LIGHT

WHAT IS LIGHT?

Answering this question is not as simple as it would seem. Colloquially, we use the word ‘light’ to refer to the luminous, often colorful, medium that allows us to see things and without which we would be cast in darkness. In a more scientific context, light is something called electromagnetic radiation, one of the four fundamental forces. It is energy that travels through space in the form of oscillating waves of intertwined electric and magnetic fields. This radiation reflects off the objects and environment around us and eventually is intercepted by our eyes and is interpreted by our brain as vision.

While electromagnetic radiation is not explicitly an astronomical concept, it does play possibly the most fundamental role in how we perceive and understand celestial objects and the cosmos through which they flow.

HISTORY OF DISCOVERY

Many ideas have been proposed throughout history to explain what light really is. Some of the earliest ideas were kind of out there but through clever experimentation and engineering, mathematicians and scientists across the world brought exponentially more sophisticated understandings.

One of the first prominent theories came from the ancient Greek philosopher Empedocles in 450 B.C. who suggested that light was actually projected from our own eyes via a “visual fire” placed within them by Aphrodite. This would much later be called emission theory. Despite being pretty off the mark, this idea held sway for centuries by other influential thinkers like Ptolemy and Euclid.

The Golden Age of Islam (7th-13th century) saw many advancements in the field of optics. In the 11th century, renowned Muslim physicist and mathematician Ibn al-Haytham disproved emission theory. In his highly influential “Book of Optics,” he accurately proposed that light reflects off objects, enters the eye where it is focused by a lens and then processed by the brain.

In the late 17th century, a debate was brewing in the scientific community about the nature of light. On one side, Isaac Newton in his book “Opticks” proposed that light is made up of tiny particles. This was the more popular theory at the time. On the other side, Christiaan Huygens authored “Treatise on Light” and suggested in opposition to Newton that light was actually made up of waves. It would later turn out that they were kind of both right. More on that later.

In the early 19th century, scientists like Thomas Young would confirm the wave-like nature of light in his famous double slit experiment. In the 1860’s, James Clerk Maxwell developed the idea that light moved as coupled waves of electricity and magnetism. He also accurately determined the speed of light.

In the early 20th century, Max Planck and later with help from Niels Bohr and Albert Einstein, developed a quantum theory of light that reconciled wave theory with particle theory by establishing a wave-particle duality. Here, light is considered to be both simultaneously but more specifically it is described as being made of photons. This theory currently represents our best understanding of light.

Electromagnetic radiation can be produced by a variety of natural and artificial sources and each of those sources can produce different wavelengths. There are two main mechanisms for the generation of electromagnetic radiation: incandescence and luminescence.

INCANDESCENCE

Light can be emitted by objects due to them being very hot. This is called incandescence, thermal radiation or “hot light.”

When you heat up matter it gains energy which it will then try to release. One of the ways this energy is released is in the form of light. The energy of the light that is emitted depends on how hot it is. The higher the temperature, the higher the energy. Even humans and other animals regularly emit energy in the form of infrared, also known as body heat, which is invisible to our eyes but we can see it using infrared sensors.

If an object gets hot enough, particularly a dense, solid object, it will begin to glow red in the visible light range. If it gets even hotter (more energetic) it will glow orange or yellow. This is also the main reason why stars are different colors. The coolest stars glow red and orange, intermediate stars glow yellow and white, while the hottest stars glow white and blue.

Some examples of incandescence would be:

🔅High-temperature rock/metal.

• Tungsten filament inside an incandescent bulb

• Nichrome coils on an electric stove

• Molten iron of a forge

• Obsidian, pumice and molten rocks (lava) from volcanic eruptions

🔅Friction and pressure.

• Meteors and spaceships entering Earth’s atmosphere at high speed

• The interiors of planets and other massive sub-stellar objects

🔅Chemical combustion.

• Burning of wood and fossil fuels (campfires, gas stoves, furnaces, automobile engines, coal-fired power plant)

• Explosion of gunpowder (guns, fireworks)

🔅Nuclear fusion.

