WAVES AND RADIATION
Day or night, we are constantly bombarded by waves of energy. Some of these
waves pass by us. Others pass right though us. Most of it we don't even notice.
But some of this constant barrage we do sense, in one way or another. We feel
heat energy through our skin, see light energy with our eyes and hear sound
energy with our ears.
Most important to our understanding of energy waves is that, although they might be conducted through matter, it is not the matter itself that moves, only the energy. Imagine holding one end of a rope in your hand and whipping it up and down. The rope is not actually travelling outwards from your hand. Energy is moving though the rope in waves.
When a stone is dropped into a tank of water, ripples (small waves) can be seen, spreading out (RADIATING) from the point at which the stone hit the water. But these waves are not made up of water travelling outwards. They are only the surface of the water rising and falling, carrying energy outwards.
The distance between one wave peak and the next is the WAVELENGTH. The rate at which the wave peaks pass a given point is the wave’s FREQUENCY. WAVELENGTH and FREQUENCY are the two key elements in our understanding of waves.
Sound and light, though we speak of them both as waves, are two very different phenomena.
Sound waves are energy moving through matter. Whether it is air, water, glass or steel, without matter there is no sound. But it is wrong to think of sound waves as rising and falling, like waves in a rope or even waves in water. Waves in water are what we call TRANSVERSE WAVES. Their movement is up and down, with crests and troughs. Sound waves are LONGITUDINAL WAVES. An individual sound wave is a chain of vibrating molecules that are pushing against and pulling on each other as energy passes through them. (Molecules are the tiny particles that make up matter.) Sound waves are, in effect, then, regions of high pressure (COMPRESSION) and low pressure (RAREFACTION) radiating from their source.
Light waves, on the other hand, rather than vibrating molecules, are vibrating electric and magnetic fields. For this reason, they are a part of the larger family called electromagnetic waves.
Electromagnetic waves travel at the speed of light (299,792.458 km/s) and can move through a vacuum. Waves in the electromagnetic spectrum differ from each other only in WAVELENGTH and FREQUENCY. The longest electromagnetic waves, radio waves, can be as much as twenty kilometres long. Gamma rays, given off by nuclear reactions, are shorter than one thousand thousandths of a millimetre.
Waves don’t just bring energy to us, they also communicate meaning. Waves that are constant, such as the steady beam of a torch, convey no information at all. But if you start flashing the torch in a certain way, the flashes (or pulses) can carry a message.
This is the basis of all wave-borne communications. Patterns of energy arrive from energy sources that are high or low, loud or soft, light or dark, one colour or another. In this way, waves can bring us music, voices and pictures, even from outer space. In Morse Code, dots and dashes represent short and long sound or light signals, a combination of which signifies a particular letter of the alphabet, as shown here.
SOUND
The feature of a sound wave that makes it seem quiet or loud is called its
AMPLITUDE. Amplitude is expressed in a unit of measure called “decibels” (dB).
An increase of ten decibels means ten times more sound; twenty decibels means a
hundred times more.
The feature of a sound wave that makes it high-pitched or low-pitched is called the FREQUENCY. Frequency is expressed in a unit of measure called “Hertz”. One Hertz (1Hz) is equal to one vibration per second. The more vibrations there are per second (i.e. the higher the frequency), the higher the PITCH of the sound.
When we play a musical instrument we are, in fact, simply varying the frequency and amplitude of the vibrations produced by the instrument to play tunes and make rhythms. Each note in the musical scale has a particular frequency. A note that has twice the frequency of another note is said to be one OCTAVE higher. Middle C on a piano has a frequency of 262 Hz. The C one octave above that has a frequency of 524 Hz. The notes in between have the following frequencies: D 294 Hz, E 330 Hz, F 349 Hz, G 392 Hz, A 440 Hz, B 494 Hz.
Sound waves are energy moving through matter. Whether it is air, water, glass or steel, without matter there is no sound. But it is wrong to think of sound waves as rising and falling, like waves in a rope or even waves in water. Waves in water are what we call TRANSVERSE WAVES. Their movement is up and down, with crests and troughs. Sound waves are LONGITUDINAL WAVES. An individual sound wave is a chain of vibrating molecules that are pushing against and pulling on each other as energy passes through them. (Molecules are the tiny particles that make up matter.) Sound waves are, in effect, then, regions of high pressure (COMPRESSION) and low pressure (RAREFACTION) radiating from their source.
