This article explains how total internal reflection (TIR) can trap light inside a material with a high index of refraction.

swimming pool under water
From underneath, the surface of the water in a swimming pool looks like a mirror
Total internal reflection (TIR) starts to occur when refraction is so strong that the light can not escape the water.

You have probably noticed that, if you are underwater in a swimming pool, you can only see out of the water if you look straight up. When you look further down the length of the pool, the surface of the water appears to be a perfect mirror, reflecting only the images of objects beneath the waves. That should strike you as odd—water is transparent and air is transparent, so how is the surface acting as a mirror?

The previous blog established a rule of thumb for refraction:

“Light refracts toward the material with the higher refractive index.”

If you think about that, you might anticipate a problem. If light is trying to move from a material with a higher refractive index into a material with a lower refractive index, but the light always bends back toward the higher index material, then there is going to be an angle at which the light cannot escape. Because the light cannot break through the surface of the water it reflects from it.

The angle at which light starts reflecting instead of refracting is called the “critical angle”. Because none of the light is leaving, it is called “total internal reflection” (TIR).

Total Internal Reflection (TIR) occurs when light cannot transmit from a high refractive index material into a lower refractive index material and is therefore reflected back into the high refractive index material.

The Critical Angle is the angle at which total internal reflection occurs, and light at grazing angles beyond the critical angle becomes trapped within the material with the higher refractive index.

So why does TIR matter?

TIR traps light within an optical fiber because the inner portion of the fiber (the “core”) has a higher refractive index than the outer portion of the fiber (the “cladding”).

TIR really is *total* reflection. For all practical purposes, TIR reflects 100% of the light—better than any kind of metal mirror. The most ubiquitous application of TIR is optical fiber. Fiber optic cables trap light and transmit it for kilometers. Fiber optics will be the subject of the next article.

Swimming pool photo copyright:  cookelma / 123RF Stock Photo

This article explains the effects of refraction and provides two rules to predict how light changes direction when it passes from one material into another.

From the previous article recall:

The Refractive index (or Index of Refraction) of a material is indicated by the letter “n.” The index impacts almost every aspect of light’s behavior, including how fast light travels through a material and how much light bends when it moves from one material into another.

Refraction is the reason that objects underwater are not always where they appear to be. A good example can be seen in this photograph of an aquarium.

Refraction causes the double image of the coral reef that can be seen by comparing the images seen through the left-hand pane of glass and the pane closer to the photographer.

From the viewpoint of the person taking the photo, all the seaweed and coral visible through the left-hand pane of glass near the children can also be seen by looking straight ahead.

Continue reading “Two Rules-of-Thumb for Refraction”

This article introduces the topic of “refractive index” and “chromatic dispersion.”

Light slows down when it travels through materials, even if the material is transparent. That includes air. The refractive index, or index of refraction, is the number that specifies how much light slows down within a material. The higher the refractive index, the slower the light moves through that material. Refractive index is generally written as “n.” As stated in previous blogs, the speed of light in empty space is 300,000,000 meters/second, and it is indicated by the letter “c.”  Inside a material, the speed of light slows down to c/n. For example, in most, glass the refractive index is near n=1.5, so light travels only about 200,000,000 meters/second. That is very fast, but it is much slower than the speed of light in open space. The refractive index of air is around 1.0003 at sea level. That is very close to 1, but the difference can be significant for applications like astronomy, where light travels through miles and miles of air before it reaches the telescope.

Light rays bend when they move from one material to another. This behavior is called “refraction.”

Continue reading “Introducing Refractive Index and Dispersion”
Two pairs of eyeglasses on a book. One pair has an anti-reflection coating that reduces glare
This photo from Zenni Optical shows how an anti-reflection coating reduces glare from eyeglasses

The previous article introduced the concept of  optical interference , which is the ability of  light waves  to interact with one another. When light waves of similar wavelengths overlap, they can either reinforce one another or cancel one another out, depending on the  phase difference  between the waves. The two cases are described as follows:

Constructive Interference occurs when light waves interact to create a combined wave with a larger amplitude than the original waves.

Destructive Interference occurs when light waves interact to create a combined wave with a smaller amplitude than the original waves.

