Introduction - radiation
The sun's radiation reaches us in the form of electromagnetic radiation. This type of radiation covers a wide spectrum (that is a wide range of frequencies), but those from the sun are mainly infra-red, visible, and ultra-violet. We and other animals have evolved to use the visible range for sight, the infra-red (that is lower frequency than red) causes warmth in objects that absorb it, and ultra-violet is at a higher frequency than visible light and causes sun burn if you get too much of it. All these frequencies travel at the 'speed of light' which is 300,000 kilometers/sec, and travel best in a vacuum.
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