What is Optical property?

The term optical property describes a material's behavior when electromagnetic radiation (light) is incident on the material's surface or, in other words, how a material interacts under an incident electromagnetic radiation. Different types of material show different optical properties due to differences in physical, chemical, and mechanical characteristics.

What is the importance of optical properties?

The knowledge of optical properties is very important in various industrial as well as in scientific applications. In the selection of material for the purpose of contactless temperature measurement devices, heat transfer methods, laser technology, etc., complete knowledge of optical properties of materials is necessary for efficient operation.

The term electromagnetic radiation refers to wave-like, electric and magnetic components that are perpendicular to each other and in the direction of propagation. For the propagation of electromagnetic radiation, a medium is not necessary. It can propagate through the vacuum at a constant velocity. In quantum mechanics, electromagnetic radiation is considered as packets of energy referred to as a photon.

The expression for the energy of electromagnetic light beam can be represented as,

$E=\frac{hc}{\lambda }$

Here, $E$ represents the electromagnetic energy of the light beam, $h$ represents  planck's constant, $c$ represents the light speed in vacuum, and $\lambda$ represents the wavelength of the light beam.

Interaction of light with solid

Whenever an electromagnetic light wave moves from one medium to another, the intensity of the light wave is equal to the sum of the intensity of transmitted, absorbed, and reflected light waves. Mathematically, it can be represented by the given expression.

${I}_{0}={I}_{t}+{I}_{a}+{I}_{r}$

Here, ${I}_{0}$ represents the intensity of incident light wave, ${I}_{t}$ represents the intensity of transmitted wave, ${I}_{a}$ represents the intensity of absorbed light wave and ${I}_{r}$ represents the intensity of reflected light wave.

Types of optical properties of materials

Different materials show different optical properties when placed in electromagnetic radiation. The basic details of different types of materials are given below in the following steps.

Reflectivity

It is the optical property of a material. When a light wave is incident on the polished surface of the material, the light returns back from the material surface and this is referred to as the reflection of light. Some of the materials show better reflectivity compared to other materials. White surface shows better reflection of light.

Absorptivity

Absorption is also a very important optical property of a material. Whenever a beam of light is incident on the surface of the material, then some part of the incident beam would be absorbed by the material. Some materials like black surface/body show better absorption of the light beam, which means good absorptivity. Solar cells are made up of a material that has good absorptivity.

Refractivity

Whenever a light ray moves from one medium to another, there is bending of light, which means that the path of incident light ray changes after interaction with another medium. This phenomenon is referred to as the refraction of light, and this property of the material is called refractivity. This property of the material depends on the value of the refractive index of the material.

Transparency

It is an optical property of the material in which if an object is placed on one side of a material and a light wave is incident on the other side, then the object can be seen clearly. Some important materials like glass, air, water, etc., are transparent.

Translucent

In the optics process, the term translucent indicates the behavior of a material in which if an object is placed on one side of the material and a light wave enters the other side, the object can not be seen clearly. Some examples are sunglasses, vegetable oils, etc.

Opaque

In the context of the optics process, some materials do not allow the passage of electromagnetic light waves which means that the light waves can not pass through them. This type of material is not transparent or translucent in nature. This material is called opaque material. Examples- wood, sand, metals, etc. The application of this type of material prevents the transmission of light.

Luminescence

The term luminescence is defined as the process in which a material absorbs energy and then immediately emits visible or near-visible radiation. This phenomenon occurs as a result of the excitation of electrons of material from the valence band to the conduction band.

Thermal emission

Whenever a material is heated (increase in temperature), the electrons of the outer shell are excited to higher energy levels, where the electrons are less strongly bound to the nucleus. These excited electrons move back to the ground state and release photons in the process that is called thermal emission.

Photo conductivity

Bombardment of semiconductors by photons, with energy equal to or greater than the bandgap may create electron-hole pairs that can be used to generate current. This process is called photoconductivity.

Electronic polarization

Whenever a rapidly fluctuating electric field interacts with a metallic material, electronic polarization induces in the material or shifts the electron cloud relative to the atom's nucleus. Due to these, some of the radiation energy may be absorbed, and light waves retard in speed.

