Ultraviolet radiation is a small slice of the energy the Sun sends us, and it carries enough punch to break the chemical bonds inside DNA. Understanding why short wavelengths do that damage is the foundation for everything else in this sub-topic.
📚 What you need to know
The Sun emits radiation across a huge range of wavelengths. Shorter wavelength means higher frequency and more energy, and more energy means more damage to living tissue.
UV comes in three bands: UVA (longest, least harmful), UVB (medium, causes burning and DNA damage) and UVC (shortest, most harmful).
Human health effects: sunburn, skin cancer, cataracts, DNA mutation and premature skin ageing.
Plants suffer reduced photosynthesis, altered metabolism and lower crop yields.
Phytoplankton are damaged too, cutting primary productivity in the oceans and affecting every trophic level above them.
Personal protection: high-SPF sunscreen, UV-blocking sunglasses, shade during peak hours, and covering up.
Why short wavelengths do the damage
Radio waves, microwaves, visible light, ultraviolet, X-rays and gamma rays are all the same thing — electromagnetic radiation — differing only in wavelength. As wavelength gets shorter, frequency and energy rise. Radio waves pass through you harmlessly. Ultraviolet carries enough energy to break bonds in the molecules that make up your cells, which is exactly why it matters.
Notice the pattern: the more harmful the band, the more completely the ozone layer removes it. That is the whole reason the next page matters.
Band
What it does
How much reaches us
UVA
The least harmful of the three, but it still ages skin and contributes to skin cancer over time
Passes through the atmosphere and reaches the surface
UVB
Causes sunburn and damages DNA directly — the band most closely linked to skin cancer
Mostly absorbed by stratospheric ozone, but some gets through
UVC
The most harmful, with the shortest wavelength and highest energy
Completely absorbed by stratospheric ozone
This is why ozone depletion is usually described as a UVB problem. UVC is already stopped completely and UVA was never blocked much anyway. It is the middle band where a thinner ozone layer changes how much gets through.
Effects on human health
Effect
What happens
Sunburn
Excessive UV triggers an inflammatory response as a defence. The skin becomes red, painful and may blister — visible evidence that cells have been damaged
DNA mutation
UV can alter DNA during cell division, producing genetic changes that disrupt normal cell growth
Skin cancer
Damaged DNA in skin cells can lead to uncontrolled growth and cancerous tumours. Risk rises with prolonged or intense exposure, especially without protection
Cataracts
Prolonged exposure clouds the lens of the eye, causing blurred vision and eventually sight loss if untreated
Premature ageing
Chronic exposure breaks down collagen and elastin fibres, giving wrinkles, sagging skin and age spots
Protection is simple and effective: sunscreen with a high SPF, sunglasses that block UV, shade during the hours around midday, and clothing that covers exposed skin. Cheap measures, large reduction in risk.
Effects on productivity
UV does not only affect people. It reaches every photosynthesising organism, and that has consequences that ripple through whole ecosystems.
Plants on land
Reduced rates of photosynthesis, so less energy is fixed.
Altered metabolism, as plants divert resources into repairing damage and producing protective pigments.
Lower crop yields, which is a food security problem as well as an ecological one.
Phytoplankton in the oceans
Phytoplankton convert sunlight, carbon dioxide and nutrients into organic matter, and they sit at the base of nearly every marine food web. UV damages their DNA and reduces their photosynthetic activity and growth, so primary productivity falls. Everything above them feels it.
Phytoplankton also absorb carbon dioxide, so anything that reduces them has a climate consequence as well as an ecological one.
Worked examples
WORKED EXAMPLE
Explain why shorter-wavelength radiation is more damaging to living organisms. [3]
Step 1: the physics
Shorter wavelength means higher frequency, and higher frequency radiation carries more energy.
Step 2: what that energy does
That energy is enough to break chemical bonds in the molecules inside cells, including DNA.
Step 3: the consequenceDamaged DNA can mutate during cell division, disrupting normal growth and raising the risk of cancer — which is why UVC would be lethal and radio waves are harmless.More energy per photon, more bonds broken
WORKED EXAMPLE
Explain how an increase in UV radiation reaching the ocean surface could reduce populations of marine mammals. [4]
Step 1: the base of the web
UV damages phytoplankton DNA and reduces their photosynthesis and growth, so primary productivity falls.
Step 2: the first consumers
Zooplankton feed on phytoplankton, so less food is available and their growth and reproduction decline.
Step 3: up the chain
Fish that feed on zooplankton have less to eat, so their populations fall too.
Step 4: the topMarine mammals depend on those fish. Less prey means poorer condition, lower breeding success and smaller populations — and reduced biodiversity overall.Less energy fixed at the base, less available at every level
💡 Exam tip
Rank the bands correctly: UVC most harmful, UVA least. Then say which are absorbed.
Link wavelength to energy explicitly. “Shorter wavelength, more energy, breaks bonds” is a chain the mark scheme follows.
For health effects, give the mechanism: DNA damage in skin cells causing uncontrolled growth, not just “causes cancer”.
Do not stop at humans. Effects on crops and phytoplankton show you understand the ecological dimension.
Name protection measures — SPF, sunglasses, shade, covering up. They are quick, easy marks.
⚠ Common mix-up
UV is not the same as infrared. Infrared is longer than visible light and is what greenhouse gases absorb; UV is shorter and is what ozone absorbs.
UVA is not harmless. It reaches the surface all the time and contributes to ageing and cancer.
Ozone depletion is not a heating problem. More UV does not warm the planet; it damages living cells.
Sunburn is not the damage itself. It is the body’s inflammatory response to damage already done.
Phytoplankton are not plants. They are microscopic photosynthetic organisms — but they do the same job at the base of marine food webs.
Up next: Ozone in the Stratosphere — how ozone forms, breaks and reforms, and how a handful of manufactured chemicals broke a balance that had held for millions of years.
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