How can glass produce glare without reflecting light?
Category: Physics Published: April 19, 2024
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Glass does indeed reflect light. In fact, glass always reflects some of the incident light (except in special situations that are rare). The process of glass reflecting light is not caused by impurities in the glass or by surface defects. Even the purest, smoothest, most transparent piece of glass reflects some of the light that shines on it. It's a fundamental effect involving the electromagnetic field of the light wave interacting with the electrons, atoms, and molecules that make up the glass. And it's not just glass that does this. All transparent materials always reflect some of the light that hits their surfaces (except in the rare cases where there is refractive index matching or V-polarized light encountering the surface at Brewster's angle). Sometimes transparent materials reflect a large amount of light and sometimes they reflect only a small amount. There may not be enough reflected light for you to notice it, but it's there.
Being perfectly transparent does not mean that the object is equivalent to nothing. In general, there are three things that can happen to light that shines on an object: (1) some or all of the light may be reflected, (2) some or all of the light may be absorbed, and (3) some or all of the light may be transmitted through the object.
1. Reflection can come in many forms—mirror reflection, diffuse reflection, scattering, diffraction—but in each case reflection involves the object sending some or all of the light back away from its surface.
2. Absorption can also come in many forms—color-selective absorption, angle-selective absorption—but in each case absorption involves some or all of the light starting to enter the object and then being destroyed. In the process of the light being destroyed, its energy, momentum, angular momentum, and information is absorbed by the object. That is why this process is usually called "absorption of light" rather than "annihilation of light," even though the light itself is technically annihilated.
3. Transmission involves some or all of the light passing through the object. The transmission process includes refraction, which is the effect where the light changes its speed and its direction of propagation. If all colors are transmitted directly through the object with no absorption, we call the object completely transparent. If only some colors are transmitted directly through the object, or if all colors are transmitted directly through the object, but only partially, then we call the object tinted-transparent. If some or all of the colors are transmitted through the object, completely or partially, but scatter off of small structures in and on the object as they go, we call the object translucent.
These three effects—reflection, absorption, and transmission—are all independently meaningful. Even if none of the light is absorbed by the object, the light can still experience reflection and transmission. In other words, zero absorption does not mean zero reflection. They are different effects. Both reflection and transmission (which includes refraction) play a role in enabling us to see transparent objects. An object cannot be perfectly transparent and perfectly non-reflective at the same time, except in the rare cases when there is refractive index matching or when V-polarized light encounters the surface at Brewster's angle. Interestingly, an object can indeed be non-transmissive and non-reflective at the same time (which means that it's completely absorptive), such as is the case with non-glossy, pure black objects. Also, an object can indeed be non-transmissive and non-absorptive at the same time (which means that it's completely reflective), such as is the case with polished silvery metals. The table below shows the various types of materials that interact with light.
| Type of Material | Reflects Light? | Absorbs Light? | Transmits Light? |
|---|---|---|---|
| Transparent materials such as clear water and clear glass | Yes | No | Yes |
| Translucent or tinted-transparent materials such as apple juice and stained glass | Yes | Yes | Yes |
| Colored opaque materials such as paint and wood, and glossy black opaque materials | Yes | Yes | No |
| True mirror materials such as polished silvery metals and during total internal reflection | Yes | No | No |
| Non-glossy, opaque, pure black materials such as carbon black | No | Yes | No |
| Non-glossy, semitransparent, pure black materials such as thin soot suspended in air | No | Yes | Yes |
| Non-reflective transparent materials: NOT POSSIBLE! (except in rare cases of refractive index matching or V-polarized light at Brewster's angle) | No | No | Yes |
| Non-reflective, non-absorptive, non-transmissive materials: NOT POSSIBLE! This would violate the law of conservation of energy | No | No | No |
Not only do transparent objects indeed reflect some of the incident light, in many cases the transparent object reflects a large amount of the light, or even almost all of the light. In the class of situations called total internal reflection, the physical surface of a perfectly transparent object literally reflects 100% of the light. However, to be clear, total internal reflection can only happen when the light strikes the inner surface of an object that is surrounded by a material with a lower index of refraction, such as air. In other words, it's only possible when the light is already traveling through the transparent object and then tries to escape out into the open air or other lower-index material; and it's only possible for angles of incidence that are beyond the critical angle. Total internal reflection is very useful in practice. It's used extensively in optics, such as in binoculars, high-end cameras, and fiber optic cables. It's also one of the dominant effects that makes diamonds sparkle.
Even for light that is striking the outer surface of a transparent object (i.e. light that is traveling in air and then encounters the surface of the transparent object), there can be almost 100% reflection of the light if it encounters the surface at near-grazing incidence.
The material property that determines how well a transparent material reflects and refracts light is called the index of refraction. In all of my explanations below, I am assuming that unpolarized light is traveling in air when it strikes the outer surface of a transparent material. The image below shows how much of the light is reflected by a transparent material as a function of the material's index of refraction, for the case of the light hitting the surface directly head-on (i.e. at normal incidence). As you can see, water reflects about 2% of the light at normal incidence and regular glass reflects about 4% to 6% of the light at normal incidence, depending on the type of glass. This may not sound like much, but it's enough that you can see your own reflection in the flat surface of a puddle or in a window pane. Transparent materials with higher index of refraction values reflect even more of the light. Adding lead to glass (to create what is called crystal glass) raises its index of refraction, causing it to reflect more light. This is part of what makes crystal glass sparkle more than regular glass. Diamond has the highest index of refraction of all of the natural transparent materials, so it reflects the most, refracts the most, and therefore sparkles the most. At normal incidence, diamond reflects 17% of the light, which is about one-sixth of the light. These values were calculated in the usual way, in terms of the percent of the light's energy that is reflected (i.e. the reflectance) using the Fresnel equations.
The image below shows the same set-up as in the previous image, but now for light that is striking the material's surface at a grazing angle of 5°. In other words, the incoming light rays are almost parallel to the material's surface. As you can see, much more light is reflected from a transparent material's surface at near-grazing incidence than at normal incidence. Whereas water reflects 2% of the light at normal incidence, it reflects 58% of the light at a grazing angle of 5°. This is why when you look out over a lake while standing at its edge, the still lake's surface looks like a good mirror. Whereas glass reflects about 4% to 6% of the light at normal incidence, it reflects 60% to 62% of the light at a grazing angle of 5°. For even smaller grazing angles, even more of the light is reflected. As the grazing angle approaches zero, the percent of light reflected approaches 100%, for all transparent materials (this is not shown in these images). Interestingly, the curve in the image below levels off for higher values of the index of refraction. This means that diamond reflects approximately the same amount of light as glass at near-grazing incidence.
It may be surprising to you that all transparent materials reflect most of the incident light at near-grazing incidence. However, if you pay attention, you will notice this effect often in everyday life. Anytime you come across a flat transparent surface, such as glass or water or anything with a glossy finish, get down low so that your line of sight is almost parallel to the surface and you will see the surface act like a good mirror.
