Chemical photography is a specific technique that allows scientists to look within objects to discover the chemicals inside. This has uses in all sorts of areas: forensic science, creating new medicines, and even discovering fake artworks!

The science behind the scenes

Chemical photography was first used as a military technique. The military use instruments called infrared light detectors to guide their missiles, and infrared light - and techniques that are used to detect it - are the basis of chemical photography.

Chemical photography has an important use as a technique to work out what an unknown sample is made from - what chemicals are present within it. The sample could be something like a fingerprint from a crime scene, or a sliver of paint from a fraudulent work of art!

How does it work?

Infrared light is used in chemical photography, because it has been found that you can get more information about the chemicals than if using light in the visible range (the light we get from the sun or a light bulb).

In infrared spectroscopy, infrared light is shone through a thin slice of the sample you are investigating, and then the light is detected after it has emerged on the other side. Comparing the light before and after it passes through the sample shows which frequencies within the light have been absorbed.

The material making up the sample will be full of chemical bonds. Chemical bonds vibrate in different ways (twisting, bending and stretching), depending on the type of bond present (e.g. CO2, N2). Some examples are shown below: CO2 can vibrate in many different ways, but N2 will only vibrate in one way.

Different types of vibrations absorb different frequencies of infrared radiation. By looking at which frequencies of the infrared light are absorbed by the sample, scientists can tell which chemical bonds are present within it. Knowing which chemical bonds are present allows scientists to work out what the sample is made from. They could answer questions like: Who was at the crime scene? Is the paint modern or old?

No vibration:

Chemicals - No vibration

Symmetric stretch:

Chemicals - Symmetric stretch

Asymmetric stretch:

Chemicals - Asymmetric stretch

Bending modes:

Chemicals - Bending modes

 

How can scientists analyse thicker objects?

One problem with infrared spectroscopy is that it can only be used with very thin samples of material, because the infrared beam of light has to pass through and out the other side where it is detected. If scientists want to investigate thicker objects - such as what a medicine tablet is made from - they can use a special technique called Attenuated Total Reflection (ATR).

In this technique, the beam of light is angled through the object and bounces off one surface of it (possibly more than once), zigzagging through the object until it emerges at another point and is detected.

In the diagram below, using ATR to examine the tablet shows what proportion of it is the active drug, how much of it is a polymer that holds the drug in place, and how much is other types of additive.

Chemicals - ATR

Watch this short animation of a tablet being analysed using infrared light:

 

 

The story behind infrared light

Sir William Herschel was a British astronomer, and a Fellow of the Royal Society, who is famous for discovering the planet Uranus, and also for discovering infrared light. He did this by passing sunlight through a glass prism. As sunlight passed through the prism, it separated into a colour spectrum, which contains all of the visible colours that make up sunlight (as in a rainbow).

Herschel was interested in measuring the amount of heat in each colour. As he measured the heat in different colours, he noticed that the temperature increased from the blue to the red part of the spectrum. He then placed a thermometer just beyond the red part of the spectrum, where there was no visible light, and found that the temperature was even higher than any he measured in the visible spectrum. Herschel realised that there must be another type of light beyond the red, which we cannot see. This type of light is known as infrared - infra is derived from the Latin word for 'below'.