Sunlight is not one thing. It arrives as a whole range of wavelengths, and the short ones carry enough energy to break the chemical bonds that living cells are built from. That single fact is why a thin layer of gas forty kilometres above your head matters to your skin, to crops, and to the base of every ocean food web.
📚 What you need to know
The Sun emits electromagnetic radiation across a range of wavelengths, from low-frequency radio waves to high-frequency gamma radiation.
Shorter wavelength means higher frequency and more energy, and more energy means more damage to living organisms.
UV damage to humans: sunburn, DNA mutation, skin cancer, cataracts and premature skin ageing.
Phytoplankton are damaged by UV, which lowers primary productivity in aquatic ecosystems.
That loss cascades up the food web through zooplankton to fish and marine mammals, reducing aquatic biodiversity.
Why short wavelengths do the damage
Energy and wavelength are inversely related. Long, low-frequency waves such as radio carry very little energy and pass through you harmlessly. As wavelength shortens, energy per photon rises, and at ultraviolet wavelengths it becomes high enough to break bonds inside molecules — including the bonds holding DNA together. Shorter still and you reach X-rays and gamma radiation, which are more damaging again.
Learn the bands in this order and the whole sub-topic clicks into place. A thinner ozone layer barely alters UVC or UVA at the surface; what it lets through is extra UVB, which is exactly the band that burns skin and damages DNA directly.
That last point is the one worth carrying into the exam. Students often write that ozone depletion “lets in more UV”. True, but vague. Writing that it increases UVB specifically, because UVC is completely absorbed anyway and UVA is barely absorbed in the first place, shows you understand the mechanism rather than the headline.
What UV does to people
Health effect
What happens
Sunburn
Excessive UV triggers an inflammatory response as a defence mechanism; skin becomes red, painful and may blister, indicating damage to skin cells
Damage to cells
UV induces mutations in DNA during cell division, causing genetic alterations that disrupt normal cell growth and raise cancer risk
Skin cancer
Damaged DNA in skin cells can lead to uncontrolled cell growth and cancerous tumours; risk rises with prolonged or intense exposure without protection
Cataracts
Prolonged exposure contributes to clouding of the lens of the eye, causing blurry vision and eventual sight loss if untreated
Premature ageing
Chronic exposure breaks down collagen and elastin fibres in the skin, producing wrinkles, sagging and age spots
Protection worth naming. High-SPF sunscreen, sunglasses that block UV, shade during peak sun hours and covering clothing. These are worth a mark in a question about reducing risk, and they are adaptation — they reduce harm without touching the cause.
What UV does to ecosystems
Excess UV reaching the surface reduces biological productivity. In plants it lowers rates of photosynthesis, alters metabolism and cuts crop yields. The effect on the oceans is arguably more serious, because it hits phytoplankton — the microscopic photosynthetic organisms that convert sunlight, carbon dioxide and nutrients into organic matter, and that sit at the base of almost every aquatic food web.
Follow the arrows and you have a complete four-mark answer. Notice the last step: this is a loss of biodiversity, not just a loss of fish, because species that depend on a narrow food source are the first to disappear.
WORKED EXAMPLE
Outline three effects of increased ultraviolet radiation on human health. [3]
Effect 1 — skin cancerUV damages DNA in skin cells, which can cause uncontrolled cell growth and the formation of cancerous tumours.Effect 2 — cataractsProlonged exposure clouds the lens of the eye, causing blurred vision and eventual loss of sight if untreated.Effect 3 — sunburnExcessive exposure triggers an inflammatory response, reddening and blistering the skin and indicating cell damage.3 / 3Each answer gives a mechanism, not just a name. “Sunburn” alone would not be enough for a full mark on some mark schemes.
WORKED EXAMPLE
Explain how increased UV radiation can reduce biodiversity in aquatic ecosystems. [4]
Step 1 — the primary producer is hitUV causes DNA damage in phytoplankton and reduces their photosynthetic activity and growth.Step 2 — productivity fallsThis lowers primary productivity, so less organic matter is produced at the base of the food web.Step 3 — the effect passes upwardsZooplankton that feed on phytoplankton have less food available, reducing their growth and reproduction.Step 4 — higher consumers and biodiversityFish and marine mammals that rely on these lower trophic levels decline, and species with narrow diets are lost, reducing biodiversity.4 / 4This is a chain question. Each mark is a link, so keep the steps in order and do not skip one.
💡 Exam tip
Learn the rule shorter wavelength, higher energy, more damage. It explains everything else on this page.
Specify UVB when discussing ozone depletion. Saying “more UV” is vaguer and scores less.
For health effects, give the mechanism: DNA damage, inflammation, lens clouding, collagen breakdown.
Use the phytoplankton chain for any “explain the ecological impact” question. It is four clean marks.
Link back to food security if the question allows it. Reduced crop yields and reduced fisheries are a human problem as well as an ecological one.
⚠️ Common mix-up
Confusing UV with heat. UV damage is chemical, caused by photon energy breaking bonds, not by warming.
Getting the bands backwards. UVA is the longest wavelength and least harmful; UVC is shortest and most harmful.
Thinking UVC is a surface problem. It is completely absorbed by stratospheric ozone.
Confusing ozone depletion with the greenhouse effect. They are separate problems with separate causes.
Saying phytoplankton are plants. They are photosynthetic organisms; describing their role as primary producers is the safer wording.
Stopping the food-web answer at “fish die”. The mark is usually for naming the loss of biodiversity as the outcome.
Up next: Ozone in the Stratosphere — what ozone actually is, where it sits, and the constant destruction-and-reformation cycle that keeps the layer stable.
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