Ultraviolet Radiation

Ultraviolet (UV) radiation occupies the region of the electromagnetic spectrum immediately beyond violet light. The term ultraviolet literally means beyond violet, referring to electromagnetic waves with frequencies higher than those visible to the human eye. Ultraviolet radiation is produced by electronic transitions in atoms and molecules, hot objects such as the Sun, electric arcs, and specialized ultraviolet lamps.

Ultraviolet wavelengths extend from approximately 400 nm down to about 10 nm, where the spectrum gradually overlaps with X-rays. Although invisible, ultraviolet radiation plays important roles in medicine, biology, astronomy, environmental science, and public health.

Ultraviolet radiation is commonly divided into three regions according to wavelength:

• UV-A (315–400 nm): the longest wavelength and lowest-energy ultraviolet radiation.
• UV-B (280–315 nm): intermediate wavelength and energy.
• UV-C (100–280 nm): the shortest wavelength and highest-energy ultraviolet radiation.

Fortunately, Earth's atmosphere absorbs nearly all UV-C radiation and most UV-B radiation before it reaches the surface. Consequently, the vast majority of ultraviolet radiation that reaches people outdoors is UV-A, with a much smaller contribution from UV-B.

One of the first dramatic demonstrations of ultraviolet astronomy occurred during the Apollo 16 mission in 1972. George Robert Carruthers designed a compact ultraviolet camera that astronauts operated on the lunar surface. Because the Moon has no atmosphere to absorb ultraviolet light, the instrument captured remarkable images of Earth that revealed atmospheric hydrogen, auroral activity, and other features that cannot be seen in visible-light photographs.

Human Exposure to Ultraviolet Radiation

Ultraviolet radiation has both beneficial and harmful effects on human health. The biological effects depend on the wavelength, the intensity of the radiation, and the duration of exposure.

Moderate exposure to UV-B radiation stimulates the production of vitamin D in the skin. Vitamin D is essential for calcium absorption, healthy bones, muscle function, and normal immune function. Only relatively small amounts of sunlight are needed for adequate vitamin D production in most healthy individuals.

Excessive ultraviolet exposure damages DNA within skin cells. Although cells possess mechanisms to repair this damage, repeated exposure increases the likelihood of permanent mutations that can eventually lead to skin cancer.

Excessive ultraviolet exposure may also cause:

• sunburn,
• premature aging of the skin (photoaging),
• wrinkles and loss of skin elasticity,
• suppression of certain immune responses, and
• eye damage, including cataracts.

The skin responds to ultraviolet exposure by producing melanin, the pigment responsible for tanning. Melanin absorbs part of the incoming ultraviolet radiation and provides some protection to deeper layers of the skin. However, a tan provides only limited protection and does not eliminate the risk of skin damage or skin cancer.

Ultraviolet radiation can also damage proteins in the eye's lens, increasing the risk of cataracts over time. Sunglasses labeled as blocking both UV-A and UV-B radiation provide effective protection.

Medical Applications of Ultraviolet Radiation

Despite its potential hazards, ultraviolet radiation has many valuable medical and scientific applications.

• UV-C radiation is widely used to disinfect hospital rooms, operating rooms, laboratory equipment, ventilation systems, and drinking water because it destroys the DNA and RNA of microorganisms, preventing them from reproducing.
• Controlled doses of UV-B radiation are used in phototherapy to treat skin conditions such as psoriasis, eczema, and vitiligo.
• Ultraviolet fluorescence is used in medical laboratories to identify certain biological molecules and microorganisms.
• Blue visible light—not ultraviolet radiation—is commonly used to treat newborn infants with jaundice by helping break down excess bilirubin in the skin.

Ultraviolet radiation can also degrade pigments, dyes, and many organic materials. Museums often protect valuable paintings, historical documents, and photographs by displaying them behind specially coated glass that absorbs ultraviolet radiation and slows fading and chemical deterioration.

Ultraviolet Radiation and the Ozone Layer

If all of the Sun's ultraviolet radiation reached Earth's surface, the consequences for life would be severe. Fortunately, a layer of ozone (O₃) located primarily in the stratosphere, about 15–35 km above Earth's surface, absorbs nearly all UV-C radiation and most UV-B radiation before it reaches the ground. This natural shield plays a critical role in protecting living organisms from harmful ultraviolet exposure.

