Universe Today

Sharpless 2-106 (Gemini Observatory/AURA, right; left: copyright Subaru Telescope, National Astronomical Observatory of Japan; All rights reserved)

In most texts that discuss light, what is often referred to is actually that very narrow band in the electromagnetic spectrum that stretches somewhere between wavelengths of 400 nm to 700 nm. That's from the colors violet to red.

It's called visible light because, well, its behaviors are easily observable through our naked eyes. You can see what really happens when rays of visible light undergo reflection, refraction, diffusion, interference, and diffraction. When you look at the mirror, see a rainbow, marvel at the tail of a peacock, or observe the bottom of a pool appear shallower than it actually is, you're witnessing the behaviors of visible light.

So if that's visible light, then what is invisible light? Invisible light may actually refer to all members of the electromagnetic spectrum whose behaviors cannot easily be perceived by the human eye. This should therefore include radio waves, microwaves, infrared, ultraviolet, X rays, and gamma rays.

While it is difficult to observe non-visible light through our naked eyes, their applications are still familiar to us.

For instance, radio waves allow us to communicate with our associates who may be located in another city or even somewhere across the globe. Cell phones, which is fast becoming (if not yet) the most ubiquitous device in the world, make use of radio waves.

Operate a remote on your TV, and most likely, you'll end up using light in the infrared. Walk out into the Sun and stay there for a long time. Pretty soon, you'll get a sunburn, courtesy of ultraviolet light.

Therefore, if visible light is the small band in the electromagnetic spectrum whose members can be easily perceived by the human eye while invisible light refers to all the other members in the electromagnetic spectrum that cannot, then it would be safe to conclude that the answer to the query, "What is light?", is the entire electromagnetic spectrum.

You might want to learn more about specific bands in the electromagnetic spectrum that are non-visible in your quest to find a deeper answer to the question "What is light?". We've got some nice articles here in Universe Today.

Why don't you start with articles about infrared and its applications in astronomy?

There are also interesting stories from NASA and PhysicsWorld:

NASA Infrared Images May Provide Clues About Mt. St. Helens' Eruption
High Performance Near Infrared Camera

Here are two episodes at Astronomy Cast that you might want to check out as well:
Infrared Astronomy
Optical Astronomy

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Extreme Ultraviolet Sun

All electromagnetic waves (EM) travel at a speed of approximately 3.0 x 10 ⁸ m/s in vacuum. Although space is not a perfect vacuum, as it is really composed of low-density particles, EM waves, neutrinos, and magnetic fields, it can certainly be approximated as such.

Now, since the average distance between the Earth and the Sun over one Earth orbit is one AU (about 150,000,000,000 m), then it will take about 8 minutes for radiation from the Sun to get to Earth.

Actually, the Sun does not only produce IR, visible light, and UV. Fusion in the core actually gives off high energy gamma rays. However, as the gamma ray photons make their arduous journey to the surface of the Sun, they are continuously absorbed by the solar plasma and re-emitted to lower frequencies. By the time they get to the surface, their frequencies are mostly only within the IR/visible light/UV spectrum.

During solar flares, the Sun also emits X-rays. X-ray radiation from the Sun was first observed by T. Burnight during a V-2 rocket flight. This was later confirmed by Japan's Yohkoh, a satellite launched in 1991.

When electromagnetic radiation from the Sun strikes the Earth's atmosphere, some of it is absorbed while the rest proceed to the Earth's surface. In particular, UV is absorbed by the ozone layer and re-emitted as heat, eventually heating up the stratosphere. Some of this heat is re-radiated to outer space while some is sent to the Earth's surface.

In the meantime, the electromagnetic radiation that wasn't absorbed by the atmosphere proceeds to the Earth's surface and heats it up. Some of this heat stays there while the rest is re-emitted. Upon reaching the atmosphere, part of it gets absorbed and part of it passes through. Naturally, the ones that get absorbed add to the heat already there.

The presence of greenhouse gases make the atmosphere absorb more heat, reducing the fraction of outbound EM waves that pass through. Known as the greenhouse effect, this is the reason why heat can build up some more.