• Cores of stars

• Nuclear reactors

Imagine the energy level like a set of stairs in which the electron can occupy one step or another but nothing in-between. The more energy the electron absorbs, the higher up the stairs it can go. If the electron can’t absorb enough energy to get to the next step, it doesn’t move. When an electron is excited it will eventually relax, release the energy and drop down to a lower step. The energy is released by emitting light, just like with incandescent light, but is done without the need for heat.

Some examples of luminescence would be:

🔅Electroluminescence

An electric current is passed through certain kinds of matter.

• Semiconductors of light-emitting diodes

🔅Chemiluminescence

A reaction caused by the mixing of specific chemicals.

• Glow sticks - hydrogen peroxide mixed with fluorescent dye

• Forensics - looking for blood at crime scenes by mixing luminol and hydrogen peroxide with the iron in hemoglobin

🔅Bioluminescence

A form of chemiluminescence found in biological organisms.

• Insects (fireflies)

• Worms (glow worms)

• Fish (angler fish)

• Fungi (panellus stipticus)

• Bacteria (noctiluca scintillans)

🔅Photoluminescence

The absorption of a particular wavelength of light (usually ultraviolet).

• Fluorescent lights and paints which relax quickly (last briefly)

• Phosphorescent lights and paints which relax gradually (last longer)

🔅Ionization

The addition, but more often removal, of an electron from an atom causes the other electrons to shift around energy levels and release energy. 

• Lightning is caused by a rapid and intense electrostatic discharge in the atmosphere that both heats and ionizes the air.

• Aurorae at the poles of planets are caused by solar wind riding Earth’s magnetic field down and ionizing the atmosphere.

• Emission nebulae are ionized by the intense ultraviolet radiation from nearby stars.

THE ELECTROMAGNETIC SPECTRUM

One of the most profound things about light is that all the light we can see with our eyes is but a tiny fraction of all the light there is. There’s an entire universe invisible to us, unless we know how to look for it. This is all part of the “electromagnetic spectrum,” a broad range of light wavelengths.

What is a wavelength?

Light is generally understood to move in a wave-like manner. These waves oscillate, forming peaks or crests in regular intervals. The wavelength of light is the distance between two consecutive peaks. If there is less distance, the wavelength is said to be shorter with higher energy. If there is more distance, the wavelength is said to be longer with lower energy. It is usually measured in nanometers (nm) or micrometers (μm). The wavelengths of light that our eyes can see directly are called visible light but the electromagnetic spectrum goes well beyond that.

The EM spectrum ranges are as follows:

• Radio waves

• Microwaves

• Infrared

• Visible light

• Ultraviolet

• X-rays

• Gamma rays

These different ranges don’t really have distinct boundaries as is sometimes portrayed but is more of a guideline. Each type has its own effects and applications which humans can take advantage of. Let’s start with the longer, low-energy wavelengths and work our way from there:

HOW LIGHT MOVES AND INTERACTS WITH MATTER

BASICS

When light is emitted by a source, it is radiated outward in every possible direction in the form of oscillating wavefronts, much like ripples spreading across the surface of a pond, except here it appears more as expanding spheres of ripples with the light source at the center. Each individual wave will move in its respective direction forever until its path is interrupted in some way.

The speed of light, as was determined by James Clerk Maxwell in the 1860’s, moves through the vacuum of space at a constant speed of about 300,000 km/s (186,000 mi/s). Astronomers often use the speed of light to measure the vast distances of the cosmos. A light year is a unit representing the distance light travels in a year which is about 9.5 trillion kilometers (5.8 trillion miles). The speed of light is often denoted by the letter “c” in equations

Light intensity decreases with the square of the distance from the source via the inverse square law. This is because every light ray is emitted outward at slightly different angles. The further away they get, the more their differing angles spread them apart into a larger and larger area. Therefore, a receiver of light will encounter less of the light, the further away it is.

