Week 9: Light Lecture 2
Light - Light, is a form of energy that is detected by the human eye. In certain conditions light appears to act like a stream of particles. In other conditions, like a wave. A way in which light differs from sound is that it does not require a medium to propagate. Light can travel from the sun, through a vacuum (outer space) and reach the Earth.
Waves - Light is a transverse wave, much the same as a ripple you would see on the surface of water. The vibrations of light waves are at right angles, up and down/side to side, to the direction of motion from the origin of the disturbance.
Light Waves - Light waves, are electromagnetic waves. This means that they are a particular form of Radio Wave and in terms of this our eyes form a Radio Receiver. The range of light that is visible to the human eye is only a small part of the electromagnetic spectrum, covering frequencies from around 400THz to 750THz. (THz = Tera-Hertz = 10 to the power of 12Hz). This range appears to us as different colours. Red at low frequencies and Violet at high. White being a mixture of many different frequency components or colours.
The Electromagnetic Spectrum - http://mynasadata.larc.nasa.gov/science-processes/electromagnetic-diagram/
Velocity of Light - In a vacuum we know that light travels at a constant speed of 300,000 km/second. So 300 multiplied by 10 to the power of 6 m/s. In air this is a little less, slowed to around 2/3rd of this. The change in velocity at air/glass interference makes glass a very useful medium for lenses. From this we know that in air, light travels at around 1,000,000 times the speed of sound. So, to travel the 3 metre length of an average living room light would take:
Frequency and Wavelength - We know that the frequency of a wave is the number of complete cycles per second. The frequency of a light wave is independent of the medium that the wave is travelling through or via. The speed at which this wave travels or it's velocity is equal to it's wavelength multiplied by it's frequency. So:
This means that a 500THz Light-Wave has a wavelength in air, 300*106 over 500*1012 m/s. So the wavelength is 600nm and appears Orange. The same light wave in glass would have wavelength of around 400nm. Note that it still appears Orange being that the frequency of the wave is independent of the medium it is travelling in.
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