Beginning in the late twentieth century, scientists observed significant depletion of stratospheric ozone, particularly over Antarctica during the Southern Hemisphere spring. This seasonal reduction, commonly called the ozone hole, is caused primarily by chlorine- and bromine-containing compounds released from human-made chemicals such as chlorofluorocarbons (CFCs) and halons.

Ultraviolet radiation breaks apart CFC molecules in the upper atmosphere, releasing highly reactive chlorine atoms. For example, one important reaction is

The chlorine atom then participates in a catalytic cycle that destroys ozone:

Because the chlorine atom is regenerated, a single chlorine atom can destroy thousands of ozone molecules before it is eventually removed from the atmosphere.

International concern over ozone depletion led to the adoption of the Montreal Protocol in 1987, an international agreement that phased out the production of most ozone-depleting substances, including CFCs. The Montreal Protocol is widely regarded as one of the most successful international environmental agreements ever implemented.

Although many ozone-depleting chemicals remain in the atmosphere because of their long lifetimes, satellite observations show that the ozone layer has been gradually recovering since the early 2000s. Scientists expect that, if current international agreements remain in place, most regions of the ozone layer will recover to approximately their 1980 levels during the second half of the twenty-first century.

X-Rays

X-rays are high-energy electromagnetic waves with wavelengths approximately between 10 nm and 0.01 nm, corresponding to frequencies higher than ultraviolet radiation. They were discovered in 1895 by the German physicist Wilhelm Conrad Röntgen, who observed an unknown form of highly penetrating radiation while experimenting with electrical discharges in evacuated tubes. Because their nature was initially unknown, he called them X-rays, where "X" represented the unknown.

As discussed in the previous section, X-rays are produced when high-energy electrons interact with matter. Medical X-ray tubes generate radiation through two mechanisms:

• Characteristic X-rays, produced when electrons transition between atomic energy levels after an inner-shell electron has been removed.
• Bremsstrahlung (braking radiation), produced when fast-moving electrons are slowed by the electric fields of atomic nuclei.

X-rays have much higher energies than visible or ultraviolet light and are therefore capable of penetrating soft tissues. Their penetration depends on both the energy of the X-rays and the density and composition of the material through which they pass. Dense materials, such as bone and metals, absorb X-rays much more strongly than soft tissues, allowing internal structures to be visualized.

How X-Rays Are Produced

X-rays are produced when high-energy electrons interact with matter. Modern X-ray tubes generate electrons by heating a metal filament and accelerating the electrons toward a metal target using a high voltage. When the electrons strike the target, X-rays are produced by two different physical mechanisms.

Characteristic X-Rays

The first mechanism occurs when an incoming electron ejects an electron from one of the atom's inner energy levels. An electron from a higher energy level then falls into the vacancy, releasing the energy difference as an X-ray photon. Because every element has a unique set of electron energy levels, the emitted X-ray energies are characteristic of the target material.

Bremsstrahlung (Braking Radiation)

The second mechanism occurs when a high-speed electron passes close to the positively charged nucleus of an atom. The electric force slows and deflects the electron. Because accelerating or decelerating charged particles emit electromagnetic radiation, the electron loses energy by producing an X-ray photon.

Unlike characteristic X-rays, which have discrete energies determined by the target material, bremsstrahlung produces a continuous spectrum of X-ray energies. Medical X-ray tubes produce both types of X-rays, and together they create the radiation used for diagnostic imaging.

Medical Imaging

The most familiar application of X-rays is diagnostic imaging. Conventional radiographs (X-ray images) provide rapid, inexpensive visualization of bones, lungs, teeth, and many implanted medical devices. Computed tomography (CT) scanners combine hundreds of X-ray images acquired from different angles to reconstruct detailed three-dimensional images of the body.

Radiation Safety

Because X-rays are ionizing radiation, they can damage DNA and other biological molecules. Excessive exposure increases the risk of cancer and other radiation-induced injuries. For this reason, medical imaging follows the principle of ALARA (As Low As Reasonably Achievable), minimizing radiation dose while still obtaining images of sufficient diagnostic quality.