If you double the distance that light travels, the intensity becomes 2 x 2 = 4 times dimmer (or 1/4 its initial intensity). If light travels 10 times further, the intensity becomes 10 x 10 = 100 times dimmer (or 1/100 its initial intensity). If this sounds familiar, it’s because this is how gravity weakens with distance too.

As the expanding spheres of wavefronts cross vast distances, they gradually resemble flat, planar wavefronts. Granted, they are always still curved but seem to flatten as they spread more and more. The curve eventually becomes negligible and appear parallel to each other.

When light does make contact with an object like a planet, a cloud of gas or some other medium, there are different ways it can react depending on the shape, density and composition of the object. The interactions we will cover are:

• The human eye

• reflection

• absorption

• transmission

• refraction

• dispersion

• scattering

• polarization

• diffraction

• interference

THE HUMAN EYE

The most important light to us is the light we can see and the only light we can see is visible light that goes directly into our eyes.

Light passes through the cornea, which is a transparent, dome-shaped structure that covers the front of our eye. The cornea helps focus the light via refraction into our pupils, which is the opening in the center of the iris, which is the colored part of the eye. This may sound like how a refractor telescope works because it is. The convex objective lens at the front of a refractor telescope focuses that light into a point.

The iris controls the size of the pupil by contracting and dilating, which controls how much light is able to pass through at once. Behind the pupil is the lens, which is another transparent, flexible structure that focuses the light coming in from the pupil towards the retina at the back of the eye.

The retina is a thin, light-sensitive layer of tissue lining the back of the eye. It contains two types of light-sensitive cells called rods and cones. Rods are responsible for distinguishing light and dark while cones are responsible for distinguishing colors. The retina’s purpose is to convert light that enters the eye into electrical signals that are sent to the brain via the optic nerves. The brain then processes and interprets these electrical signals and the end result is our visual perception.

The human eye is a complex and crucial tool for living our lives, but it is very limited in terms of the kind of light it can see. The very narrow range of the electromagnetic spectrum called visible light (from violet at 400 nanometers to red at 700 nanometers) covers all the light that the human eye is capable of processing. Any light outside of that range is more or less invisible to us and we must use technology to detect it.

REFLECTION

When light strikes the surface of an object, it can be reflected. The angle of reflection (θr) depends on the angle of incidence (θi). This means that the light will bounce off the surface at the same angle it hit the surface, just in the opposite direction. The angle of incidence is measured between the path of light and the normal line (a line perpendicular to the surface at the point of incidence). For example, if light hits a flat surface at a 30° angle with respect to the normal, it will also be reflected at a 30° angle with respect to the normal. Alternatively, if light hits a flat surface head-on at a 90° angle with respect to the normal, it will be reflected back in the same direction it came from.

Whether or not light is reflected off an object’s surface depends on the object’s reflectivity. This is usually expressed as a percentage where 100% means a surface reflects all light that makes contact with it and 0% means a surface absorbs all light. Most mirrors have a reflectivity in the 90-95% range. Reflectivity depends on a number of factors including the object’s composition, surface roughness, the angle of incidence and the wavelength of the incoming light.

Reflectivity is crucial as the only way we are able to see the objects and environments around us is if light is first reflected off those objects and environments and then directly enter our eyes (unless, of course, the object itself is a light source).

ABSORPTION

No object has 100% reflectivity. Even the most complex, well-crafted mirrors can only reflect up to 99.999% of the light it receives. Any light that is not reflected, transmitted or other otherwise allowed to escape the object is absorbed by the object. When light is absorbed, the object becomes imbued with energy. The electrons of the object’s atoms become excited and move to a higher energy level before eventually releasing the energy as heat. This is why darker-colored objects feel hotter than brighter-colored objects.

Note: This is not to be confused with atoms emitting light from relaxing electrons as seen in luminescent objects.