Optical properties of nonmetals

In the context of optics, nonmetals may be transparent to visible light. The optical behavior of nonmetals in the presence of electromagnetic waves is discussed in the following steps.

Refraction

Whenever photons are transmitted through a material, they cause polarization of the electrons in the material, and by interacting with the polarized materials, photons lose some of their energy. As a result of this, the speed of light is reduced, and the beam of light changes direction.

The mathematical expression of refractive index of a material can be represented as,

$n=\frac{\mathrm{sin}{\theta }_{i}}{\mathrm{sin}{\theta }_{r}}$

Here, $n$ represents the refractive index of the material, ${\theta }_{i}$ represents the angle of incident beam from normal and ${\theta }_{r}$ represents the angle of refracted beam from normal.

Reflection

When light radiation passes from one medium to another, some light scatters at the interface between the two mediums. If the deviation of light from its original path is more it means the material's reflectivity is more. The expression of reflectivity can be represented as,

$R=\frac{{I}_{r}}{{I}_{0}}$

Here, $R$ represents the reflectivity of material, ${I}_{r}$ represents the intensity of the reflected light beam, and ${I}_{0}$ represents the intensity of the incident light beam.

Scattering

The radiation emanating from the oscillating electrons that travel in all directions represent the scattered radiation. The blue color of the visible light beam scatters more than other colors of light.

Absorption

Non-metallic materials may be transparent or opaque to visible light. If it is transparent, it appears as colored. The absorption of the light beam takes place through two mediums. One is electronic polarization, and the other is valence band conduction.

Applications of optical properties of materials

There is a wide range of applications of optical properties of materials that are given below in the following steps:

• Development of communication infrastructures and information technology
• In medicine manufacturing
• In the development of devices like television, microscope, solar cells, etc.
• Luminescence
• In the development of photoconductive devices
• In solar cells
• In optical communications fibers

Common Mistakes

• Students sometimes get confused regarding the difference between reflection and refraction. However, reflection refers to the return back of a light beam when it strikes a smooth surface, whereas refraction refers to the bending of the light beam when it moves from one medium to another.
• Sometimes, students also get confused about the difference between transparent material and translucent material. However, the image of an object from transparent material is clear, whereas the image generated from translucent material is not clear (blur).
• Students also get confused about the difference between the optics phenomenon in metals and the optics phenomenon in non-metals. However, non-metals may have transparent or opaque nature; if they are transparent, they appear colored.

Context and Applications

The optical properties of materials are very significant in several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical and Electronics Engineering
• Light emitting diodes
• Opacity and translucency
• Color phenomenon of material
• Semiconducting materials
• Electronic polarization
• Snell's law

Practice Problems

Q1. Which of the following optics phenomenon suited for metals?

1. Reflect the light beam
2. Refract the light beam
3. Transmit the light beam
4. None of these

Correct option: (a)

Explanation: Metals are expressed as the substances that have greater conductivity as well as malleability as compared to other substances. Most of the metals exhibit the phenomenon of reflection of the light beam.

Q 2. Which is/are the optical characteristics of metallic material?

1. Translucent
2. Transparent
3. Opaque
4. None of these

Correct option: (c)

Explanation: Metals are composed of partly loaded conduction bands, as photon moves towards metals, their energy excite electrons, placing the electrons in unoccupied states. This makes the optical characteristic of metallic metals as opaque.

Q3. Which of the following option is correct regarding transmission through metallic material?

1. Visible light
2. Microwaves
3. X-ray

Correct option: (c)

Explanation: Compared to other rays like visible light, radio waves, and microwaves, X-rays are transmitted through metallic material.

Q4. Which of the following option relates to Snell's law?

1. Light refraction
2. Light reflection
3. Light absorption
4. Light transmission

Correct option: (a)

Explanation: Snell's Law is related to the refraction of light. It is the second law of refraction of light.

Q5. The phenomenon of luminescence is due to

1. Photons stimulated by photons
2. Knocking out of electrons by photons
3. Photons are emitted while excited electrons drop-down
4. All

Correct option: (c)

Explanation: Luminescence is the phenomenon which occurs when photons are emitted as a result of the electrons dropping down from higher energy level to lower energy level.

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Optical Properties of Materials

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