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What Is Light?

Using Slooh’s Online Telescope and integrated NGSS aligned Quest learning activities, you can capture your own images of different astronomical sources of light while learning more about the nature of light. Light: The Language of Astronomy is one of 60+ curriculum-aligned STEM Quest learning activities on Slooh for students 4th grade to college.

Slooh’s Online Telescope:

For Everyone For Educators

If you are an educator looking for fun, interactive ways to teach NGSS: MS-PS4-2, as well as other standard curriculum codes related to astronomy, keep reading to the end for more information on the Light: The Language of Astronomy Quest and ways to easily integrate Slooh into your classroom.


What Is Light?

Light is electromagnetic radiation, a type of energy that provides sustenance to plants on Earth and gives us the ability to see the world around us. Our eyes can only detect one of the seven types of electromagnetic radiation, the one known as visible light, which is also what Slooh's telescopes capture. The types of light in order from most to least energetic are gamma rays, X-rays, ultraviolet, visible, infrared, microwave, and radio waves. This entire range of light is called the electromagnetic spectrum.

Light can travel through space without any need for a physical medium. In fact, it travels at its highest speed in empty space; any form of matter that light interacts with slows down its journey. In a complete vacuum, the speed of light, 299,742,458 meters per second (around 670 million miles per hour), is the upper limit of all motion. Nothing can travel faster than it!

Note that, despite how quickly light travels in space, it can take billions of years for the light from distant stars and galaxies to reach us; think about how incredibly far these objects must be from us for that to be true! Because of the vast distances between objects in space, the light from many of the celestial objects you can see through a telescope is from the distant past, and, in many ways, can be viewed as a time capsule from the year in which the light was first generated.

Fun fact: Astronomers use light to define the primary unit of measurement used in astronomy, the light-year! A light-year is the distance that light can travel in one year.

Wave or Particle?

One of light’s seemingly paradoxical features which has been debated for millennia is that sometimes it appears to act like a wave of energy and, at other times, like a collection of discrete particles. Let’s look at each definition and how they interact with each other.

An important characteristic of the wave nature of light is its wavelength. The wavelength is the distance over which the electromagnetic fields associated with the light wave repeat themselves, like the distance between crests of waves in an ocean. The wavelengths of light are tiny, typically between 450 and 650 nanometers! As seen in the figure below, red light has the longest wavelength of visible light, and blue has the shortest.

When light is looked at as a group of particles, those particles are called photons, which are produced from the movement of atoms at high temperatures. Photons have no mass, and each one carries a specific amount of energy.

So which definition is true? The answer is both! The physical particles of light, photons, can behave like waves, causing light to sometimes seem like a wave, and sometimes like particles, depending on the situation and effect being observed. Quantum mechanics tells us that light can also behave like a wave and a particle simultaneously, but this duality is very difficult to observe; most experiments are only capable of showing it acting as one or the other at a time. Our understanding of the nature of light is still far from complete, and theories surrounding these ideas continue to be developed and debated in the relatively young field of quantum mechanics.

How Is Light Produced?

The light that is most important in astronomy is produced mainly by two processes: incandescence and ionized gas emission. Incandescence is the light generated by hot, dense bodies like the Sun. When any material is heated to a sufficiently high temperature (above around 500°C), its constituent atoms and ions vibrate so violently that they spontaneously emit light.

Ionized gas emission, on the other hand, is light generated by hot, diffuse ionized gasses. New stars form in large clouds of gas known as emission nebulae. These young stars give off intense radiation, exciting hydrogen atoms and molecules in the cloud. Sometimes electrons become so excited that they are stripped from their atom, in which case we say that the atom is ionized. When these highly excited electrons return to a lower energy level, they give off the additional energy in the form of light.

The Color of Light

The light emitted from incandescent objects such as our Sun contains all the colors of the rainbow because of how densely the atoms in the body are packed. In fact, rainbows are a manifestation of all the different components of sunlight; falling raindrops that happen to be illuminated by sunlight disperse the light into its constituent colors, resulting in a rainbow!

In contrast, the light produced by hot, ionized gasses only contains very specific colors, or wavelengths, of light. These distinct colors depend on the types of gas present in the cloud, allowing scientists to draw important conclusions about the chemical composition and history of the cloud!

Light's Interaction with Matter

When traveling through a vacuum, light moves in a straight line, but what happens when it interacts with matter? All matter absorbs and reflects light, with the material’s absorptivity determining the amount absorbed and the amount reflected dictated by an object’s albedo.

Light absorption is the process in which the energy from light is stored within a substance’s atoms. The light energy that comes into contact with the object is transferred to electrons within the material, where it is stored in a manner similar to how energy is stored in a stretched spring. The material's atoms are then referred to as being in an excited state. Eventually, the excited electrons return to their lowest energy state or ground state. The excess energy is sometimes released as invisible infrared, but can also be released as visible light through light reflection. Other times, the stored energy is released as random motion or kinetic energy, which is what we typically think of as what happens when light is absorbed.

Light reflection is similar to light absorption in that, during the process of reflection, the electrons within the material are affected by the incoming light. In response, however, they emit light of their own, which may or may not have the same wavelength as the incoming light; the wavelength emitted is more heavily dependent on the composition of the material. For example, planet Mars appears reddish to us since there is a large amount of reddish iron oxide (rust) in its soil. When the white light of the Sun is incident upon the iron and oxygen atoms within the iron oxide materials, they reflect primarily the reddish color light that you see. Remember that sunlight contains all the colors of the spectrum. What happened to the other colors? The blue and green wavelengths of light are mostly absorbed.

Let's put light reflection and absorption in perspective using the optics of Slooh's Online Telescope. The large, metalized light-gathering mirrors in Slooh's reflector telescopes are designed to reflect the light that they gather into the telescope's sensor. However, when the light reaches the telescope's sensor, the sensor absorbs nearly all the incoming light through electron excitation. The results of the excited electrons are ultimately how the images you see are formed. In this specific case, the conversion of light into excited electrons is called photodetection.


More About Slooh's Light: The Language of Astronomy Quest

Understand the fundamentals of light – astronomy's language. Learn how light is created, reflected, and absorbed by astronomical bodies to better appreciate what you see when you observe objects with Slooh's telescopes.

Learning Objectives

By the end of this Quest, students will be able to answer the following questions:

  • What are the fundamental properties of light?

  • How does light reflect, absorb, transmit, emit, and generally interact with various celestial objects in the Universe? And what do they look like?

  • How can light inform astronomers of what's taking place in these glorious regions of the Universe?

Vocabulary Words

Standards Addressed

NGSS Performance Expectations



RST.9-10.1, RST.9-10.2, RST.9-10.3, RST.9-10.4, RST.9-10.7, RST.9-10.10

Related Slooh Quests

  1. Nifty Nebulae

  2. Mystery of the Changing Moon

  3. Stars Like Ours

More About Slooh’s Astronomy NGSS Aligned Learning Activities

Slooh’s Online Telescope is a learning platform designed to support any educator in teaching astronomy to meet NGSS requirements by collecting and analyzing real-world phenomena. No previous experience with telescopes is necessary to quickly learn how to use Slooh to explore space with your students.

You can join today to access Slooh's Online Telescope and all 60+ Quest learning activities if you are able to make astronomy a core subject of study for the semester or year. If you only have a few weeks to study astronomy, we also have a curriculum designed to fit your busy academic schedule and budgetary limitations. To learn more about our offers, click here.