1. Reflection and Refraction of light

Reflection of Light in Gemstones

The manner in which gemstones are cut is one of the more aesthetically important and pleasing applications of the principles of light reflection. Particularly in the case of diamonds, the beauty and economic value of an individual stone is largely determined by the geometric relationships of the external faces (or facets) of the gem. The facets that are cut into a diamond are planned so that most of the light that falls on the front face of the stone is reflected back toward the observer (Figure 6).

A portion of the light is reflected directly from the outside upper facets, but some enters the diamond, and after internal reflection, is reflected back out of the stone from the inside surfaces of the lower facets. These internal ray paths and multiple reflections are responsible for a diamond’s sparkle, often referred to as its “fire” . An interesting consequence of a perfectly cut stone is that it will show a brilliant reflection when viewed from the front, but will look darker or dull from the back, as illustrated in Figure 6.

Image showing reflections in faceted diamonds

Light rays are reflected from mirrors at all angles from which they arrive. In certain other situations, however, light may only be reflected from some angles and not others, leading to a phenomenon known as total internal reflection. This can be illustrated by a situation in which a diver working below the surface of perfectly calm water shines a bright flashlight directly upward at the surface.

If the light strikes the surface at right angles it continues directly out of the water as a vertical beam projected into the air. If the light’s beam is directed at a slight angle to the surface, so that it impacts the surface at an oblique angle, the beam will emerge from the water, but will be bent by refraction toward the plane of the surface. The angle between the emerging beam and the surface of the water will be smaller than the angle between the light beam and the surface below the water.

If the diver continues to angle the light at more of a glancing angle to the surface, the beam rising out of the water will get closer and closer to the surface, until at some point it will be parallel to the surface. Because of light bending due to refraction, the emerging beam will become parallel to the surface before the light below the water has reached the same angle. The point at which the emerging beam becomes parallel to the surface occurs at the critical angle for water. If the light is angled still further, none of it will emerge. Instead of being refracted, all of the light will reflect at the water’s surface back into the water just as it would at the surface of a mirror.

1. Reflection and Refraction of light

How Do Mirrors Reflect Light?

Perhaps the best example of specular reflection, which we encounter on a daily basis, is the mirror image produced by a household mirror that people might use many times a day to view their appearance. The mirror’s smooth reflective glass surface renders a virtual image of the observer from the light that is reflected directly back into the eyes. This image is referred to as “virtual” because it does not actually exist (no light is produced) and appears to be behind the plane of the mirror due to an assumption that the brain naturally makes. The way in which this occurs is easiest to visualize when looking at the reflection of an object placed on one side of the observer, so that the light from the object strikes the mirror

The type of reflection that is seen in a mirror depends upon the mirror’s shape and, in some cases, how far away from the mirror the object being reflected is positioned. Mirrors are not always flat and can be produced in a variety of configurations that provide interesting and useful reflection characteristics. Concave mirrors, commonly found in the largest optical telescopes, are used to collect the faint light emitted from very distant stars. The curved surface concentrates parallel rays from a great distance into a single point for enhanced intensity. This mirror design is also commonly found in shaving or cosmetic mirrors where the reflected light produces a magnified image of the face. The inside of a shiny spoon is a common example of a concave mirror surface, and can be used to demonstrate some properties of this mirror type. If the inside of the spoon is held close to the eye, a magnified upright view of the eye will be seen (in this case the eye is closer than the focal point of the mirror). If the spoon is moved farther away, a demagnified upside-down view of the whole face will be seen. Here the image is inverted because it is formed after the reflected rays have crossed the focal point of the mirror surface.

Reflection of light example image: Convex and concave reflections on the inside and outside of a spoon bowl

Another common mirror having a curved-surface, the convex mirror, is often used in automobile rear-view reflector applications where the outward mirror curvature produces a smaller, more panoramic view of events occurring behind the vehicle. When parallel rays strike the surface of a convex mirror, the light waves are reflected outward so that they diverge. When the brain retraces the rays, they appear to come from behind the mirror where they would converge, producing a smaller upright image (the image is upright since the virtual image is formed before the rays have crossed the focal point). Convex mirrors are also used as wide-angle mirrors in hallways and businesses for security and safety. The most amusing applications for curved mirrors are the novelty mirrors found at state fairs, carnivals, and fun houses. These mirrors often incorporate a mixture of concave and convex surfaces, or surfaces that gently change curvature, to produce bizarre, distorted reflections when people observe themselves.

