3.2 Interactions of light and matter

Light, or visible radiation, interacts with parts of our eyes and is translated by our brains into the perception of colours.

Wavelengths starting at about 7 x 10^-7 m (700 nm) are perceived as the colour red. As the wavelength decreases to about 4 x 10^-7 m (400 nm), the colour we perceive changes from red through to violet. Daylight is composed of all of the radiation in the visible region, which results in 'white' light.

When light, or any other form of electromagnetic radiation, interacts with any kind of matter, including living matter, the light can be:

• reflected
• absorbed
• transmitted (pass through unchanged)
• absorbed at one wavelength and emitted at another (longer) wavelength.

These phenomena are exploited in many ways in forensic science and analytical science generally. They are especially useful because different materials will interact highly specifically with radiation of different wavelengths.

If you are wearing a blue piece of clothing and are looking at it in daylight, the principles of the scientific use of radiation can be seen. When the daylight (visible radiation) hits your blue clothes, all (or most of) the colours except blue are absorbed (by the blue dye) and the blue light is reflected back into your eyes (Figure 6).

Figure 6 Reflection of light. If white light is directed onto a blue material, all the wavelengths apart from blue are absorbed by the blue dye. The blue light is reflected.

Different-coloured materials absorb and reflect different wavelengths. Consider what happens when a piece of coloured glass is held in front of your eyes.

The process by which light is absorbed at one wavelength and emitted at another, longer wavelength is called fluorescence. This principle is used in 'optical brighteners' in washing powders. These emit light in the blue region of the visible spectrum which reduces the appearance of 'yellowing' in white material and the emitted light increases the amount of light reaching the eye, making white garments washed in them appear bright and white.

So, by a complex process, the molecules of the optical brighteners absorb radiation in or near the ultraviolet region. Some body fluids can also fluoresce, as you will see later.

These interactions of electromagnetic radiation with matter - reflection, absorption, transmission and fluorescence - are exploited in many ways in forensic science in a set of techniques collectively called spectroscopy.

Figure 7 Transmitted light. A transparent material appears blue by transmitted light when all wavelengths are absorbed except blue, which passes through the material.

In spectroscopy, matter is exposed to radiation and the pattern of absorption, transmission, reflection or fluorescence emission of this radiation by the material is measured and interpreted. The patterns obtained can reveal a great deal about the chemical and physical structures of substances.

In fingerprint analysis the observation of fluorescence can be used to visualise some fingerprints that are otherwise invisible to the eye. This technique involves shining ultraviolet radiation onto the sample. Since biological fluids, such as semen, saliva, urine and others fluoresce, and any of these fluids could find their way onto the fingertips that leave fingerprints at crime scenes, an 'invisible' or latent fingerprint may be revealed by fluorescence (Figure 8). It can then be photographed for later comparison with prints from suspects and others.

Figure 8 A fingerprint on a mug has been made visible in fluorescent light. To see the fluorescence clearly it is necessary to work in a dark room using UV light. To avoid damage to eyes from the UV radiation, protective goggles need to be worn.