1. What are ultraviolet rays?
Ultraviolet (UV) rays are electromagnetic waves with wavelengths shorter than visible violet light but longer than X-rays. They carry more energy than visible light and can cause chemical changes in materials, including skin and biological tissues.
Although we cannot see UV rays directly, their effects are noticeable in sunlight, fluorescent materials, and many natural and artificial processes.
1.1. Position in the electromagnetic spectrum
Ultraviolet rays lie between visible light and X-rays. They begin at wavelengths shorter than violet light (~400 nm) and extend down to around 10 nm.
1.2. General characteristics
- Wavelength: 400 nm to 10 nm
- Frequency: \(7.5 \times 10^{14}\) to \(3 \times 10^{16}\,\text{Hz}\)
- Higher energy than visible light
- Mostly invisible to the human eye
2. Types of ultraviolet rays
Ultraviolet radiation is often divided into three categories based on wavelength and biological effect.
2.1. UVA (long-wave UV)
Wavelength range: 400–315 nm. These rays penetrate deeply into skin and contribute to tanning and aging effects.
2.2. UVB (medium-wave UV)
Wavelength range: 315–280 nm. These rays cause sunburn and play a major role in DNA damage in skin cells.
2.3. UVC (short-wave UV)
Wavelength range: 280–100 nm. These rays have very high energy but are mostly absorbed by the Earth's atmosphere and do not reach the surface.
3. Sources of ultraviolet rays
UV rays come from both natural and artificial sources. The Sun is by far the strongest natural source.
3.1. Natural sources
- The Sun (primary source)
- Stars and hot celestial objects
- Electrical discharges (lightning)
3.2. Artificial sources
- Mercury vapor lamps
- Fluorescent lamps
- UV sterilizers
- Welding arcs
- Blacklight lamps
4. Interaction of ultraviolet rays with matter
UV rays are energetic enough to cause chemical bonds to break and initiate photochemical reactions. This makes them useful in some technologies but also potentially harmful to living beings.
4.1. Absorption in the atmosphere
Most UV radiation from the Sun is absorbed by oxygen and ozone in the Earth’s atmosphere. The ozone layer is especially effective at absorbing UVC and most UVB rays.
4.2. Interaction with skin and eyes
UVB and UVC rays can break chemical bonds in DNA, which may cause mutations. UVA penetrates deeper layers of skin and can damage collagen.
4.3. Fluorescence
Some materials absorb UV light and emit visible light. This effect is used in security markings, fluorescent paints, and blacklight decorations.
5. Effects of ultraviolet rays on living beings
Depending on intensity and duration, UV exposure can have beneficial or harmful effects on living organisms.
5.1. Beneficial effects
- Helps the body produce vitamin D
- Used in controlled medical treatments (phototherapy)
- Assists in sterilizing surfaces by killing microbes
5.2. Harmful effects
- Sunburn and skin damage
- Accelerated skin aging
- DNA mutations
- Increased risk of skin cancers
- Eye damage such as cataracts with prolonged exposure
6. Uses of ultraviolet rays
Despite their potential hazards, UV rays are extremely useful in science, medicine, and industry.
6.1. Sterilisation and disinfection
UVC radiation is highly effective at killing bacteria, viruses, and fungi. UV lamps are commonly used to sterilise equipment, water, and air.
6.2. Forensics and material testing
UV light reveals fluorescent marks, forged documents, and hidden stains that are invisible under normal lighting.
6.3. Sunbeds and tanning lamps
These devices use UVA or UVB radiation to artificially tan the skin by stimulating melanin production.
6.4. Scientific applications
- Studying atomic spectra
- Analysing minerals and compounds
- UV curing of inks and adhesives
7. Example: UV in sunlight
When sunlight reaches Earth, only a small part of its UV radiation makes it through the atmosphere. This remaining UV is responsible for tanning and sunburn. Sunscreen lotions protect the skin by absorbing or reflecting harmful UV rays before they penetrate the skin.