Spoons can be employed to simulate convex and concave mirrors, as illustrated in Figure 4 for the reflection of a young woman standing beside a wooden fence. When the image of the woman and fence are reflected from the outside bowl surface (convex) of the spoon, the image is upright, but distorted at the edges where the spoon curvature varies. In contrast, when the reverse side of the spoon (the inside bowl, or concave, surface) is utilized to reflect the scene, the image of the woman and fence are inverted.

The reflection patterns obtained from both concave and convex mirrors are presented in Figure 5. The concave mirror has a reflection surface that curves inward, resembling a portion of the interior of a sphere. When light rays that are parallel to the principal or optical axis reflect from the surface of a concave mirror (in this case, light rays from the owl’s feet), they converge on the focal point (red dot) in front of the mirror. The distance from the reflecting surface to the focal point is known as the mirror’s focal length. The size of the image depends upon the distance of the object from the mirror and its position with respect to the mirror surface. In this case, the owl is placed away from the center of curvature and the reflected image is upside down and positioned between the mirror’s center of curvature and its focal point.

The convex mirror has a reflecting surface that curves outward, resembling a portion of the exterior of a sphere. Light rays parallel to the optical axis are reflected from the surface in a direction that diverges from the focal point, which is behind the mirror (Figure 5). Images formed with convex mirrors are always right side up and reduced in size. These images are also termed virtual images, because they occur where reflected rays appear to diverge from a focal point behind the mirror.

1. Reflection and Refraction of light

How Many Types of Reflection of Light Are There?

The reflection of light can be roughly categorized into two types of reflection. Specular reflection is defined as light reflected from a smooth surface at a definite angle, whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions (as illustrated in Figure 3). There are far more occurrences of diffuse reflection than specular reflection in our everyday environment.

To visualize the differences between specular and diffuse reflection, consider two very different surfaces: a smooth mirror and a rough reddish surface. The mirror reflects all of the components of white light (such as red, green, and blue wavelengths) almost equally and the reflected specular light follows a trajectory having the same angle from the normal as the incident light. The rough reddish surface, however, does not reflect all wavelengths because it absorbs most of the blue and green components, and reflects the red light. Also, the diffuse light that is reflected from the rough surface is scattered in all directions.

Image showing specular and diffuse reflection
1. Reflection and Refraction of light

Who Discovered the Reflection of Light?

Some of the earliest accounts of light reflection originate from the ancient Greek mathematician Euclid, who conducted a series of experiments around 300 BC, and appears to have had a good understanding of how light is reflected. However, it wasn’t until a millennium and a half later that the Arab scientist Alhazen proposed a law describing exactly what happens to a light ray when it strikes a smooth surface and then bounces off into space.

The incoming light wave is referred to as an incident wave, and the wave that is bounced away from the surface is termed the reflected wave. Visible white light that is directed onto the surface of a mirror at an angle (incident) is reflected back into space by the mirror surface at another angle (reflected) that is equal to the incident angle, as presented for the action of a beam of light from a flashlight on a smooth, flat mirror in Figure 2.

Thus, the angle of incidence is equal to the angle of reflection for visible light as well as for all other wavelengths of the electromagnetic radiation spectrum. This concept is often termed the Law of Reflection. It is important to note that the light is not separated into its component colors because it is not being “bent” or refracted, and all wavelengths are being reflected at equal angles. The best surfaces for reflecting light are very smooth, such as a glass mirror or polished metal, although almost all surfaces will reflect light to some degree.

Because light behaves in some ways as a wave and in other ways as if it were composed of particles, several independent theories of light reflection have emerged. According to wave-based theories, the light waves spread out from the source in all directions, and upon striking a mirror, are reflected at an angle determined by the angle at which the light arrives. The reflection process inverts each wave back-to-front, which is why a reverse image is observed. The shape of light waves depends upon the size of the light source and how far the waves have traveled to reach the mirror. Wavefronts that originate from a source near the mirror will be highly curved, while those emitted by distant light sources will be almost linear, a factor that will affect the angle of reflection.