Modern digital imaging systems, automatic exposure controls, shielding, and careful quality assurance have significantly reduced patient radiation doses compared with earlier generations of X-ray equipment. Although CT examinations generally involve higher radiation doses than conventional radiographs, the diagnostic benefits often greatly outweigh the small associated risks when the examination is medically justified.

Other Applications of X-Rays

X-rays are used in many fields beyond medicine. In industry, they inspect welds, aircraft components, and manufactured parts without damaging them. Airport security systems use X-rays to examine luggage for prohibited items.

In science, X-ray diffraction allows researchers to determine the arrangement of atoms in crystals and large biological molecules. This technique played a central role in determining the double-helical structure of DNA and continues to be indispensable in structural biology, chemistry, and materials science.

X-ray fluorescence (XRF) is another analytical technique in which X-rays excite atoms to emit characteristic X-rays. Because each element produces a unique set of energies, XRF allows scientists to identify and quantify the elemental composition of materials without destroying the sample.

Gamma Rays

Gamma rays (γ rays) are the highest-energy electromagnetic waves in the electromagnetic spectrum. They were first identified during studies of natural radioactivity in the late nineteenth century. Because their origin was initially unknown, they were given the name gamma rays, following the earlier discoveries of alpha and beta radiation.

Gamma rays are produced by changes within an atomic nucleus. They commonly accompany radioactive decay but are also generated during nuclear reactions, particle interactions, and some of the most energetic events in the universe, such as supernova explosions and neutron star mergers. The lowest-energy gamma rays overlap the highest-energy X-rays, and the distinction between them is based primarily on their origin rather than their energy. X-rays are generally produced by electrons outside the nucleus, whereas gamma rays originate from the nucleus itself.

Like X-rays, gamma rays are ionizing radiation. Their high energies allow them to penetrate deeply into matter and remove electrons from atoms, producing ions. This property makes gamma rays both extremely useful in medicine and potentially hazardous if appropriate safety precautions are not followed.

Medical Applications

Gamma rays play an important role in modern healthcare. Small amounts of radioactive materials are used in nuclear medicine to produce gamma rays that can be detected outside the body. These techniques allow physicians to evaluate organ function, blood flow, and metabolism.

Gamma rays are also used in radiation therapy to treat cancer. Carefully controlled, high-energy beams damage the DNA of rapidly dividing cancer cells while minimizing damage to surrounding healthy tissue.

Industrial and Scientific Applications

Gamma rays are used to sterilize medical equipment, pharmaceuticals, and certain food products by destroying bacteria, viruses, fungi, and insects without making the products themselves radioactive. Food irradiation is considered safe by major international health organizations and helps reduce foodborne illness while extending shelf life.

Gamma rays are also used in industrial inspection, materials testing, and scientific research because of their ability to penetrate thick materials.

Detecting Electromagnetic Waves from Space

Astronomers study the universe using the entire electromagnetic spectrum. Different astronomical objects emit different wavelengths, so combining observations from radio waves through gamma rays provides a much more complete picture of the universe than visible light alone.

Radio telescopes detect long-wavelength radio emissions from stars, galaxies, pulsars, and clouds of interstellar gas. Observations of the cosmic microwave background—the faint microwave radiation left over from the Big Bang—have provided strong evidence for the origin and evolution of the universe.

Infrared telescopes observe heat emitted by cool stars, planets, dust clouds, and newly forming stars. Because infrared detectors are sensitive to thermal radiation from the instruments themselves, they are often cooled to very low temperatures to reduce unwanted background signals.

Visible-light telescopes remain essential tools for astronomy, while ultraviolet, X-ray, and gamma-ray observatories are generally placed on satellites because Earth's atmosphere absorbs these high-energy forms of electromagnetic radiation before they reach the ground.

Examples of modern space observatories include the Hubble Space Telescope, which observes visible and ultraviolet light; the James Webb Space Telescope, which primarily observes infrared radiation; the Chandra X-ray Observatory, which studies high-energy X-ray sources; and the Fermi Gamma-ray Space Telescope, which detects gamma rays from some of the most energetic events in the universe.