ELECTROMAGNETIC RADIATION: THE USE OF PHOTODYNAMIC THERAPY (PDT) AS A HEALING LIGHT

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Life on Earth depends on one chemical in particular – chlorophyll. It can Interact with red light in sunlight; in a photochemical reaction, its electrons become more energetic. A chain of reactions forms sugar from water and Carbon dioxide in the surroundings, with oxygen as a useful by-product.

A photochemical reaction is now being applied to the treatment of cancers, in a procedure called photodynamic therapy (PDT). Its key process parallels the action of chlorophyll. A patient is given a harmless drug designed to build up in cancerous tissue. The drug is a small part of the very complex chlorophyll molecule and acts as a light-absorbing dye. Dyes absorb certain frequencies from white light directed at them, so the light they reflect lacks those frequencies, which is why a dye looks coloured: leaves look green because green light is, roughly, white minus the red.

High-intensity blue light (425 nm) used for the treatment of acne. Skin58 - Own work, CC BY-SA 4.0

In PDT, light comes from a laser tuned to a frequency absorbed by the drug. The laser produces a low-energy dose of red light which penetrates quite deeply into the body and causes no harm to ordinary tissue. The important thing is what happens to the energy carried by the absorbed radiation.

In typical laser treatment, absorbed radiation heats up target tissue enough to kill the cells, both ordinary and cancerous. In PDT, the less energetic radiation that the drug absorbs triggers a photochemical reaction which releases a poison consisting of single atoms of oxygen. In contrast to the harmless diatomic oxygen molecules we breathe from the air, single oxygen atoms are highly reactive and are able to kill living cells.

Photodynamic therapy is effective because, since the poison only forms at the point where the dye accumulates, only the diseased cells are killed.

THE IDEAS IN THIS ARTICLE

We can say that the Universe consists of matter – in one form or another – and electromagnetic radiation. Light is just one small part of the range of electromagnetic radiation that exists. It has just the right character to get through the Earth’s atmosphere – a useful property that living things have taken advantage of. It is visible because our eyes are adapted to respond to it.

Electromagnetic radiation can fool us into thinking that it can be both a wave and a particle, though maybe not at the same time. The particle – photon – aspect of electromagnetic radiation will be discussed in my next article, which deals with quantum theory; It will also feature the photon, as one of the family of particles making up the Universe of matter.

This article will just cover the wave aspect of the range of radiations observed in the electromagnetic spectrum, their speed of propagation, how they are produced and detected, and how we make use of the wave nature of the radiations. So we deal here with diffraction, interference and polarization.

THE ELECTROMAGNETIC SPECTRUM

A very hot object such as a star produces a range of radiation; the spectrum of the Sun is shown below. Visible light is the tiny fraction that we detect with our eyes. White light can be separated into a range of colours to form a visible spectrum, from violet with a wavelength of about 4 × 10^-7 m, to red of about 7 × 10^-7 m.

A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths. Inductiveload, NASA - self-made, CC BY-SA 3.0

WHY ‘ELECTROMAGNETIC’?

The radiations are produced and detected by the acceleration or sudden movement of electrons (or occasionally of other charged particles). For example, a radio signal is produced by electrons as they oscillate to and fro in an aerial. The result is an interlocked pair of fields, electric and magnetic, oscillating at the frequency of the electron current. We can think of the electric field as being produced by the charge on the electrons.

A magnetic field is produced whenever charges move, and is proportional to the size of the current. What is surprising (and hard to explain) is that the interlocking fields move away from the aerial at a speed which in a vacuum is the same for all electromagnetic radiations, whatever their frequency. This is the speed of light, labelled with the standard symbol c. These moving fields behave like waves: they can be diffracted, they interact with each other to show interference, and they can be polarised. These terms will be explained later.

When the waves reach another metal rod, the electric field component exerts a varying electric force on the electrons in the metal which oscillates in time with the variations of the field. A loop-shaped metal aerial encloses the varying magnetic field of the waves and, by the laws of electromagnetic induction, an e.m.f. is induced in the loop.

Electromagnetic induction network. Gepsimos - Own work, CC BY-SA 3.0

Infrared radiation, light and the shorter wavelengths beyond the visible spectrum are also made by electrons moving within atoms.

PRODUCING AND DETECTING ELECTROMAGNETIC WAVES

Radio waves

Radio waves are produced by the accelerated motion of free electrons as explained above and are used mainly in communication systems. A process was developed in the 1930s to provide short-wavelength radio waves – microwaves – to help in the medical treatment of damaged tissue. Most school laboratories now have low power microwave sources for experiments on the nature of electromagnetic waves. Microwaves are stil used in physiotherapy, as well as in cooking and also as message carriers.

Radar

The potential of these very short radio waves for the radio detection and ranging of aircraft in the Second World War was quickly exploited with the development of radar. In radar transmitters, electrons oscillate in small metal cavities called magnetrons which contain a strong magnetic field.