Optical interference has many applications. One use that many people see every day (or rather *don’t* see!) is anti-reflection (AR) coatings, which are commonly used to reduce glare from eyeglasses. When light hits the front of the eyeglasses, about 2% 4% of the light reflects away from the lens. Reflection occurs at both the front and the back surface of the lens, so the net effect is that a total of 4% 8% of the light reflects away from the lens. The reflected light obscures the wearer’s face. Furthermore, although 4% light-loss is not really noticeable for someone wearing eyeglasses, for larger optical systems, such as cameras, often have several lenses. Losing 2% 4% of the light at every surface adds up quickly. Furthermore, the unwanted reflections create “stray light,” which can have all sorts of problems.

Continue reading “An Application of Optical Interference: Antireflection Coatings”

This article introduces the topic of optical interference and phase.


In water, waves interact when they overlap, creating intricate patterns on the water’s surface.

The topic of the previous article was diffraction—the phenomenon that causes waves to bend around corners. Another behavior easily observed with water is the general ability of waves to interact with one another, creating complicated patterns on the surface. That mutual influence among waves is called “interference.” The phenomenon also occurs with light waves and is called optical interference.

Waves with the peaks separated by various distances
Phase is the relative distance between the crests of two waves. Phase is an angular measurement from 0° to 360°.

When waves cross paths, they add together. The result of the addition depends on whether the waves are in “phase”— the relative difference between the peaks of two waves. Phase is measured like an angle, from 0° to 360°.1 As the phase between two waves increases, the peaks eventually realign so that the waves are in phase again, just like how something winds up facing the original direction after making a 360° turn.

Continue reading “Wave Behavior of Light, Part 2: Optical Interference”

This article introduces the topic of diffraction—lights curious capability to bend around corners.

Previous articles described light in three different ways depending on the circumstances:  

What does it mean for light to behave like a wave?

Continue reading “Wave Behavior of Light, Part 1: Light Diffraction”


This article will define a light wave in terms of three characteristics and compare that definition to the features of a light ray.

A cross-section of an ocean wave at sunset above a red sine wave with black lines illustrating amplitude, wavelength, and speed
Waves are repeating patterns defined by wavelength, amplitude, and speed.

I previously wrote that light behaves differently depending on the circumstances. Usually it is easiest to think of light as a ray of energy that travels in straight lines; however, in some instances, that simple behavior breaks down. This change in behavior is particularly noticeable for laser light or for light passing through very small openings. In those situations, light interacts with itself in ways that are best understood by thinking of light as a wave rippling through space. As an analogy, consider waves in water. 

Continue reading “Three Characteristics That Define a Light Wave”


This article gives a practical definition of “light ray,” from a technical perspective. 

Scenic forest of fresh green deciduous trees framed by leaves, with the sun casting its warm rays through the foliage with the rays highlighted by red arrows
Rays extend from the sun in straight lines.

In the previous article I defined photons:

Photons are sub-atomic particles that are the fundamental building blocks of light in the same way that electrons are the fundamental building blocks of negative electric charge.

When I proposed that definition, though, I gave the caveat that there is still considerable debate about how to define a photon. That ambiguity arises because light acts in different ways under different conditions. Specifically, light can behave like a particle (photon), a ray, or a wave. The terminology “ray” and “wave” is probably already familiar because “light ray” and “light wave” are common phrases that people use interchangeably. There are significant technical differences, though, between a ray and a wave, so it is important to define them clearly

Continue reading “Four Obvious But Important Features of Light Rays”

This article defines the word photonics and explains the origin of the word.

Before getting into a definition of photonics, we first have to define what a photon is.

Good luck with that.

Every year at the SPIE Photonics West conference, there is a late-night panel called “What is a Photon?” at which more-or-less sober physicists still argue about the exact nature of photons. Trying to nail down a technical definition of a photon quickly spirals into abstract particle physics, and that’s not useful from a business perspective. A practical definition of a photon is this:

Photons are sub-atomic particles that are the fundamental building blocks of light in the same way that electrons are the fundamental building blocks of negative electric charge.

I will leave it at that.

The origin of the word photonics can be traced to a 1974 conference in France called, appropriately, “Photonics.” At the meeting, an international cross-section of specialists set out to define a growing area of research that, up to that point, was ambiguously named (and hyphenated) as either electro-optics or opto-electronics. The conference was meant to settle the terminology and be “the birth certificate for a new technological field which will certainly be developed in the near future.” Continue reading “Why Do We Call Photonics “Photonics”?”

3… 2… 1… Lift-Off!

Thank you to everyone who attended my class at Photonics West 2018. This website is a resource for the “Very Least You Need to Know about Optics” course. All of the class content will eventually be up here as individual, searchable articles. Contact me through the site with any questions.