Waves and the Electromagnetic Spectrum
What are waves?
Waves carry energy from one place to another - if you've ever thrown a stone in a pond then you'll have seen waves in action. The ripples which you see coming from where the stone enters the water carry energy with them, and as they spread across the pond, the energy is also spread out until it eventually reaches the edge of the pond.
Sound also travels by waves - air is made up of molecules, and it is the vibration of these molecules which causes sound waves to travel. As the waves passes through the molecules, they vibrate and bump into their neighbouring molecule, passing on their energy; this molecule then starts vibrating, bumps into its neighbour and passes energy along, and so on. In fact, sound waves can travel through any substance which contains molecules - air, water, even custard! It is only in a vacuum where there are no molecules for waves to travel through, that sound waves would not be able to travel. That's why you don't hear any sound in space.
So, if you can't hear sound waves in space, how do you see light waves? Well, there are 2 different types of waves - longitudinal and transverse.
Longitudinal waves carry energy by vibrating along the same direction in which the wave is travelling. Examples of longitudinal waves are sound waves, and p-waves which are a type of seismic wave which travels through the Earth during an earthquake.
Transverse waves carry energy by vibrating at right angles (or 90 degrees) to the direction that the wave is travelling. Examples of transverse waves are s-waves which are a type of seismic wave which travel through te Earth during an earthquake (and are slower than p-waves), and light waves.
See how transverse waves move by clicking here.
See the difference between longitudinal and transverse waves using this applet. Keep an eye on the red molecule. - for the longitudinal wave, the molecule moves from side to side about a fixed point, and the wave itself can be seen as where there is a compression of molecules, and is carried along in the same direction. In the transverse wave, the red molecule moves up and down but the wave travels towards the right of the page.
Watch a simple animation of transverse and longitudinal waves in a spring on YouTube by clicking here.
The electromagnetic (EM) spectrum is the complete range of frequencies that electromagnetic waves cover. All waves in this spectrum are transverse waves - they transfer energy from one place to another by vibrating at right angles to the direction of travel.
The visible part of the EM spectrum is only a very small part of it - this is the part that we can see with our eyes. Here, our eyes are acting as detectors, and we detect the EM waves that an object emits. For our eyes, the waves which we detect are from the visible or optical part of the EM spectrum.
But, we can 'see' other parts of the EM spectrum
When astronomers "see" objects in space, they use specially designed detectors to detect EM waves which can be from any part of the EM Spectrum, not just the visible part.
For example, some objects in space, such as regions where stars are being born, are very dusty, and so the visible light can't pass through to reach our detectors. However, infra-red light can pass through this dust easily, so astronomers can use special detectors to "see" this light, and see where stars are being born.
The picture above shows that radio, microwaves, visible and Xrays are all the same thing - they are all waves. The main difference between them is just how long they are.
The length of a wave (given by the symbol λ) is measured as the distance from one crest or peak of the wave to the next, as shown below. The longer the distance between the 2 peaks, the longer the wavelength.
Energy and Frequency of a wave
The frequency of a wave (given by the letter, f) is simply the number of peaks or crests that pass by a certain point, per second. It has the units Hertz (Hz).
Using the applet below, investigate what happens as you increase the frequency of the wave...
The frequency of a wave is also related to its energy - the higher the frequency of a wave, the higher its energy. Conversely, the lower its frequency, the lower its energy.
Mathematically we can write this as:
E ∝ f
or, if we put a constant into the equation:
where h is Planck's constant (6.626 068 96 × 10−34Js).
You may find it easier to remember this equation by thinking of of a wave as a tightly coiled spring - it takes more energy to squash the coils of the spring together once they get close to each other. So, the closer together the coils are in a given space, the higher their frequency, and the more energy it takes to squash them even closer...
Or you may find this equation easier to remember by thinking of the mnemonic....Elephants have feet!
There is also an equation which links the velocity of a wave, with its frequency and wavelength:
v=velocity of the wave (m/s)
f = frequency of the wave (Hz)
λ=wavelength of the wave (m)
Explore the properties of waves here.
|In a nutshell...|
|The EM spectrum covers waves of all frequencies|
|Waves can be described by their energy, frequency and wavelength|
Equations which describe waves are:
v=fλ and E=hf