What light gets absorbed or reflected depends on the wavelengths of the light versus the element of the atom. Remember that electrons can only be moved to a higher energy level if they absorb a sufficient amount of energy. Some energy levels require more energy than others, depending on the element of the atom. So, if the energy of the light’s wavelength matches the energy needed to level up the electron, the light is absorbed. If it is just shy of the amount needed, the light is reflected or transmitted.

The absorption of some wavelengths of light combined with the reflection of other wavelengths is what gives an object its color. For example, a leaf on a tree appears green to us because it absorbs all the Sun’s wavelengths of white light except for green. The green wavelengths are instead reflected outward and some of it enters our eye and we perceive the leaf as that color. If an object absorbs all wavelengths of light, that object will appear black. However, just as there are no real-world examples of a perfect reflector, we also don’t have a perfect absorber. The closest thing we have to the latter are black holes.

TRANSMISSION

Some materials can allow light to pass through it instead of being reflected or absorbed. This is called transmission and the ability of a material to allow this is called transparency.

Transparent material that allows the transmission of light tends to have:

• Simple atoms with a dense, highly ordered, uniform structure allowing for light transmission without disruption (silicon, carbon, hydrogen, oxygen, nitrogen, some rare earth elements).

• Tightly bound electrons with a large energy threshold for excitation, meaning they are less likely to absorb light (see absorption).

• A low refractive index that prevents the bending of light (see refraction).

• Low polarizability which will not prevent light from passing through based on the orientation of its oscillating waves.

• Minimal impurities, irregularities or defects that prevent internal scattering that turns the object hazy or opaque (see scattering).

Some materials are more transparent than others.

Materials with a high level of transparency will transmit most or almost all light that interacts with it. Remember that just as no known material is a perfect reflector or perfect absorber, so too is no known material perfectly transparent.

Some natural examples of transparent material: Diamonds (transparent solid), pure water (transparent liquid), air (transparent gas)

Some manufactured examples of transparent material: glass (for windows and lenses), synthetic plastics (such as acrylic)

Materials with a more moderate level of transparency, called semi-transparent or translucent, will only allow some light to pass through it while the rest is reflected, absorbed or otherwise diverted away. Some materials may only allow through certain kinds of light such as certain colors or light with certain wave orientation (polarization).

Some natural examples of semi-transparent material: seawater (due to salt and particulates), quartz crystals, marine organisms (jellyfish)

Some manufactured examples of semi-transparent material: telescope filters, sunglasses, stained or frosted glass, etc.

Objects that transmit no light are considered opaque.

REFRACTION

When light transmits at an angle from one medium to another, such as from air to water or glass, the light is bent or refracted. This is because the speed of light changes as it crosses the boundary between the two mediums. Why? Different mediums have a different refractive index. This property quantifies how much the speed of light is reduced when traveling through the medium compared to the speed of light in a vacuum. Two different mediums will have two different refractive indices and will allow light to move at different speeds. Moving from a lower refractive index (like air) to a higher refractive index (like water or glass) will result in the light slowing down. Likewise, moving from a higher refractive index to a lower refractive index will result in the light speeding up.

Why does the change in the speed of light change the direction of the light?

Imagine wavefronts of light transmitting through a slab of glass at an angle (pictured). As the wave enters the glass, a portion of it will be in the glass while the other is still in the air. Light travels more slowly in glass, so the wavefronts are closer together in glass than they are in air. The only way the wavefront can remain continuous is if the wavefront tilts in the glass. Which means the beam as a whole bends down. According to Snell’s law, the bigger the difference between the two mediums’ refractive indices, the more the light is refracted.

Again, this bending only happens when the surface between two mediums is at an angle to the beam of light. If the surface was perpendicular to the light beam, the light would simply pass straight through, albeit at a different speed. This is why if you look through a glass of water only the edges appear distorted, but the middle appears mostly normal.

