17 August, 2011

What do the spectrums mean?

Previously I have shown images that I have taken of the spectrum captured using my home-made spectrograph and plain and simple diffraction grating slides, but what do the images captured actually mean?

I will start by explaining what visible light is in relation to the electromagnetic spectrum then

Visible light:
This is one section of the Electromagnetic (EM) spectrum which is visible to the human eye. Light (and other electromagnetic radiation) travels as a wave but unlike sound energy, which vibrates the air particles to transport the signal away from the source, it does not need particles to travel through since it is a vibration of the magnetic and electric fields. This enables light to reach us from the sun, through the vacuum of space. 

This image shows the different wavelengths of the visible light spectrum. Red light has a lower frequency (therefore a larger wavelength ~700nm) and violet light has a higher frequency (smaller wavelength ~400nm), outside of these wavelengths the human eye is unable to pick up the signal contained by these waves. 

Colours are perceived by the eye due to different proportions of the different wavelengths being absorbed and reflected by different surfaces. A surface which appears Red will be reflecting the EM radiation which corresponds to the red section of the spectrum whilst absorbing the other wavelengths. 
Colours which are not a direct wavelength colour (Red, Orange, Yellow etc) can be made up by mixing light of different wavelengths. 

'White light' is made in a similar fashion and is composed of light of all the different wavelengths. 

Images of the spectrum:

The fact that white light is made up of the whole spectrum can be shown simply by shining a light through a prism which uses the difference in densities of the glass and air to bend the light. Light of different wavelengths reacts to the change in densities by a different amount so they bend at different angles causing the spectrum to spread out. 

This process is replicated in the diffraction grating but in a way that means it is possible to have a 'flat' material rather than the traditional bulky prism. 

The images I took previously of the spectrum from an incandescent bulb shows the same complete coverage of the spectrum and all at a similar intensity.
Whereas the spectrum of the energy saving light bulb shows bands of higher and lower intensity throughout the visible spectrum. 
The differences between the two spectra are due to the processes that the bulbs use to create 'white light' (I will discuss this in a later post). But what do the actual spectra mean?

Well, certain elements will give off light at specific wavelengths, called an emission spectrum, which is controlled by the actual structure of the atom in question. 

here is an example of an emissions spectrum for Carbon:

This shows the intensity peaking at specific wavelengths. Using spectroscopy scientists can ideantify the elements in a substance. This is mostly used in astronomy, where scientists will use the emissions spectra for stars or distant galaxies to identify what elements are present and giving off light.

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