The Earth is not the only planet that experiences the greenhouse effect. Read about the greenhouse effect taking place in Venus here in Universe Today. We've also got an interesting article that talks about a real greenhouse on the Moon by 2014.

Here's a simplified explanation of the greenhouse effect on the EPA's website. There's alsoanother one at NASA.

Relax and listen to some interesting episodes at Astronomy Cast. Want to know more aboutUltraviolet Astronomy? How different is it from Optical Astronomy?

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Composite image of the nearby Circinus Galaxy. Credit: X-ray (NASA/CXC/Columbia/F.Bauer et al); Visible light (NASA/STScI/UMD/A.Wilson et al.)

Of all electromagnetic waves, it is those that belong to the visible spectrum that are most familiar to us. They're primarily responsible why we can see things. They're also the reason why objects have different colors. That is, each frequency in the visible spectrum is processed and interpreted as a particular color by our eyes and brain.

Our eyes are sensitive only to electromagnetic wavelengths found roughly between 400 nanometers (violet) to 700 nanometers (red). These wavelengths are usually collectively known as visible light or simply light.

The unique set of colors that serve as the signature of gases, known as spectral lines, have enabled scientists to determine the composition of distant stars and other celestial bodies.

Practically all electromagnetic waves possess the same qualities. However, since those that belong to the visible spectrum are the ones easiest to observe, they're the ones frequently used in high school and college experiments and demonstrations.

Thus, you might very well remember how your physics teacher discussed reflection and refraction as well as interference and diffraction. Virtually all these optical phenomena were easily observed through your unaided eyes.

Isaac Newton is perhaps the most notable scientist who conducted thorough studies on visible light. He published his findings in a book named "Opticks" in 1704. The topics discussed in that compilation of observations belong to the specific branch of physics now known as physical optics.

Among his discussions there are fascinating explanations regarding rainbows and prisms, among many others.

Early astronomers used telescopes that could detect visible light. The telescopes used by Galileo Galilei, Edwin Hubble, and many others were all dependent on visible light. Even the Hubble Space Telescope produced images using waves from the same spectrum.

By using these kinds of telescopes, astronomers have learned a lot about the Universe. That it is expanding, that the rate of expansion is increasing, how stars and planetary systems are formed, and so on.

The use of such telescopes continue even up to this day. For example, you don't need X-rays, infrareds, or ultraviolets to appreciate Jupiter's Great Red Spot, the Moon's craters, Saturn's rings, or even the Ring Nebula and Orion Nebula. All you need is a clear night, a decent telescope, and light from the visible spectrum.

We have some articles in Universe Today that are related to the visible spectrum. Here are two of them:

• Visible Light
• Where Does Visible Light Come From?

visible spectrum articles brought to you by NASA. Here are the links:

• Visible Light Waves
• What Wavelength Goes With a Color?

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Ultraviolet Astronomy
• Nebulae

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Sunlight passing through a prism. Image credit: NASA

When we see objects, it's because they're being illuminated by visible light. When we see that the sky is blue, or the grass is green, or hair black, or that an apple is red, that's because we're seeing different wavelengths within the 400nm-700nm band. Because of the waves in this band, a lot has been learned about the properties of electromagnetic waves.

Through visible light, reflection & refraction are easily observed. So are interference and diffraction. Mirrors, lenses, prisms, diffraction gratings, and spectrometers have all been put to use to understand and manifest the qualities of the light that we see through our naked eyes.

Galileo's telescope, which was composed of a simple set of lenses, made use of the refractive properties of light to magnify distant objects. Today's  binoculars and periscopes capitalize on the optical phenomenon called Total Internal Reflection by using prisms to improve on what early refractive telescopes were capable of achieving.

As mentioned earlier, visible light is made up of the wavelengths that range from 400 nm to 700 nm. Each wavelength is characterized by a unique color, with violet on one end (adjacent to ultraviolet light) and red on the other (adjacent to infrared light). When all these wavelengths are combined together, they make up what is known as white light. 

