Reflection/Refraction Overview

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Reflection/Refraction

Introduction

Reflection is the return of light from a surface. When light strikes various parts of a character at different angles, it is reflected in multiple directions. For instance, the smooth surface of a mirror tends to reflect light from different angles. Similarly, the rough surface of a textbook reflects the light that strikes it. This means that the only way the human eye can see an object that does not emit light on its own is through the reflection of light by that object. On the other hand, refraction refers to the incidence where the path of lightwave bends while passing a boundary that separates two different media. Refraction occurs when the speed of a wave changes when it enters an other medium. For example, the waves tend to travel faster in the deep waters than in the shallow waters. The best case in point is when an ocean wave approaches a beach at an angle, the part that seems farther from the shoreline will move faster than the part that is closer in. Hence, the wave will swing around to move in the direction that is at a right angle with the shoreline.

Another example is when light enters a prism and gets refracted. It will result in the separation of light into its component wavelengths. Reflection and refraction play an essential role in helping the human see things in sufficient detail.

The Laws of Reflection

The behavior of light is known to be very predictable. For instance, if a ray of light approaches and reflects on a flat smooth surface such as a mirror, its behavior as it reflects follows a predictable law termed as the law of reflection.

Diagram 1.0

As shown in diagram 1.0, the ray of light that approaches the polished metal surface is known as the incident ray. The beam of light leaving the metal surface is known as the reflected ray. Furthermore, the line that is perpendicular to the metal’s body at the point where the beam strikes the metal is known as a standard line. This line separates the angle between the end of incidence and the moment of reflection into two grades; the angle of incidence and the angle of reflection, respectively (Keller, 1953). Based on the first law of thinking, the angle of incidence is equal to the angle of reflection when a ray of light reflects off an object’s surface.

The second law of reflection states that the incident ray, the reflected ray, and the normal occur on the same plane (Yu et al., 2011). Also, a change in the direction of the incident ray leads to a change in the plane’s angle. If a person were to sight along a line at a location that is different from the image location, it would be impossible for a ray of light to emanate from the object, reflect off the mirror based on the law of reflection, and afterward travel to the person’s eye (Carrasco, Tamagnone, & Perruisseau-Carrier, 2013). It means that only when a person sights an image, does light coming from the object reflect off the mirror in harmony with the law, and travel to the person’s eye.

Diagram 1.1

If light from an object is forced to reflect in a manner that would make the angle of incidence to appear lesser than the angle of reflection to reflect and travel to a person’s eye, it has not obeyed the law of reflection. Alternatively, if an eyesight an object at a position that appears to be below the reallocation of the image, the light will be forced to reflect in a manner that the angle of incidence appears greater than the angle of reflection. Such an occurrence will not have followed the law of reflection (Keller, 1953). In both cases, an image cannot be seen when sighting along an indicated line of sight. Therefore, the law of reflection demands that the eye sights at the location of an image to eventually view the object’s image in a mirror. According to the third law of reflection, both the incident ray and the refracted ray appear on different normal line (Yu et al., 2011). As diagram 1.0 illustrates, the incident ray and the reflected ray must appear on different normal line sides.

The laws of reflection are applied in the day-to-day activities of human beings. For example, dentists apply these laws when examining their patients (Yu et al., 2011). With the use of a dental mirror, a dentist can see underneath and behind a patient’s teeth and inspect any crevice. A dentist can only see the hidden parts of a tooth by adjusting the mirror’s position to make the angle of incidence and reflection equal to both the eye and the mirror.

Refraction

There are so many questions that can be addressed by examining the crucial concept of refraction. For example, people need to understand why their legs may appear bent as they dangle then in water or why fish seem to change positions as people observe them from different points of view in the pond or aquarium. Refraction involves a process whereby light bends when encountering a different medium other than the one through which it has been traveling.  The point at which two different media meet is called the interface. Both reflection and refraction of light are said to occur at the interface. For light to refract, it has to be incident at a transparent surface. This transmitted element of light then changes direction at the interface.

Diagram 1.2

As shown in diagram 1.2 above, the refracted ray changes direction and deviates from a straight extension of the incident ray at the interface. This change in light direction while traveling from one medium to another is linked to changes in wavelength and velocity. However, the energy of the light remains unchanged while it travels through one medium to another.  For example, when the visible light traveling through air enters a medium such as glass, its velocity reduces to 75 percent of its air velocity. Besides, a significant decrease may be experienced in other materials.

Furthermore, because a change in the speed of light occurs at the interface, the wavelength of light changes. As it enters the medium, the wavelength reduces as the light wave takes a different direction.  This concept is well illustrated in diagram 1.3 below.

Diagram 1.3

Lightwave change direction when they enter a new medium because the lines are so close together, as in diagram 1.3. Hence, when the first line hits the water, the line will slow down. So, in order to maintain their synonymous relationship, all the lines must turn once they hit the water. And they must turn towards the normal line that runs perpendicular to the interface between the different media.

Refractive Index

The refractive index is the measure of the bending of rays of light when they travel through one medium to another. When light travels through an interface of two media with different indexes of refraction, it bends.

Diagram 1.4

The refraction index is useful in the design of optical lenses since it allows for the calculation of the bend angle of a ray of light as it goes through a transparent medium to another (Shrivastava, 2018). Additionally, these indices could be useful in quality control as well as in the identification of various materials with transparent or translucent characteristics to the rays of light.

When the light in a denser medium approaches a less dense medium, a complete reflection of the rays of light occurs. This complete reflection occurs within one medium, such as water from the surrounding surfaces back into the medium. The phenomenon of complete reflection is referred to as total internal reflection (Shrivastava, 2018). This phenomenon occurs at the interface between two transparent media when rays of light in one medium with a greater refractive index approach the second medium at an angle of incidence that appears to be greater than the critical angle. Refractive indices rely on wavelength; therefore, the critical angle will also vary slightly with color as it does with wavelength.  Refraction and reflection seem to occur in varying proportions at angles that are less than the critical angle.

References

Carrasco, E., Tamagnone, M., & Perruisseau-Carrier, J. (2013). Tunable graphene reflective cells for THz reflectarrays and generalized law of reflection. Applied Physics Letters, 102(10), 104103.

Keller, J. B. (1953). Parallel reflection of light by plane mirrors. Quarterly of Applied Mathematics, 11(2), 216-219. https://www.ams.org/journals/qam/1953-11-02/S0033-569X-1953-55188-8/S0033-569X-1953-55188-8.pdf

Shrivastava, A. (2018). Introduction to plastics engineering. William Andrew.

Yu, N., Genevet, P., Kats, M. A., Aieta, F., Tetienne, J. P., Capasso, F., & Gaburro, Z. (2011). Light propagation with phase discontinuities: generalized laws of reflection and refraction. Science, 334(6054), 333-337. DOI: 10.1126/science.1210713