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When we look at a sunset we see an array of colors, a spectrum of light. Light traveling from the Sun to our eye brings information about the Sun itself and the air, clouds, etc that the light traveled through to get to us.
When we look at the night sky through a telescope, we see stars and galaxies. The light that we gather from our observation allows us to to gain understanding about them. Almost everything we know about stars and galaxies comes from their light. Here, we will learn about the properties of light and how we gain information from it.
When we speak of light, we often refer to it as electromagnetic radiation. This is not to be confused with particle radiation as from a nuclear fission power plant.
Also, when we consider light (EM radiation) we can refer to all wavelengths of light, not just visible light. The image of the sunset above shows us the visible spectrum of light, but there are also other forms of light that our eyes cannot detect coming from the Sun.
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Light can be seen as a particle or a wave, depending on the method we are using to measure it. When we consider light as a particle, we call that particle a photon. Photons are discrete objects, or quanta. This means they can be counted with whole numbers. Waves are analog by comparison. They do not have sharp boundaries and can be counted using fractions.
The difference between particle and light is similar to the difference between a digital clock and an analog clock. A digital clock shows each increment as a number, while an analog clock sweeps out every fraction in between.
The diagram above shows a waveform with some of its properties labeled. The height above the medium position, or equilibrium, is called the amplitude. The distance between successive peaks is called the wavelength. The time is takes for a wave to repeat itself is called its period. The frequency of a wave is how often it repeats itself in time. The wave number is how often a wave repeats itself in space.
The speed of anything is a measure of distance divided by time. The time it takes a wave to travel one wavelength is one period, so we can define the speed of the wave as the wavelength divided by the period.
The frequency of a wave is the inverse of the period, so we can also write the speed of a wave in terms of its frequency.
The speed of light is a constant we call c. We typically use the Greek letter lambda to denote the wavelength.
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Light waves are also known as electromagnetic waves. Light waves do not need a medium for propagation, they can move through the vacuum of outer space. The fluctuation of the EM wave is a fluctuation of the electric field and the magnetic field, which fluctuate at right angles to each other, as shown in the above diagram.
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Visible light has wavelengths between 400 nanometers and 700 nanometers. Violet and blue light have shorter wavelengths, and red light has a longer wavelength. The shorter the wavelength of the light, the higher the energy it carries.
The equation shown above allows us to calculate the energy carried by a light wave of a given frequency. It uses h, a constant of proportionality known as Planck's constant.
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The range of all electromagnetic waves is called the EM spectrum. Visible light is only a very narrow portion of the EM spectrum. Waves with slightly shorter wavelengths than visible light are called ultraviolet, or UV waves. They have higher energy than visible light waves, and can penetrate through some materials. X-rays have even higher energy and can penetrate through soft materials but not hard materials like bone. Gamma rays have the shortest wavelengths, and highest energy, and can penetrate many materials.
Infrared waves have lower energy than visible light, along with microwaves and radio waves. As you can see from the diagram, not all wavelengths of light are able to pass through Earth's atmosphere. For example, visible light and radio waves are able to pass through just fine, so we can have optical and radio telescopes on Earth's surface. Gamma ray telescopes must be in orbit, since gamma rays are largely blocked by our atmosphere.
© Kathryn Hadley PhD 2020