Sofia Martinez
Plastic is an interesting material that can be transparent, translucent, or opaque, and its interaction with light depends on various factors. When light hits a plastic surface, a portion of it is reflected, and the rest is transmitted through the material. The amount of light reflected and transmitted depends on the smoothness of the plastic surface. Rough plastic scatters light in all directions, making it visible to the human eye, while smooth plastic reflects light internally, only becoming visible when it reaches the edge and escapes. Additionally, the ability of plastic to reflect light can be altered by modifying the surface, such as by adding roughness or treatments. Furthermore, plastic can also block certain types of light, like UV light, and its ability to do so depends on the specific type of plastic and its properties.
| Characteristics | Values |
|---|---|
| Transparency | Plastic containers can be transparent and reflect light simultaneously |
| Reflection | Rough plastic scatters light in all directions, while smooth plastic reflects light that strikes at a grazing angle back inside |
| UV Light | Plastic can block UV light |
| Polarization | Reflected light is polarized, allowing you to see the stresses in molded plastic |
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What You'll Learn
Rough plastic scatters light in all directions
When light hits any object, some of it is reflected and some transmitted. The amount of light reflected and transmitted depends on the properties of the object. Smooth plastic reflects light that strikes it at a grazing angle back inside. This is known as specular reflection, where light is reflected at just one angle.
However, rough plastic scatters light in all directions. This is due to the phenomenon of diffuse reflection, where light is reflected at many angles. The surface roughness of the plastic causes the irregularities on the surface to scatter light in multiple directions, reducing the predictability of its path. This is also known as surface roughness scattering or interface roughness scattering, where particles are scattered against a rough solid surface.
The scattering of light by rough plastic can be observed in everyday life. For example, when light passes through a plastic container, some of the light is reflected, and some is transmitted through the plastic. The transmitted light then bounces back and forth within the plastic, with a portion being leaked out with each bounce. This results in a mixture of specular and diffuse reflection, where some light is reflected off the smooth parts of the plastic, while the rest is scattered by the rough parts.
The degree of scattering by rough plastic can be influenced by various factors. One factor is the wavelength of light, with shorter wavelengths such as blue and violet scattering more than longer wavelengths like red or infrared light. Additionally, the size of particles or irregularities in the plastic can impact light scattering, with particles similar in size to the wavelength of light efficiently scattering it. This is known as Mie scattering, which is responsible for the white colour of water droplets in clouds.
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Smooth plastic reflects light internally
When light hits any surface, a portion is reflected and a portion is transmitted. The reflection of light depends on the colour of the material. White objects reflect all wavelengths of visible light, whereas black objects absorb them.
Smooth plastic reflects light that strikes its surface at a grazing angle back inside. This phenomenon is observed when the plastic is smooth. When light hits rough plastic, it scatters the light in all directions, and some of it reaches your eye. However, in the case of smooth plastic, the light that strikes the surface is reflected internally and is not visible to the observer.
The reflection of light by smooth plastic can be observed at the edges. When light reaches the edge of the plastic, it escapes, and this is where you see it. This is because the light is no longer reflected internally, and some of it escapes and reaches your eye.
The colour of the plastic also plays a role in how much light is reflected. For example, a white piece of plastic will reflect more visible light than a black piece of plastic. This is because white objects reflect all colours of light, while black objects absorb most colours and reflect very little.
Additionally, the thickness of the plastic can also affect the reflection of light. If the thickness of the plastic is similar to the wavelength of light, interference effects come into play. These effects can alter the way light is reflected and transmitted by the plastic.
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Plastic can block UV light
When light hits a plastic surface, a portion of it is reflected, and a portion is transmitted. The ability of light to penetrate plastic depends on the chemical composition of the material. Plastic is known to block or transmit UV light, depending on its type and additives used.
UV light is defined as light with wavelengths between 100 nanometers (nm) and 400 nm. It is further divided into UV-A, UV-B, and UV-C light. UV-C light, with a wavelength of 100-290 nm, is a very energetic form of light that is not visible to the human eye. Standard acrylic plastic sheets will not allow UV-C light to pass through. Even very thin acrylic sheets of less than 5 millimeters (mm) do not let UV-C light penetrate.
