Shorter wave, more energy, more danger: Ultraviolet (UV) sits just past violet light. Its short waves carry more energy than light, enough to damage the molecules that cells are made of.
The points to remember
- The shorter the wavelength, the higher the frequency.
- The higher the frequency, the more energy the radiation carries.
- UV has a shorter wavelength than visible light, so it carries more energy.
- That energy can break the bonds in molecules such as DNA and proteins.
- So UV poses a greater danger to life than visible light or infrared.
Shorter wavelength
UV waves are shorter than the waves of visible light.
Higher frequency
More waves pass each second.
More energy
Each burst of UV carries more energy than visible light.
Danger to life
Enough energy to break bonds in DNA and proteins.
Remember it as: Short wave, strong punch.
Real example: New York City cleans its drinking water with UV. Since 2013 the Catskill-Delaware plant has shone UV lamps on up to about 2 billion US gallons a day for over 9 million people. The UV damages the DNA of microbes, so they cannot multiply.
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UV is split into three bands by wavelength. Each one damages living things, but how much of each reaches the ground depends on the ozone layer.
The points to remember
- UVC (100-280 nm): the shortest wavelength and the most energy; all of it is absorbed by ozone and oxygen.
- UVB (280-315 nm): most is absorbed by ozone; the rest causes sunburn, DNA damage, skin cancer and cataracts.
- UVA (315-400 nm): mostly passes the ozone layer; about 95% of the UV that reaches the ground.
- UVA goes deep into the skin: it causes premature ageing and adds to the risk of skin cancer.
- All three damage living things; the shorter the wavelength, the more harm per unit of radiation.
UVC
- 100-280 nm: most energy.
- All absorbed high up.
UVB
- 280-315 nm: most absorbed.
- Sunburn, skin cancer, cataracts.
UVA
- 315-400 nm: mostly passes.
- Ages skin; adds to cancer risk.
Real example: sunbeds give out mostly UVA. In 2009 the World Health Organization's cancer agency put UV tanning beds in its highest cancer group, alongside tobacco smoke: even the 'gentlest' band harms.
Not 'blocks all UV': Ozone absorbs all UVC and most UVB. It does not block all UV: most UVA, and some UVB, still reach the ground.
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How this comes up: A short Paper 2 question asks why one type of radiation is more harmful than another. Build the chain: wavelength, frequency, energy, then the harm.
UVC lamps in hospitals kill bacteria on surfaces in minutes. Yet the Sun's UVC never harms people on the beach.
Outline why UVC from the Sun does not harm life at Earth's surface, even though it carries the most energy.
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See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.