The waves produced have a frequency determined by the size of the cavity and the strength of the magnetic field, depending on how long it takes an electron to complete an oscillation. The waves are led out of the cavity through a metal tube called a waveguide. The metal walls reflect the waves and keep them moving in the right direction, towards a small aerial at the focus of a metal mirror. They are then emitted as a beam. The transmitting aerial and mirror can also act as the main receivers of the radiation, with a detector at the focus.

Radar dish. Pixabay

Microwave ovens

Similar magnetrons are used in microwave ovens. The frequency of the microwaves is selected to match the resonance frequency of water molecules so that energy from the waves is transferred efficiently to the kinetic energy of the molecules. This raises the temperature of any food containing water.

Infrared radiation

Infrared radiation (IR) overlaps with short microwaves in the electromagnetic spectrum, but in practice, the term describes the (invisible) hot body emissions that have wavelengths just beyond the visible red. Infrared is readily absorbed by matter. By giving its energy to cause movement in the molecules of a body, thus raising the body’s temperature. This is why IR is often called ‘heat rays’ or thermal radiation.

Being sensitive to near (i.e. short wavelength) infrared, the human skin is a simple detector – we feel the ‘heat’. A thermometer with a blackened bulb (or sensor surface) will show a rise in temperature when placed in the red region of a white-light spectrum. But it shows a greater temperature rise when placed in the dark region just outside the red – as was first shown in 1800 by the astronomer Sir William Herschel (Germany/Britain, 1738-1822).

Special film takes photographs in infrared, but is difficult to use and has largely been replaced by electronic detecting methods. Modern infrared detectors use solid state (electronic) detectors which act rather like TV cameras. In industry, they monitor processes that cause temperature differences. They are used in Earth satellites both for military purposes and to observe the growth of crops, measure surface temperature, etc.

Infrared detectors can be made sensitive to a narrow wavelength band: in the rescue cameras used to find fire or earthquake victims, the detectors are most sensitive to the infrared radiation that is characteristic of body temperature.

Electronic devices (e.g. semiconductor light-emitting diodes) also emit infrared, and are widely used in the remote controls of household electronic systems such as TV sets, DVD players/recorders and hi-fi systems.

Light

Light is emitted from matter when it is made hot enough – as in the Sun, in flames and in filament lamps. It is also emitted in certain chemical changes – in the photoluminescence of fireflies and glow worms, for example, and – as a result of electron movements in stimulated atoms – as in lasers and ordinary fluorescent lighting. Visible radiation is detected by both human and animal eyes and also by a range of devices including ordinary photographic film, photoelectric cells of various types and by very sensitive ‘charge-coupled devices’ (CCDs).

SHORT WAVE AND IONISING RADIATION

Ultraviolet radiation (UV) is produced by changes in the energy levels of atomic electrons, as we shall see below, which happens in very hot bodies. UV was discovered during early photochemical experiments in which it was found that light blackened silver chloride. Studying the effects of a spectrum of white light in 1801 Johann Ritter (Germany, 1776-1810) found that a type of radiation beyond violet in the visible spectrum had even more effect on silver chloride than visible light. As explained below, the shorter the wavelength, the more energetic the radiation, so UV is very effective at exposing photographic film. UV is good at making chemicals called phosphors glow, and low-intensity UV are used for special stage effects. Ordinary fluorescent (‘daylight’) tubes and compact (‘low-energy’) lamps emit light from a phosphor coating, stimulated by UV light from excited mercury atoms. UV is also energetic enough to ionize atoms and so can harm living tissue, causing sunburn and skin cancers.

X-rays have shorter wavelengths than ultraviolet. They are produced when electrons decelerate very rapidly, such as when high-speed electrons are stopped by colliding with a metal target. They were first discovered, accidentally, by William Röntgen (Germany, 1845-1923). He was experimenting with cathode rays (electron streams in an evacuated tube) and noticed that they caused a phosphor screen some metres away to glow. He soon found that the rays could pass through soft tissue but were selectively absorbed by denser material such as metal and bone. Within weeks of the discovery, X-rays were being used in hospitals for diagnosis and treatment – rather too soon and too dangerously.

Pixabay

Nowadays carefully monitored doses of X-rays are used for medical diagnosis. They are also used to inspect metal objects for flaws, and they have played a significant role in the science of X-ray crystallography which, amongst other things, led to the discovery of the helical structure of DNA.

Gamma radiation is produced by changes in the internal energy of an atomic nucleus in radioactive decay. Gamma rays are very energetic and penetrating: photographic film and ionization detectors such as Geiger counters will detect them.

DAMAGE CAUSED BY IONIZING RADIATION

Because of the high energy associated with photons of such short wavelength radiation, UV, X- and gamma radiation can all damage living tissue. These radiations ionize atoms and molecules in living cells and disrupt their biochemical processes. The cells may die, or worse, they may become cancerous. Small changes to the DNA in sperm or ovum cells are particularly dangerous since they may cause genetic mutations to appear in offspring.