I went to a job interview the other day, the guy asked me if I had any questions , I said yes, just one, if you're in a car traveling at the speed of light and you turn your headlights on, does anything happen? He said he couldn't answer that, I told him sorry, but I couldn't work for him then.
-- Steven Wright
Light is one of the most familiar things in our lives. We see because we have organs (our eyes) that sense the intensity (brightness) and wavelength (color) of light. We experience light in a variety of other ways as well. For example, we sense radiant heat when our skin is near a warm object. This is due to our skin's reaction to infrared radiation.
We learn almost all of what we know about the world around us from the interaction of the objects in the world with electromagnetic radiation. Often, the word 'light' is used a little more broadly, to include electromagnetic radiation that is just outside the range we can see, in the ultraviolet and infrared.
Electromagnetic radiation refers to a phenomenon that moves energy from one place to another, and carries with it an electric field and a magnetic field. There are several kinds of electromagnetic radiation (radio waves, microwaves, x-rays, etc.), but light is the part we can see.
Electromagnetic radiation travels at about 186,000 miles per second (300,000 kilometers per second), so light from the sun takes about 8 minutes to go 93 million miles (149 million kilometers) to earth. If you could drive to the sun at 60 mph (100 kph), it would take you 177 years to get there! In one second, light can go around the earth 7 times!
Properties of Light
Under normal circumstances - i.e. travelling through a uniform medium, light travels in a straight line. Our visual systems rely heavily on this fact, 'back-projecting' rays that enter our eyes, to the probable origin of the light rays. Objects that we see around us can usually be assumed to be where they appear to be, as long as the light from them has travelled to our eyes in a straight line. However, the following phenomena can alter the path or nature of the light.
Light falling on an object may be absorbed, transmitted, or reflected. What happens to it depends on the color of the object: a red object reflects red light and absorbs much of the rest of the other colors that we see. The color of an object is that color which is reflected rather than absorbed.
Reflection of light is the most familiar property of light, since it is what enables us to see objects around us! Light from a source, such as the Sun, or a lamp, travels in a straight line until it strikes an object, at which point it may be either absorbed, transmitted, or reflected.
Reflection occurs when waves encounter a boundary that does not absorb the radiation's energy and bounces the waves off the surface. The incoming light wave is referred to as an incident wave and the wave that is bounced from the surface is called the reflected wave.
Those surfaces which reflect the most light appear white, or silver. A highly polished, smooth and flat silver surface acts as a mirror, reflecting a perfect image of the world around it.
Light that is transmitted through a medium will usually be deviated somewhat from the straight path it was previously following. This phenomenon is familiar with transparent objects such as glasses and lenses - objects seen through them appear larger, smaller, or distorted. Place a stick partially into water and it appears to be bent at the surface.
Refraction is an important characteristic of lenses that allows them to focus a beam of light onto a single point. Refraction occurs as light passes from a one medium to another when there is a difference in the index of refraction between the two materials.
Interference is the net effect of the combination of two or more wave trains moving on intersecting or coincident paths. The effect is that of the addition of the amplitudes of the individual waves at each point affected by more than one wave.
If two of the components are of the same frequency and phase (i.e., they vibrate at the same rate and are maximum at the same time), the wave amplitudes are reinforced, producing constructive interference; but, if the two waves are out of phase by 1/2 period (i.e., one is minimum when the other is maximum), the result is destructive interference, producing complete annulment if they are of equal amplitude.
One of the best examples of interference is demonstrated by the light reflected from a film of oil floating on water or a soap bubble, which reflects a variety of beautiful colors when illuminated by natural or artificial light sources.
Diffraction occurs when a light wave passes by a corner or through an opening or slit that is physically the approximate size of, or even smaller than, that light's wavelength. This is a specialized case of light scattering in which an object with regularly repeating features (such as a diffraction grating) produces an orderly diffraction of light in a diffraction pattern. In the real world most objects are very complex in shape and should be considered to be composed of many individual diffraction features that can collectively produce a random scattering of light.
Natural sunlight and most forms of artificial illumination transmit light waves whose electric field vectors vibrate in all perpendicular planes with respect to the direction of propagation. When the electric field vectors are restricted to a single plane by filtration then the light is said to be polarized with respect to the direction of propagation and all waves vibrate in the same plane.
Light may be regarded as a flood of particles, called photons, or as a wave. In either case, it carries energy through a vacuum at a velocity which is a universal physical constant, and is the same for all observers and for all colors.
Light frequently behaves as a particle. Although individual photons all travel with velocity c, a long wavelength, low frequency photon carries little energy; while a short wavelength, high frequency photon has a high energy. Redder light has lower energy per photon; bluer light has higher energy per photon.
Light is a complex phenomenon that is classically explained with a simple model based on rays and wave fronts.
Light also behaves like a wave in many contexts. Either the distance between crests, (the wavelength), or the number of crests to pass the observer second (the frequency) may be used to describe the color of the light. All colors propagate with the same velocity c in vacuo. Wavelength, frequency and photon energy are all interrelated.
This prism is made using optical glass and will demonstrate that white light is made up of the colours of the rainbow. This is known as a spectrum. Sir Isaac Newton in the 17th century was the first to prove that white light was in fact made up of different colours. He used a glass prism in his experiments. The prism is 5cm (2 in) long. Each side is about 2.5 cm (1 in).