You can separate these wavelengths (and the corresponding colors) by letting them pass through either a prism or a diffraction grating. The magnificent array of colors that we see in a rainbow, on a diamond, or even a peacock's tail are examples of this separation.

All phenomena of visible light such as reflection, refraction, interference, and diffraction are also exhibited by non-visible wavelengths. Hence, by understanding these phenomena, and applying them to the non-visible wavelengths, scientists were able to unearth many of nature's secrets. In fact, if we trace back the roots of modern physics, particularly the wave-particle duality of matter, we will be led back to its manifestation in visible light. 

The study of visible light falls under the realm of optics. Among the scientists who have contributed substantially to the development of optics are Christiaan Huygens for his wavelets and a wave theory of light, Isaac Newton for his contributions on reflection and refraction, James Clerk Maxwell for the propagation of electromagnetic waves as explained in a series of equations, and Heinrich Hertz for verifying the truth of those equations through experiments.

You can read more about visible light here in Universe Today. Want to know where visible light comes from? How about a visible light image of a distant galaxy?

There's more about it at NASA and Physics World:
Visible Light Waves
The special effect of physics

Here are two episodes at Astronomy Cast that you might want to check out as well:
Optical Astronomy
Interferometry

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An infrared false color image of the massive stellar cluster RCW 38, where 317 young stars have been classified. Credit: DeRose et al., and ESO.

Infrared (IR) light is part of the electromagnetic wave spectrum that borders the lowest frequency/longest wavelength among those that make up visible light. Just like all members of the electromagnetic wave spectrum, the IR frequencies overlap with its adjacent members. However, most of IR is between 750 nm and 1,000,000 nm.

Infrared radiation is what is emitted by a warm or hot body and is where the term 'red hot' was derived from.

Optical infrared radiation is divided into three bands by the International Commission on Illumination:

IR-A for wavelengths between 700 nm and 1,400 nm – Some applications include fiber optic communication and night vision goggles.
IR-B for wavelengths between 1,400 nm and 3,000 nm – This band is most noted for its application in long-distance telecommunications.
IR-C for wavelengths between 3,000 nm and 1,000,000 nm – Typically used in the military, specifically in 'heat seeking' missiles as well as in thermal imaging.

Astronomers typically make use of practically all three bands to observe a wide variety of temperature ranges.

Some Applications of Infrared Light

Night Vision

Night vision devices make use of infrared by either capturing the naturally emitted IR from the subjects or by using infrared light sources that are not as noticeable as visible light. Devices like these are mostly used in the military.

Thermal imaging

Devices that make use of thermal images are able to display outlines of the subjects based on the amount of infrared radiation (which corresponds to the amount of heat) emitted by the body. Applications range from medical diagnostic to search and rescue devices. These are highly favored devices because of their non-invasive capabilities.

Communications

You're probably most familiar with this than other applications. Before the introduction of Bluetooth, IR was the dominant wireless means of transferring data between two mobile devices or between mobile devices and a computer. This method required line-of-sight and was only possible at short distance separations (a few centimeters).

Astronomy

Infrared telescopes are pretty much like ordinary telescopes such that they make use of components like mirrors and lenses in addition to digital detectors. To avoid the interference of water vapor found in the atmosphere, most of these telescopes are placed at very high altitudes.

There are many advantages of infrared telescopes over those that rely on visible light. One of them is the better penetration capability of IR. Infrared telescopes can either be used to detect the IR that are emitted by the celestial bodies themselves or the IR that is bounced off a cooler celestial body from a nearby star.

You can read more about infrared light here in Universe Today. Want to know how a balloon experiment solved the mystery of far infrared background? We've also written about the crucial role played by infrared in explaining why galaxies are smooth.

There are also interesting stories from NASA and PhysicsWorld:

NASA Infrared Images May Provide Clues About Mt. St. Helens' Eruption

High Performance Near Infrared Camera

Here are two episodes at Astronomy Cast that you might want to check out as well:
Infrared Astronomy
Optical Astronomy

Filed under: Astronomy | Comments Off