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Optical properties of materials

What are the optical properties of materials?

optical properties of materials
Fluorescence and birefrigeration in a calcite crystal, two of the optical properties of materials. The laser beam splits in two as it passes through the glass. Source: Wikimedia Commons.

The optical properties of materials are those that become apparent when matter interacts with electromagnetic radiation. These properties explain various phenomena such as color, transparency or opacity. Optical properties of materials

The different structures of materials, at the molecular level, cause light to be absorbed and reflected in different ways, producing varied effects. Understanding these phenomena is essential in many current technologies, such as those based on optical fibers.

Now, electromagnetic radiation and particularly light, which is the visible part of the spectrum, interacts with the medium in three different ways: Optical properties of materials

  • Absorption, part of the incident beam is totally absorbed by the medium.
  • Reflection, another fraction of the incident energy is reflected back to the original medium.
  • Transmission, the rest of the energy passes through the medium and is transmitted to another medium.

Thanks to this, from an optical point of view, materials are classified into: Optical properties of materials

  • Transparent, those that the light passes through completely, allowing objects to be seen clearly through them.
  • Translucent, they absorb a part of the incident light and transmit another, in such a way that any object seen through them appears diffuse.
  • Opaque, it is not possible to see through them, since they completely absorb the incident light. Optical properties of materials

Most important optical properties

1. Brightness Optical properties of materials

This quality refers to the appearance of a surface when light is reflected on it. If reflections occur, the surface is shiny, regardless of its color, and if, on the contrary, it looks dull, it is a matte surface.

Metals shine because light interacts with their free electrons, increasing their degree of vibration, which results in the reflection of their particular light waves.

2. Color Optical properties of materials

Objects are the color of the light they scatter. White light contains all wavelengths and each of these is perceived as a different color: blue, green, yellow, red … The sky looks blue, because the molecules in the atmosphere preferentially scatter that wavelength, absorbing the others.

In contrast, water droplets and ice crystals in clouds scatter virtually all wavelengths out of them, and thus appear white.

On the other hand, metals such as gold and copper absorb the wavelengths of blue and green, reflecting those of yellow and red. And silver, steel, and aluminum reflect all visible wavelengths and therefore appear silver-white. Optical properties of materials

The materials that allow all the visible light that falls on them to pass through are transparent. Such is the case with liquid water, transparent acrylic sheets and glasses lenses. On the other hand, materials that do not are considered opaque, for example metal or wooden pieces.

Translucent materials have intermediate characteristics, they absorb a part of the light that passes through them and transmit the rest. Examples of this class of substances are some oils and ice crystals. Optical properties of materials

It is important to note that some materials are opaque at certain wavelengths and transparent to others. One case is the atmosphere of Earth, which is largely opaque to infrared radiation emitted by the planet, being transparent change to light coming from the sun .

4. Luminescence Optical properties of materials

Some substances exposed to certain energetic stimuli have the ability to absorb energy and then spontaneously emit a part in the range of visible light or close to it. For some materials, exposure to sunlight is sufficient, others require more energetic radiation, such as X-rays.

Not only electromagnetic radiation gives rise to the emission of light, but also mechanical, electrical, thermal stimuli and more.

This luminous phenomenon has its origin in the fact that the electrons in the atoms are arranged in discrete or quantized energy levels. If they absorb energy, they are able to go from a lower energy state to a higher one, and when they later return to the original state, they emit the excess energy in the form of light. Optical properties of materials

  • Fluorescence and phosphorescence

Fluorescence is the emission of light that occurs within 10 – 8 seconds after the material is exposed to the source of energy. In contrast, phosphorescence occurs when the light emission from the luminescent material lasts for more than 10 – 8 seconds.

  • Thermoluminescence

Some insulating or semiconductor materials are capable of emitting light by continuously heating below red. Because of this the solid emits light later. Optical properties of materials

This phenomenon should not be confused with incandescence, such as that which occurs when an electric current passes through a conductive tungsten filament, in a conventional light bulb.

Thermoluminescence is frequently used to date ceramic objects that contain certain minerals, since the intensity of the light emitted by these is proportional to the time that has elapsed since they became part of the object by another previous heating. With this method, samples up to 500,000 years old can be dated. Optical properties of materials

  • Triboluminescence Optical properties of materials

Some types of quartz and cane sugar crystals emit light when they are crumbled, rubbed, or deformed in some way. Sometimes, some earthquakes are accompanied by light phenomena associated with the triboluminescence of the rocks in the earth’s crust.

  • Electroluminescence

They are semiconductor substances that emit light when a potential difference is applied to it. The effect is widely used on car dashboards, toys, and decorative items.

  • Chemiluminescence and bioluminescence

Certain chemical reactions release energy in the form of light and if they occur in living things, it is called bioluminescence, observed in insects such as fireflies and in much marine life.

Chemiluminescence is used in forensic science through luminol, a substance that makes it possible to determine whether blood has spread on a surface, even if it has been carefully cleaned. Luminol reacts with small amounts of iron in the blood and produces a faint glow when the room is dark. Optical properties of materials

5. Dichroism Optical properties of materials

Some substances show different colors depending on the angle from which they are viewed, that is, they scatter a certain wavelength at a certain angle.

6. Birefringence or double refraction

They are substances in whose interior the speed of light is not the same in all directions.

A light wave front striking a material of this type generates two sets of secondary waves tangent to each other, along a specific direction, called the optical axis. The effect is that two slightly displaced images of the same object are seen through a birefringent glass. Optical properties of materials

Examples of birefringent substances are calcite and crystalline quartz.

7. Photochromism

It is the color change in certain substances, caused by the interaction with some type of electromagnetic radiation or another type of external stimulus of a physical or chemical type, such as the passage of an electric current, friction, a change in pH or heat.

These materials are used for various purposes, such as in the manufacture of glasses for the selective improvement of visual acuity, protective glasses for homes and patches indicating the degree of exposure to explosions, among other applications.

8. Polarization

The electromagnetic fields that make up unpolarized light can move in any direction perpendicular to the direction of propagation. But there are substances that when they are crossed by nonpolarized light, they only let through the light that vibrates in a certain direction.

One way to obtain polarized light is by passing it through a birefringent crystal, and eliminating one of the two components, as in the case of the Nicol prism.

A tourmaline crystal can absorb light that vibrates in all but one direction, which is why the crystals from which Polaroid sheets are made use tourmaline. Optical properties of materials

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