Other animals can hear far beyond the level that humans can. Cats, for example, can hear up to about 60,000 Hz. Bats can make and hear frequencies even higher, up to 120,000 Hz.
HEARING
Sound is matter vibrating and causing other matter to vibrate. Usually it is
solid matter, such as wood or metal or plastic, which is vibrating and causing
the air around it to vibrate. Our perception of these vibrations in matter and
air is what we call hearing. If something vibrates faster than twenty times a
second (20 Hz), we can hear it. This is, in fact, the deepest note that the
human ear is capable of hearing. As the vibrations speed up (that is, as the
frequency increases), the pitch gets higher. At 20, 000 Hertz, the human ear can
no longer hear the noise. We say it is too high-pitched. Click on the buttons
opposite to hear sounds at different frequencies.
Other animals can hear far beyond the level that humans can. Cats, for example, can hear up to about 60,000 Hz. Bats can make and hear frequencies even higher, up to 120,000 Hz.
LIGHT
All objects emit electromagnetic waves; stars, plants, even animals. We can't
normally see these waves because of their low frequency. But when an object is
heated it gives the waves more energy and they become shorter (their frequency
increases). If the object is heated enough, visible light is produced, first as
a dull red, then, as the object gets hotter, all the way through the spectrum to
violet. It is this narrow band of frequencies in the electromagnetic spectrum
(from red to violet) that we call light and that we are capable of seeing. The
mixture of all of these colours produces white light.
A prism splits white light into its component colours because it refracts (bends) the different wavelengths of light by different amounts. White light is a mixture of all wavelengths, from red (long wavelength) to violet (short wavelength).
Colour Addition
Red, green and blue are known as the primary colours. All other colours can be
made by adding them together.
Colour Subtraction
Things that don't produce light themselves get their colours through a process
called "colour subtraction". They absorb some colours of light and reflect
others. It is this "reflected" light that we see and which gives the object its
colour. For example, a green leaf looks green because it reflects green light
while absorbing blue and red. A ripe tomato looks red because it absorbs green
and blue and reflects red light. A yellow alien absorbs blue light and reflects
red and green. So, if a magenta (red and blue) light is shining on it, the alien
will reflect only the red light, if a cyan (green and blue) light is shining on
it, the alien will reflect only the green light and if a yellow light is shining
on it, the alien will reflect red and green light, making yellow. In a red
light, the alien will look red, in a blue light the alien will look black, in a
green light the alien will look green and in a white light the alien will look
yellow.
Colour mixing with paint works by subtraction. Magenta, cyan and yellow dyes absorb only one primary colour each from white light. Mixing any two of these colours together produces a primary colour. Mixing all three produces black.
Light waves, on the other hand, rather than vibrating molecules, are vibrating electric and magnetic fields. For this reason, they are a part of the larger family called electromagnetic waves.
LASERS
LASER stands for Light Amplification by Stimulated Emission of Radiation. What
distinguishes laser light waves from ordinary light waves is a property called
COHERENCE. Whereas ordinary light waves are jumbled, a laser's light waves are
all travelling in step with each other, having exactly the same wavelength and
vibrating together. Lasers produce a thin beam of light that travels for long
distances without spreading out. The heat of a powerful infra-red laser beam is
sufficient to cut metal. Other laser beams are used for eye surgery, surveying,
carrying television and computer signals along optical fibres, or reading
information from bar codes and compact discs.
To produce a laser beam:
1. Electrical energy is used to excite (give extra energy to) atoms of the laser material. The chemical composition of the laser material (which can be solid, liquid or gas) will determine the colour of the laser light.
2. These excited atoms release that extra energy as photons (tiny pulses of light), which stimulate more atoms to emit more photons travelling in the same direction.
3. These photons bounce back and forth between mirrors at either end of the tube. One of the mirrors, however, is only partially silvered, so it reflects some light but lets the coherent, amplified light escape as a thin, bright beam.
