All matter has an internal property of energy. This energy is related to the electrons traveling in orbits around the nucleus of the atoms. As the electrons change orbital levels, energy is emitted or absorbed. For example, if we burn a sheet of paper, energy is given off, which we see as a flame. The matter in the filament of a light bulb emits light when it is excited by an electrical current.
The term that we use for emitting energy from matter is called radiation. Radiation is often referred to as electromagnetic radiation because the emitting energy has both electrical and magnetic properties.
The emitted energy is transmitted at a wavelength determined by the nature of the matter and the forces acting on it. The human eye can only detect a very small range of wavelengths of radiation. What it detects we call light. Physicists often refer to it as visible light.
The electromagnetic spectrum, shown in Figure 12.1, includes the energy emitted at all wavelengths. This includes radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, gamma rays, and other electromagnetic radiation of longer and shorter wavelengths.
Radio waves, which we use for AM, FM, TV, and shortwave radio, range in length from around 1 centimeter to a kilometer long. Some naturally occurring radio waves are much longer, actually thousands of kilometers long.
At the opposite end of the spectrum are the gamma rays, which are radioactive and have a wavelength of less than a trillionth of a centimeter.
Note that the names given to various portions of the spectrum are arbitrary labels chosen by scientists for convenience. By setting up these artificial divisions, scientists can describe the specific nature of the waves they are examining. The various portions of the spectrum blend into one another. For example, a microwave and a radio wave at their point of contact would be identical (Chen, Kao, & Liu, 1999).
Electromagnetic radiation has three properties: wavelength, amplitude, and polarization. Wavelength is the length of the wave, which determines its properties. In visible light the wavelength determines the color. The amplitude of a wave determines the brightness of the light. Polarization is the angle at which the wave is vibrating.
For us to appreciate the nature of polarization of light, we need to think in three dimensions rather than two dimensions as on a piece of paper. When we show waves on a sheet of paper, the wave is shown moving up and down. In reality, it can move at any angle from the paper. A polarized light filter such as that used on sunglasses or photographic cameras is designed to filter out the waves that are not parallel to the filter plane. This can reduce the glare off a body of water or other glaring objects. When two polarized filters are placed in front of one another, and one of the filters is rotated, that is, when the two filters become 90° apart, all of the light is filtered out and no light can pass through.
Light’s Enigma: The Duality of Wave and Particle Theories of Light
Scientist have long debated whether light is a wave of energy or a particle of matter. That light demonstrates characteristics of both light and matter is accepted by most physicists. The particle characteristic of light is that particles called photons travel at the speed of light. Light is the only wave that can travel though a vacuum.
As previously mentioned, visible light is the small range of electromagnetic radiation that is detected by our eyes.
The spectrum of visible light is further subdivided into various colors, with red having a longer wavelength and violet having a shorter wavelength. The colors are separated into the basic colors of red, orange, yellow, green, blue, and violet, ROY G. BV. Often a vowel is added to BV, making it BIV with the I standing for indigo. However, in recent years, the inclusion of indigo has been discontinued.
The Speed of Light
Visible light and all electromagnetic radiation travel at the same speed. This speed is measured by many different methods. The speed of light in a vacuum is normally rounded to 300,000 kilometers per second or 186,000 miles per second.
Light travels in a straight line. However, if it could curve around Earth, it could make more than seven trips in 1 second. Light can travel to our Moon in just over 1 second. It takes light from the Sun approximately 8 1/2 minutes to get to Earth.
Examining light from stars becomes an interesting process. Our nearest neighboring star is Proxima Centauri, which is visible in the Southern Hemisphere. Even though it is the nearest star, it takes the light from that star 4.3 years to get to Earth. Other objects in our sky, such as some distant galaxies, are so far away that it takes 13 billion years for the light to reach us. That means that we see that galaxy as it was 13 billion years ago.
The Normal Sighted Eye
The eye is an amazing organ that detects and positions light so that we can see. Our eyes collect light so that we can see. Our eyes collect light energy that travles through the lens of the eye and is detected by the retina. These responses are sent to the brain where they are decoed into vision.
The light receptors of the eye are of two types: cones and rods. Cones are specialized cells that not only detect light but also measure the wavelength so that the color of the light is recognized. Cones are concentrated at the fovea area of the retina. Rods detect light and are concerned with black and white vision. Because rods cover most of the retina, the objects that we see at the perimeter of our sight tend to be viewed as black and white. The objects viewed at the center of our vision tend to be more color vision.
For detecting dim objects, the rods are more critical because they do a better job of collecting light. It is interesting that observational astronomers watching stars in the nighttime sky tend to look at an object a few degrees away from the star they are viewing so that the rods come more into play than the cones. The object that they want to see will become much brighter than if they looked directly at the object.
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