum.”

The graduation is measured in nanometers, which are fractional parts of a meter and provide scientist with a more exacting system of measurement of these small light waves.  Armed with this very basic knowledge, it is easier to understand some of the characteristics of light. As photographers, we are primarily concerned with the visual results of light striking matter. This interaction of light and matter can be classified into three conditions.

Absorption:  When light strikes a medium and is neither reflected nor transmitted (passed on), it is said to be absorbed.  Black cloth or areas of dark forest absorb more light than objects such as a white sheet or a beach.  When light comes in contact with the surface of an object, a certain degree of reflection, and some absorption, always takes place.

A medium that does not allow light to pass through it is opaque. An opaque material may also reflect light. When an object is opaque and the light is not reflected, it is absorbed by the object.  When light is absorbed, its energy is converted and it no longer exists as light.  The color of an object is determined by the way it absorbs light falling upon it (incident light).

A woman’s dress appears red when it absorbs the blue and green rays of white light and reflects the red waves. A lawn appears green because the grass blades absorb the red and blue rays of light and reflect the green rays.

Transmission:  In addition to being reflected and absorbed, light rays may be transmitted. They may also pass through some medium they encounter. When objects can be clearly  seen  through  the  medium,  the  medium  is transparent.

A transparent medium transmits light rays in a regular, or uniform, pattern. When the medium transmits light but breaks up the orderliness of the pattern, sending the transmitted rays in many directions, the medium is translucent.  A medium is said to be translucent when light is visible through it, but objects  are not  clearly  distinguishable.

Thin  fabrics and frosted glass are examples of translucent materials that allow the passage of diffused light.

Refraction: The change of direction that occurs when a ray of light passes from one transparent substance into another substance of different density is called refraction.  It’s an important form of transmission.  Refraction enables a lens to form an image. Without refraction, light waves behave as X rays and pass in straight lines through all suitable substances without any control of direction, and only shadow patterns can be made with them.

Refraction occurs because light travels at different speeds in different transparent substances.  The speed of light in each transparent substance is called the index of refraction for that substance.  Light travels about 1 1/2 times as fast in air as it does in glass, so the index of refraction for glass is about 1.5.

Refraction, or change of direction, always follows a simple rule. “In passing from one transparent substance into another of greater density, refraction is toward the normal.  In passing from one transparent substance into another of lesser density, refraction is away from the normal. ”In this rule the normal is defined as a line perpendicular (90°) to the surface between the mediums.

Dispersion:  The speed of light in a medium depends on the wavelength of the light. As light enters a more dense medium, the short waves, such as blue, are slowed more than the long waves, such as red. Thus the index of refraction  of  a  medium  varies  with  the  wavelength,  and the different colors of light are bent different amounts.

This changing index of refraction or the breaking up of white  light  into  its  component  colors  is  dispersion.  The colors of light in a prism create a spectrum from white light. The prism is able to create this spectrum because of dispersion.

Diffraction:  We have said that light travels in a straight line. Well,that is not always true. An exception to this rule occurs when light travels close to an opaque edge. Because of the wave nature of their travel, light rays passing near an opaque edge are bent ever so slightly. This bending is called diffraction and is evidenced by the formation of a shadow with a fuzzy edge when light passes an opaque object.

In this case, the outside edge of the shadow is light and indistinct, but it gradually darkens into the true black of the shadow that indicates that some of the light is scattered into the shadow area.  Unlike  refraction,  in  diffraction  the  long  wave-lengths of light are bent the most.  Diffraction is important to the photographer when the light passes the edges of a lens diaphragm. When the lens diaphragm is opened fully, the amount (actually the percentage) of diffracted light is quite small.

But when the  diaphragm  is  closed  to  a  small  opening,  the percentage  of  diffracted  light  is  quite  large  and  reduces the sharpness of the image formed by the lens. In otherwords,  a  small  aperture  opening  interferes  with  the image-forming  light  more  than  a  large  aperture  does.

Polarization: Energy in the form of wave motion radiates from its source and travels through a medium. For example, when a section of line is secured at one end and the free end is held in your hand and given a shake, a wave travels down the length of the line from the end that was shaken to the secured end just like an oscillator.

A light source acts as an oscillator. The wave motion in the line does not represent the true wave motion of light because light waves move in all possible directions at right angles to their direction of travel.  A much clearer picture of light wave motion can be seen by having a number of parallel lines with each one being shaken in a different direction-one up and down, one sideways,and the others at various angles in between.

Ordinarily, light waves vibrate in all directions at right angles to their direction of travel. However, when light waves strike a series of parallel microscopic slots,all the light that passes through vibrates in one direction.This is polarized light.  Filters that polarize light, termed polarizing filters, have a practical use in photography.

Specular  reflected  light,  from  a  nonmetallic  surface at any angle between 32° and 37°, is polarized in such a way  that  the  light  rays  vibrate  in  a  direction  parallel to the reflecting surface. Light reflected in this manner is said to be plane polarized and is seen as glare. There is no polarization whatsoever produced by reflections from metallic surfaces.

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