The Sun's radiation at Higher Level: The same spectrum as SL, with different examples: the James Webb and Hubble telescopes, and the UV patterns bees see on flowers. At HL, keep the link 'shorter wavelength, more energy' ready: it explains why UV breaks the bonds in ozone and oxygen later in this topic.
Practise this as you read
- Rank the types of radiation by wavelength, frequency and energy.
- Link each type of sunlight to its role in the biosphere.
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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: the James Webb Space Telescope looks at space in infrared, the long waves given out by cool dust and very distant galaxies; the Hubble Space Telescope sees mostly visible light and UV. Same kind of radiation, different wavelengths, so they see different things.
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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.
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: honeybees can see UV. Many flowers, such as the black-eyed Susan, have UV patterns we cannot see, called nectar guides, which lead bees to the nectar.
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.
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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.
At the top of the atmosphere, about 8% of the Sun's energy is ultraviolet (UV). At the ground it is only about 4%.
Explain why living things at Earth's surface are exposed to much less UV than the Sun gives out.
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