Brightness
Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light.
One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though.
(1) Luminosity
This is a more physical measurement. It measures the energy output of the object. This is done by looking at how much light is given out by the object over time.
The units of luminosity are more standard, given in Watts - much like a light bulb! A typical LED light bulb in a house might be 20 Watts - the Sun is 20 million billion billion (2 x 1025) times brighter!
Electromagnetic Spectrum
This light that we see is just one small chunk of light which is made by stars. We often call the small range of wavelengths our eyes can detect, optical, or visible light.
- Properties of each Electromagnetic Wave
Moving through the electromagnetic spectrum we go from short to long wavelengths and the frequency of the waves gets smaller. We go from high-energy to low-energy.
Wave-Particle Duality
To understand what happens to light when it reaches a mirror, we can say that photons bounce off the surface of the mirror. This treats the light as a particle like a ping-pong ball bouncing back. If we pass light through 2 holes then at the other side of the holes we see it interact like waves on the surface of a pond.
When we take a photograph on a modern camera, or on a telescope camera, we count these photons. The more we count the brighter an object is - we call this measurement the flux.
Spectra
Even though light often looks white, it is made up of lots of different colours all added together.
Astronomers often want to know exactly what colours of light there are coming from a star or galaxy. To measure this, they use an instrument called a spectroscope. These have prisms, or gratings, inside them which separate the light. They then record the data.
Light
When we talk about light we usually mean the light we can see with our eyes. This is also called visible or optical light.
- How does light travel?
For us to see, light must travel from a light source to our eye. The light we see on Earth travels in straight lines.
Anything in the path of light has an effect on what we see. For example, a window is transparent – this means that the light can pass straight through it.
Magnetism
Since ancient times, people have known about some rocks that could attract iron. When these rocks were rubbed on other types of metal, they had the power to make them attractive too. These items could also push each other away, repelling each other.
- Magnetic Poles
Ancient Chinese thinkers figured out that a magnetic needle in water would always move to the same position. It would line up from north to south. This discovery led to the invention of the compass, which has been used ever since to help people find their way. The reason this works is that the Earth itself is a giant magnet.
Climate Change
The Earth's climate is always changing. It shifts through a regular cycle of temperatures. These cycles last around 100,000 years. They move the Earth through warm periods, then cold ones. This is known as climate oscillation.
A big effect of climate change is the rise in global temperature. Humans have released greenhouse gases into the atmosphere (read more about the Greenhouse effect below). As greenhouse gases rise, so do the average temperatures on Earth. Not all areas on Earth will experience climate change in the same way.
Some areas will become much warmer. Others will become colder. Higher temperatures cause glaciers to melt. When they melt, more freshwater ends up in the ocean. This contributes to sea-level rise and changes crucial processes in the ocean.
Light and Telescopes
One of the first people to study light was Ibn al-Haytham (known as Alhazen). He was a Muslim, living in Egypt in the early 11th Century. Alhazen is first person we know of who used the scientific method. He wanted to know why our eyes could see things.
Hans Lipperhey (also spelt Lippershey) was the first person to write down a design for a telescope. He made spectacles for a living, and in 1608 he applied for a patent for his new tool. He said it was "for seeing things far away as if they were nearby". Hans did not get the patent because other people were making similar tools. But, Hans was paid for his design. The first telescopes, known as Dutch Telescopes, contained both a convex and a concave lens.