One of the greatest innovations in the history of astronomy is the invention of lenses, specially crafted pieces of glass which refract light in a controlled manner. A convex lens, like the one shown in this graphic, is curved outward in a way that it refracts light inward with the outer edge refracting light at the greatest angle and the center allowing light to pass straight through. This focuses all the incoming light into a small point. Telescopes that use these convex lenses are called refractor telescopes because they use the refraction to funnel light into the eyepiece so that the image can be magnified.

DISPERSION

As you know by now, when light is passed through from one medium to another, the light is refracted or bent. You should also know that white light is a combination of all the different wavelengths, or colors.

When white light is refracted in this way, there is an interesting phenomenon in which the different wavelengths that make up the white light are refracted at different angles. Red wavelengths are refracted at the least angle while violet wavelengths are refracted at the greatest angle. These different angles allow the wavelengths to separate from each other and spread outward. This phenomenon is called dispersion.

This effect is most famously demonstrated using an object called a prism. These are pieces of glass with a high refractive index and crafted into a triangular shape. This creates an exaggerated refraction and dispersion effect. Prisms can be used in other forms as part of a spectroscope or pair of binoculars.

Dispersion happens in nature as well. In the presence of mist, such as following a rainstorm or when standing near a waterfall, many small droplets of water vapor are floating around the air. As white sunlight passes from the air through the water droplets, it is refracted and dispersed into its separate colors. From inside the water droplet, the light is then reflected off the back side and finally exits the droplet. This disperses the light even more as it changes mediums again. This dispersed light being reflected back at us is what creates rainbows. This will appear as a colorful arc in the sky in which the colors are arranged in a particular order: (from top to bottom) red, orange, yellow, green, blue, violet. A similar phenomenon is the halo of light that occasionally appears around the moon as the moon’s light is refracted off high-altitude ice crystals in Earth’s atmosphere.

Dispersion is also why you can see the color spectrum in the reflection of water or in soap bubbles.

SCATTERING

Light coming into contact with small particles or molecules in the atmosphere, as opposed to large mediums, can cause the light to redirect its component wavelengths in separate directions. This is called scattering. This is distinct dispersion wherein light is refracted upon passing through material.

There are different types of scattering that occur under different circumstances but the most common are Rayleigh scattering and Mie scattering.

Rayleigh scattering occurs when the scattering particles are much smaller than the wavelength of light. Shorter, blue wavelengths are higher energy and are thus scattered much more efficiently by the smaller particles than longer, red wavelengths. A prolific example of this is the air molecules in Earth’s atmosphere (molecular nitrogen and oxygen). When the white light from the Sun enters Earth’s atmosphere, the blue wavelengths are scattered much more and thus the sky appears blue in the daytime. As the Sun descends toward the horizon, the light is reaching through the atmosphere at a lower and lower angle, scattering more and more of the blue light away. By the time the sunlight reaches you, most or all of the blue has been scattered away leaving only red light remaining.

Mie scattering occurs more predominantly when the scattering particles are comparable in size to the wavelength of light or larger. This would be instances in which the air is littered with thicker particulate matter such as water droplets, dust or smoke. This primarily occurs in the lower atmosphere where larger particles tend to settle. While Rayleigh scattering will scatter the light in every direction, Mie scattering will scatter light more heavily in the direction of the incidence (the direction the light was initially moving). Mie scattering is not strongly wavelength dependent and will scatter wavelengths more evenly across the spectrum. This is where we get the pale hues of clouds, mist and fog. It also produces the white glare around the Sun and the haloes we see around the Sun and Moon when the air is thick with ice crystals.

Note: Scattering happens to non-visible light as well. With ultraviolet wavelengths, the effects of Rayleigh scattering are even more pronounced than blue visible light while infrared light is scattered less than red visible light.

DIFFRACTION

SPECTROSCOPY

There is more to light than meets the eye.

If you or anyone you know are looking learn more, members of the Harford County Astronomical Society are here for you to provide our enthusiastic knowledge and experience.

Come find us at future events or fill out an outreach request form and ask for assistance and we will do what we can!