One kind of energy, many wavelengths: Light, heat from the Sun and radio signals are all electromagnetic radiation. They differ only in their wavelength.
The points to remember
- The Sun gives out electromagnetic radiation: energy that travels as waves, even through empty space.
- It covers a range of wavelengths: from radio waves (longest) to gamma rays (shortest).
- In order: radio, microwave, infrared, visible light, ultraviolet (UV), X-ray, gamma.
- Long wavelength = low frequency; short wavelength = high frequency.
- Wavelength = the distance from one wave peak to the next; frequency = how many waves pass each second.
Remember it as: Long and lazy: radio. Short and strong: gamma.
Real example: BBC Radio 4 on long wave broadcasts at 198 kHz: its waves are about 1,500 m long. A hospital X-ray uses waves shorter than a millionth of a millimetre. Both are electromagnetic radiation; the X-ray has a far higher frequency and more energy.
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The Sun gives out the whole spectrum, but not equally. Almost all of its energy arrives as three types: infrared, visible light and UV.
The points to remember
- Most of the Sun's energy arrives as infrared, visible light and UV.
- At the top of the atmosphere: about 49% infrared, 42% visible and 8% UV.
- At the ground, UV is only about 3 to 5%: the atmosphere (the ozone layer) absorbs most of it.
- The Sun gives out the most energy as visible light: our eyes evolved to see the strongest part of sunlight.
Infrared
About half of the Sun's energy, both above the air and at the ground.
Visible light
About 42%: the biggest single band, and the part we see.
Ultraviolet
8% at the top, only about 4% at the ground: the rest is absorbed.
Read the chart: Compare the two bars for each type: the only big change is UV, which halves on the way down.
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Each of the three types of sunlight does a different job for life on Earth, the biosphere.
The points to remember
- Infrared = heat: it warms land, sea and air; Earth sends it back out, and greenhouse gases trap some of it.
- Visible light drives photosynthesis, so it is the energy source of almost every food chain.
- Visible light also lets animals see, and day length sets seasons for flowering and migration.
- UV has the most energy of the three: it damages cells and DNA, so too much is harmful.
- A little UV (UVB) is useful: it makes vitamin D in human skin.
- Some animals see UV: bees use UV patterns on flowers to find nectar.
Infrared
- Warms land, sea and air.
- Drives the water cycle and winds.
Visible light
- Photosynthesis: the start of food chains.
- Sight and day length.
Ultraviolet
- Most energy: damages cells and DNA.
- Makes vitamin D in skin.
Real example: the NHS tells everyone in the UK to think about a vitamin D supplement from October to early March. In winter the Sun is low, its light passes through more air, and too little UVB reaches the ground for skin to make enough vitamin D.
Do not mix up the ozone layer and the greenhouse effect: The ozone layer (stratosphere) absorbs incoming UV from the Sun. Greenhouse gases (troposphere) absorb outgoing infrared from Earth's surface. Different radiation, different layer.
How this comes up: Paper 2, Section A: a short question with 2 or 3 marks. Name the type of radiation, then say what it does: one point per mark.
Plants in a greenhouse in the Netherlands grow under sunlight, and the greenhouse warms up on sunny days.
Outline the role of visible light and infrared radiation in the biosphere.
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