How much of the world do you think we can see in visible light? The answer is: about 0.0035%. That’s just a very, very tiny portion of the electromagnetic spectrum that is the light that’s visible to our eyes! Everything else is energy, or electromagnetic radiation, of wavelengths that are not visible to the human eye. We need special equipment to detect electromagnetic radiation of all these other wavelengths. Let’s have a look at the electromagnetic spectrum, the electromagnetic waves, and the applications of electromagnetic waves in cultural heritage and in our daily lives.
What is the electromagnetic spectrum?
The electromagnetic spectrum consists of all the electromagnetic waves of all possible energies moving through space at the speed of light. The different regions of the electromagnetic spectrum are: gamma rays, X-rays, UV rays, visible light, IR light, microwaves, and radio waves. All of these electromagnetic waves have applications in cultural heritage.
What are electromagnetic waves?
Electromagnetic waves are composed of oscillating electric and magnetic fields, as shown in the figure below. The electric field (in blue) and magnetic field (in red) are perpendicular to one another and perpendicular to the direction of propagation of the wave. These waves travel in vacuum at the speed of light, and they carry electromagnetic energy. Their wavelength, defined as the distance between two successive peaks of the wave, can be very short or very long, or anything in between. The shorter the wavelength, the higher the frequency of the wave, and the higher the energy of the electromagnetic radiation.
The gamma rays are the highest energy waves of the entire electromagnetic spectrum. That means they have the highest frequency and the shortest wavelengths.
They are produced in the radioactive decay of atomic nuclei and in nuclear explosions. Even though we can’t see them with the naked eye, using specialized equipment, we can detect the gamma rays produced by grand events, such as supernova explosions to smaller-scale events, like the radioactive decay of radioisotopes.
In cultural heritage, the gamma radiation can be used to determine the composition of certain artifacts because of the unique energy signature of each chemical element in a gamma-ray spectrum.
X-rays are also very high in energy, but not quite as high as the gamma rays.
You may be familiar with the X-rays from the medical field, where X-rays are used to acquire images of your bones. Another familiar application of X-rays is in airport security, where the contents of your luggage are scanned with X-rays.
In cultural heritage, we can use X-rays in X-radiography to obtain images of paintings in X-rays. This can sometimes reveal other paintings beneath the visible paint layer.
X-rays can also be employed in cultural heritage through a method called X-ray fluorescence, or XRF (my favorite scientific method used in cultural heritage). Using XRF, we can determine the elemental composition of objects of cultural heritage. This can help us identify the materials used in creating those objects. And knowing the chemical composition of heritage objects can help us find the best conservation conditions for those objects.
The Ultraviolet (or the UV) region is the region between the visible and the X-ray regions of the electromagnetic spectrum.
Probably the most common topic where UV comes up is the UV rays we receive from the sun. On the one hand, this UV light is beneficial because it helps the organism produce vitamin D. On the other hand, overexposure to the sun’s UV light can lead to sunburn, or more severe consequences, such as skin cancer. So be careful with the exposure to UV light, whether that’s from the sun or from artificial sources like tanning beds.
In cultural heritage, UV lamps can be used to examine the surface of art objects. For example, they can be used to detect retouches and restorations of art objects, as these are done on the object surface. This way, through the details revealed in UV light, we can learn about the history of a certain object.
The visible region of the electromagnetic spectrum is composed of electromagnetic radiation whose wavelengths we can detect with our eyes.
There are many sources of visible light. Some of these include the sun, the aurora borealis and australis, or the fireflies.
In cultural heritage, in visible light, we can observe and admire our favorite works of art. But, visible light can also damage the objects. That’s why you see certain objects, like old books and parchments in museums, kept in a dimmer light.
The infrared (or IR) radiation is lower in energy than the visible light.
In the infrared region, the most common application is the TV remote. But it can also be used in thermal imaging, which works by detecting the radiation with infrared wavelengths, which is emitted by hot objects, including human bodies. This is very useful currently, during the COVID-19 pandemic, to detect the high temperature in people.
In cultural heritage, IR spectroscopy can be used to identify the chemical composition of art objects. And infrared images can be used to view the sketch layers of paintings, underneath the painting layers that we can observe with our eyes. This way, we can see what the original sketch looked like, and, by comparing it to the final product, we can see if the artist changed his or her mind while creating the painting.
With lower energy than the IR radiation, this is where our microwave ovens work. But these waves are also used in communications and radars.
In cultural heritage, microwaves treatment of artwork can help with the disinfestation from various biological agents infesting these artworks.
The radio waves, at the very left of the electromagnetic spectrum, are the least energetic electromagnetic waves.
Their most common applications are in radios, communications, and air-traffic control. One of my favorite applications of radio waves is in the search for extraterrestrial intelligence (or SETI). This is a project that uses big radio antennas to detect signals from outer space. This signal is then analyzed for any patterns that might indicate intelligent communication from someone from outer space. Let’s keep on searching for messages from E.T.!
In cultural heritage, radio waves are used through a scientific method called Nuclear Magnetic Resonance, or NMR. Mobile NMR (NMR in low magnetic fields) can reveal the stratigraphy of paintings through the use of the Profile NMR-MOUSE. It can also be used to study the aging of heritage objects and to monitor the water penetration in wall paintings, which can lead to their deterioration.