However, there are certain types of plastics that are designed to transmit UV light, such as UV transmitting acrylic sheeting, which is often used in indoor tanning beds and greenhouses. These plastics have no additives to block UV light and can allow up to 92% UV ray transmission.
To achieve good resistance to UV rays in plastics, certain additives can be used. For instance, carbon black at around a 2% level can provide protection by blocking UV light. Other additives like titanium dioxide, benzophenones, and benzotriazoles are effective absorbers that selectively absorb UV light and re-emit it at a less harmful wavelength, usually as heat.
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Plastic can change the energy level of light
When light hits any surface, a portion is reflected, and a portion is transmitted. For example, if 10% of light is reflected at the interface of air and plastic, 90% of it enters the plastic. Plastic can be manipulated to reflect and refract light. The roughness or smoothness of the plastic surface determines how light is reflected. Rough plastic scatters light in all directions, while smooth plastic reflects light that strikes at a grazing angle back inside.
Recent research by Florida State University has discovered that low-energy light can be used to manipulate photopolymers or plastic films. This finding has significant implications for technologies that use light as an energy source to create shape-shifting structures. The research team, led by Associate Professors of Chemistry and Biochemistry Ken Hanson and Justin Kennemur, and Professor William Oates, developed a new process that harnesses low-energy light and uses it to bend plastic films.
The process involves using a photopolymer based on the chemical compound stilbene, which can be used to make dyes, optical brighteners, or dye lasers. By applying a triplet sensitizer mechanism to the polymer, the plastic can absorb low-energy light and convert it into high-energy mechanical work. This causes the plastic films to bend in response to the energy transfer.
Additionally, some plastics can store light and slowly release it over time. This phenomenon is observed in phosphorescent materials, where electrons are energized to a higher orbit, resulting in a "forbidden transition" that gives lasers their exact wavelengths and glow-in-the-dark materials their distinct colour.
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Reflected light off plastic is polarised
When light hits a surface, a portion of it is reflected, and a portion is transmitted. This is true for plastic as well as other materials. However, reflected light off plastic is polarised. This is due to the phenomenon known as Brewster's angle or the polarizing angle. Brewster's angle is the angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the reflected light from the surface is perfectly polarized.
Étienne-Louis Malus first observed the phenomenon of light being polarised by reflection from a surface at a particular angle in 1808. Brewster's angle was then defined by Scottish physicist Sir David Brewster in 1815. Brewster's angle is often used in optics and laser physics to eliminate unwanted reflections. For example, when recording a classical hologram, the bright reference beam is arranged to strike the film in the p polarization at Brewster's angle, eliminating reflection of the reference beam at the transparent back surface of the holographic film.
The polarization of light is always perpendicular to the direction in which the light is travelling. When light is reflected at Brewster's angle, the dipoles that generate the reflected light do not radiate any energy in the direction of the dipole moment, resulting in no reflection. This is why reflected light off plastic is polarised.
Additionally, the surface of the plastic can also affect how light is reflected. Rough plastic scatters light in all directions, while smooth plastic reflects light that strikes at a grazing angle back inside. This is why you may see light reflected off a rough plastic surface but not a smooth one.
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Frequently asked questions
Yes, a plastic container will reflect light. The amount of light reflected depends on the smoothness of the plastic. Rough plastic scatters light in all directions, while smooth plastic reflects light that strikes at a grazing angle back inside.
When light hits a rough surface, it is scattered in all directions, and some of it reaches our eyes. On the other hand, smooth plastic reflects light internally, so less light is visible to us.
Plastic can block UV light to some extent. The energy from UV light can be absorbed and re-emitted as light of a different wavelength, which may appear less bright to the human eye.
Yes, you can see the reflection in a plastic container. The reflected light is polarized, so you may be able to observe stresses in the molded plastic in the reflection.
In a dark room, hold a piece of paper with a whitening agent like TiO2 (commonly found in printer paper) up to the light, inside and outside the plastic container. Compare the brightness to judge the difference.