Image showing equal angles of reflection

According to particle theory, which differs in some important details from the wave concept, light arrives at the mirror in the form of a stream of tiny particles, termed photons, which bounce away from the surface upon impact. Because the particles are so small, they travel very close together (virtually side by side) and bounce from different points, so their order is reversed by the reflection process, producing a mirror image. Regardless of whether light is acting as particles or waves, the result of reflection is the same. The reflected light produces a mirror image.

The amount of light reflected by an object, and how it is reflected, is highly dependent upon the degree of smoothness or texture of the surface. When surface imperfections are smaller than the wavelength of the incident light (as in the case of a mirror), virtually all of the light is reflected equally. However, in the real world most objects have convoluted surfaces that exhibit a diffuse reflection, with the incident light being reflected in all directions. Many of the objects that we casually view every day (people, cars, houses, animals, trees, etc.) do not themselves emit visible light but reflect incident natural sunlight and artificial light. For instance, an apple appears a shiny red color because it has a relatively smooth surface that reflects red light and absorbs other non-red (such as green, blue, and yellow) wavelengths of light.

1. Reflection and Refraction of light

Reflection of Light Example

The simplest example of visible light reflection is the surface of a smooth pool of water, where incident light is reflected in an orderly manner to produce a clear image of the scenery surrounding the pool. Throw a rock into the pool (see Figure 1), and the water is perturbed to form waves, which disrupt the reflection by scattering the reflected light rays in all directions.

Reflection of light example image: Light reflected on the surface of water]
1. Reflection and Refraction of light

Scattering of light

Some light is scattered in all directions when it hits very small particles such as gas molecules or much larger particles such as dust or droplets of water.

The amount of scattering depends on how big the particle is compared to the wavelength of light that is hitting it. Smaller wavelengths are scattered more.

“Why is the sky blue?” is a common question. Light from the sun is made of all the colours of the rainbow. As this light hits the particles of nitrogen and oxygen in our atmosphere, it is scattered in all directions. Blue light has a smaller wavelength than red light, so it is scattered much more than red light. When we look at the sky, we see all the places that the blue light has been scattered from.

This is similar to the question: “Why are sunsets red?” When the Sun appears lower in the sky, the light that reaches us has already travelled through a lot more of the atmosphere. This means that a lot of the blue light has been scattered out well before the light arrives at us, so the sky appears redder.

Clouds appear white because the water droplets are much larger than the wavelengths of light. For this situation, all wavelengths of light are equally scattered in all directions.

1. Reflection and Refraction of light

Specular reflection

The angle at which light hits a reflecting surface is called the angle of incidence, and the angle at which light bounces off a reflecting surface is called the angle of reflection

Lake reflection

This photo of Lake Matheson shows specular reflection in the water of the lake with reflected images of Aoraki/Mt Cook (left) and Mt Tasman (right). The very still lake water provides a perfectly smooth surface for this to occur.

If you want to measure these angles, imagine a perfectly straight line at a right angle to the reflective surface (this imaginary line is called ‘normal’). If you measure the angle of incidence and the angle of reflection against the normal, the angle of incidence is exactly the same as the angle of reflection. With a flat mirror, it is easy to show that the angle of reflection is the same as the angle of incidence.

Water is also a reflective surface. When the water in a lake or sea is very still, the reflection of the landscape is perfect, because the reflecting surface is very flat. However, if there are ripples or waves in the water, the reflection becomes distorted. This is because the reflecting surface is no longer flat and may have humps and troughs caused by the wind.

It is possible to make mirrors that behave like humps or troughs, and because of the different way they reflect light, they can be very useful.

1. Reflection and Refraction of light

Reflection of light

Reflection is when light bounces off an object. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. This is called specular reflection.

Types of reflection

Light reflects from a smooth surface at the same angle as it hits the surface. For a smooth surface, reflected light rays travel in the same direction. This is called specular reflection. For a rough surface, reflected light rays scatter in all directions. This is called diffuse reflection.

Diffuse reflection is when light hits an object and reflects in lots of different directions. This happens when the surface is rough. Most of the things we see are because light from a source has reflected off it.

For example, if you look at a bird, light has reflected off that bird and travelled in nearly all directions. If some of that light enters your eyes, it hits the retina at the back of your eyes. An electrical signal is passed to your brain, and your brain interprets the signals as an image.