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What is light?

Light and the electromagnetic spectrum is a topic often taught at upper secondary level. Here's a short video to help explain what light is.
Light is a strange thing. It isn’t just what we can see. Light can be broken up into different types, and we call the whole family the electromagnetic spectrum. So even when it’s dark, light is still all around us. Light can be created by making an electron oscillate. This creates an oscillating magnetic field and an oscillating electric field, which we call an electromagnetic wave, or light. We often represent light as a wave. Just like water waves, light has a wavelength, a frequency, and a speed. The different colours of optical light have different wavelengths and so does the rest of the electromagnetic spectrum. Gamma rays are the most energetic whereas radio waves have the least energy.
Even so, light is the fastest thing in the universe. In a vacuum all light waves, even radio waves, travel at an incredible speed of 300 million metres per second.
The Royal Observatory is overrun with squirrels. They’re hard to miss during the day because they reflect visible light from the sun. But at night, it’s more difficult. However, they do emit infrared light. And if our eyes could detect it, we would be able to see them in the dark. If we pass high energy x-rays through them, the squirrels look very different. Humankind has invented lots of different instruments to show us what our eyes can’t see, like our Great Equatorial Telescope, which captures huge amounts of light and allows us to see objects that are tens of millions of trillions of kilometres away. We can see matter interact with light and change it.
The gases in the Earth’s atmosphere scatter blue wavelengths of sunlight, giving us blue skies during the day and red skies at sunrise and sunset. We can watch sunlight reflect off planets and the moon. During a lunar eclipse, red sunlight changes direction as it passes through the Earth’s atmosphere and reaches the Moon. And although visible light from stars can be absorbed by dust in space, we can detect the infrared light which gets through to see baby stars hiding in nebulae. Light can even appear to stretch out. Since our universe is expanding and galaxies are moving away from each other, the light they emit is stretched to longer wavelengths. We call this redshift.
Some galaxies are so far away that their light has shifted and stretched from visible to infrared, so we can no longer see them. Stranger still, the universe has unknown amounts of dark matter and bizarre black holes. So even if we can detect all the different types of light, there are still some things in the universe that will remain invisible to us.

Light, in all its forms, is the astronomer’s tool for understanding our Universe and the objects we find in it. Although we can only see a small portion of the full electromagnetic spectrum with our eyes, we have built telescopes and detectors that enable us to study our Universe at every wavelength of light.

But what is light and what are its properties? We answer these questions in our aptly titled video – “What is light?”. You can use this video in the classroom and pause it at various points to reiterate specific concepts in class.

Here are the main science points mentioned in the video:

  • Light is an electromagnetic (EM) wave.
  • Electromagnetic waves can be produced by the oscillation of an electron.
  • Light has a wavelength, a frequency and a speed.
  • It is the fastest thing in the Universe – in a vacuum it travels at a mind-blowing 300,000,000 metres per second!
  • The electromagnetic spectrum is the entire range of electromagnetic waves.
  • Each part of the EM spectrum has a different wavelength of light.
  • Gamma-rays have the shortest wavelengths and the most energy, while radio waves have the longest wavelengths and the least energy.
  • Light can interact with matter through processes like scattering, reflection and absorption.
  • Objects that might not be observable in visible light can often be observed by switching to a different wavelength, such as infrared.
  • The expansion of our Universe causes light from galaxies to be stretched to longer wavelengths. This is known as redshift.
  • We can’t directly observe black holes as they don’t emit any detectable type of light.

You can download the ‘Colour and wavelengths in space’ resource below to use with this video in your classroom.

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Physics, Astronomy, and Space: Teaching Secondary Science

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