Wave Motion and the Electromagnetic Spectrum
Energy is often transmitted in the form of waves, that is , back and forth or up and down vibrations. The energy of a wave travels, but the medium in which it travels only vibrates or oscillates.
There are two main types of waves. In transverse waves, the vibrations of the medium are perpendicular in the direction of wave motion. A common example is a boat bobbing up and down on water waves pass. The boat moves at right angles to the direction of the waves. In longitudinal waves, the vibrations of the medium are parallel to the direction of the waves. Sound is a longitudinal wave.
Sound
Is the rate at which waves are produced in a unit of time. The distance from one point on a wave to a corresponding point on the next wave is the wave-length. Wavelength determines the pitch of the sound. The greater the frequency, the shorter the wavelength, and the higher the pitch of the sound produced. The amplitude, or loudness, of a sound is determined by the distance the particles of the medium are displaced from their original undisturbed position. A blast of a loud horn will displace particles of air much farther than a small whistle or toot.
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When a tuning fork is struck it starts to vibrate a frequency that depends upon the size of the fork and the material it is made of . This vibration sets the air particles around the tuning fork vibrating back and forth at the same frequency. When the energy reaches us, our eardrums vibrate, and we hear a tone.
Electromagnetic Waves
These are transverse vibrations; that is, the oscillations are perpendicular to the directions of the wave. Light is a small portion of a large range, or spectrum, of transverse waves called electromagnetic waves because they combine both electrical and magnetic properties. Strictly speaking, however, electromagnetic waves are not what we normally think of as electricity, which is produced by moving electrons, or magnetism, which is a field that can be produced by moving electrons or in other ways. Electromagnetic waves are the wave interpretation of the motion of a particle called the photon.
The electromagnetic spectrum is divided into regions according to ranges in wavelength (or ranges in frequency). The units used to measure the wavelengths of the shorter waves are the micron; milimicron; and angstrom.
In order of increasing wavelength, the electro magnetic spectrum can be divided into gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves, and radio waves. Note that each type of radiation merges gradually into the next and that all electromagnetic radiations travel at the same speed in a vacuum, the speed of light.
The Properties of Light
The speed of light is accepted today as 2.99793 times 10^8 meters per second in a vacuum (approximately 186,000 miles per second). In a more dense medium, such as glass, all electromagnetic waves, including that portion called light or the visible spectrum, will slow down. The change in direction of waves as they pass obliquely from one medium to another is called refraction. The index of refraction is an indication of how much light will bend in passing from one substance to another.
Refraction enables us to bend and focus light with lenses, making photography, microscopes, and telescopes possible. Prisms have enabled us to separate the visible spectrum into the various wavelengths we see as colors. The wavelengths we perceive as colors travel at the same speed in a vacuum but at different speeds in matter. Red, the longest visible wavelength , is approximately 0,00007 cm in length and travels about 1 percent faster in glass than does violet, the shortest visible wavelength (0.00004 cm).
Lenses
The purpose of a lens is to change the curvature of light waves, usually to form an image. The ability of a lens to form an image is measured by its focal length. The distance of the principal focus from the lens is the focal length of that particular lens. Its value depends on the curvature of the two surfaces of the lens and the refractive index of its material. Convex lenses form real images. A real image can be focused on a screen and is inverted.
The Law of Reflection
Reflection is the return of an object or a train of waves from a surface that acts as a boundary between two media. If the surface is smooth, the incoming ray of light, the incident ray, and the ray when reflected, the reflected ray, form equal angles with the normal. The normal at the point where the rays intersect is a line perpendicular to the surface. The angle of incidence equals the angle of reflection, and both angles and the normal are on the same plane. Curved mirrors called reflectors are used to focus light in large telescopes. Mirrors do not distort images as much as refracting lenses do.
The Law of Inverse Squares
This law explains why we often move an object closer to a light source to see it better. It states that the intensity of certain effects, including the illumination of a surface, gravitational force, and sound, is inversely proportional to the square of the distance from the source. The intensity decreases as the distance increases. At 4 meters, the intensity is 1/16 of thaw is at 1 meter.
Applications
The high accuracy of the speed at which electromagnetic radiation travels makes it impossible to measure the distance of objects such as the moon by radar. Radar is an instrument that emits microwaves, bounces them off an object, and measures the time for the echo to return. Knowing the speed of the radiation, the distance to the object can be calculated with great precision.
Radar is also used by highway officers to get an instantaneous reading of the speed of automobile by taking into account the Doppler effect. If an object emitting a tone is receding from us, the pitch is lower than that heard when the object was at rest; that is, the wavelength appears to increase. If the object is moving toward us, the wavelength is decreased and we hear a higher pitch. The speed of a moving car can be ready by the increase or decrease of the wavelength of the radar waves that are bounced off it. The Doppler effect has also been used to determine that the universe is expanding. Light of known wavelengths emitted by distant galaxies is shifted toward longer wavelengths, that is, toward the red end of the visible spectrum, indicating that these galaxies are receding from us.
The laser is a device that amplifies focused light waves and concentrates them in a narrow, very intense beam. The emitted light, called coherent light, does not spread out, so it does not lose intensity. It can deliver great energy to a small area.