Light is the connection between humans and the universe. It is through life that we could experience distance, objects, colors, our world, communicate, support life and even look back to the beginning of existence. Light is the electromagnetic radiation detected by the human eyes, its interaction with the matter have helped us understand the shape of the universe and it is light that provides us with a window to study phenomena on atomic and cosmological scales (Woodford). Light surrounds us as makes possible to know the world as we used to. Light has been subject to study by scientists and physicist for centuries. Particularly, three phenomena are responsible for allowing our eyes to devise the world and in some cases cause illusions.
The phenomenon responsible for human vision is known as reflection. Light travels through a medium, commonly air, and hits the surface of an object, which causes the wave to bounce at their surface. Then, the wavelength travels from the object to our eyes creating the shape and color of objects. When the light enters the eye, it is absorbed by the retina, passing electrical signals to the brain and the brain interprets these signals as an image. These bouncing waves of light are called reflection.
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There are two types of reflection, specular reflection of light occurs when the light waves encounter a surface that does not absorb the energy of radiation, causing that these waves bounce away. Most common surfaces for this phenomenon to take place are glass, polished metal or water, as these surfaces reflect at the same angle. Diffuse reflection occurs when the surface is rough; in these cases, the light is dispersed through different directions creating the shapes we perceive as objects.
In astronomy, for example, reflection is the phenomenon that allows us to watch the moon, comets, planets, asteroids, etc. We are able to see them because the light from the sun is reflected in their surface. The sun and other stars glow as a consequence of nuclear reactions occurring on the inside that make them hot enough to emit radiation (Stille).
Another possibility is that these light waves are bent. The refracted light does not bounce back; they continue going forward but in another direction. This phenomenon is known as the refraction of light. Often this causes that objects are perceived differently than their real form. But also, the refraction in our eyes focuses light, allowing us to see the object clearly (Stille).
Waves are bent when they enter a medium where their speed is different. In this order of ideas, the phenomenon takes place when light passes from a fast medium to a slow medium, proportionally shortening the wavelength and bending the light towards the normal to the boundary between the two media. The amount of bending depends on constants particular for each surface, known as indices of refraction.
Light refraction is responsible for dessert mirages. In the desert, the sand is heated by the sun to very high temperatures. This hot sand heats the air closer to the surface, when the temperature differential among two layers of air reach a considerable degree the air is separated in hot air (closer to the surface) and cold air (farther from the surface). The index of refraction in cold air is higher than hot air. As a result, the particles of light travel through the hot air faster, because hot air is less dense.
This condition is not processed by the human brain; it assumes that light travels in a straight line. Therefore, when a viewer is standing in the desert and sees the sky reflected in the sand and the brain interprets it as water on the sand because subconsciously the brain calculates that water would reflect the light from the sky in the same way that this vertical gradient does (Meyer).
Another important phenomenon that is important to study is total reflection. It occurs when a ray of light is completely reflected within a medium from the surrounding surfaces backs into the medium. This phenomenon takes place when the angle of incidence is greater than the critical angle, in most cases at the boundary between two transparent media. It is observed when a ray of light travels through a medium with a higher index of refraction approaching to another medium at an angle of incidence greater than the critical angle (Encyclopaedia Britannica).
Prisms are shaped to produce this effect, in order to be employed as optical instruments such as binoculars, telescopes, and periscopes.
Perhaps one of the most popular and well-known effects caused by total internal refraction is the characteristic shine of diamonds. The sparkles we see in diamonds are a consequence of this light phenomenon. Diamonds are one of the toughest objects in the world, it is tremendously dense and with a high index of refraction. As a consequence, the critical angle is smaller; when light enters a diamond the waves of light are very likely to cause total internal reflection and showing what we see as sparkles.
In conclusion, it is possible to state that physics is present in every aspect of our lives, whether we acknowledge it or not. Often the physics theory is misunderstood and might go unnoticed by unfamiliarity with the basic concepts that drive major effects. The truth is that physics rule most basic interactions of bodies, from our perception of the universe, in the form of light, to the movement of celestial bodies, such as planets, asteroids, and comets.
- Encyclopaedia Britannica. «Total internal reflection.» Encyclopaedia Britannica (2018).
- Meyer, Edwin. «What causes a mirage?» Scientific American (2003).
- Stille, Darlene. Manipulating Light: Reflection, Refraction, and Absorption. Capstone, 2005.
- Woodford, Chris. Light: Investigating Visible and Invisible Electromagnetic Radiation. The Rosen Publishing Group, 2012.
