Absorption

Absorption is a concept we experience regularly in our everyday lives. Without it, seeing or hearing would be quite difficult. Absorption describes how waves are transferred to a medium. In this case, our eyes and ears act as the medium. Our eyes enable us to see light waves, absorb them, and in turn convert them into an electrical impulse that our brains interpret as an image. Our ears capture sound waves and amplify them inside our ear canal which enables us to hear. Hence, let's use this as a starting point in understanding absorption.

Absorption Definition

Absorption is one of the six main properties associated with light or sound waves.

Mediums can be solid, liquid, or gas. Common mediums include:

• Wood: Sound is produced as a result of the sound wave striking the wood. When this occurs, the wood amplifies or absorbs the sound wave.
• Air: Sound travels shorter distances in the air because the collision of particles is less. This results in the sound wave losing energy faster. Imagine hearing a sound but then quickly walking away. The farther you are from the source of the sound, the less loud the sound is.
• Water: Sound travels farther in water because particles can more easily collide with each other. This allows the sound wave to maintain its energy longer. Take the sound of humpback whales which travels thousands of miles in water.

Light waves do not need a medium to propagate, unlike sound waves which do.

Absorption Coefficient

When studying the interaction between waves and mediums, we must be aware of the absorption coefficient.

The absorption coefficient depends on the material and the wavelength. High absorption coefficients indicate a material absorbs more readily in comparison to materials with smaller coefficients.

Importance of Coefficient to Light and Sound Waves

To better understand this coefficient's importance, let us look at the examples of solar panels for light waves and soundproofing for sound waves. Engineers use this coefficient to determine what material should be in a solar panel. Solar panels function as a result of two layers of a semiconducting material that is usually silicon. The area in which these layers meet is crucial as it is the space where solar radiation (sunlight) is absorbed and converted into electricity. Similarly, engineers use this coefficient to determine what material best suits a recording studio. Recording studios use soundproofing as a way to maximize the absorption of sound waves while minimizing the reflection of sound waves. Reflected sound waves create echoes which in turn disrupt the production of music. Now that we have discussed absorption, let us specifically discuss the absorption of sound and light waves and their corresponding processes.

Absorption of Sound Waves

Sound is a word used in everyday language. However, most may not be familiar with its technical definition.

Sound travels as a longitudinal mechanical wave. Longitudinal refers to the wave propagating parallel to the direction of the molecules while mechanical refers to the transport of energy through the medium due to the vibration of molecules. Sound waves travel the quickest through solids as a result of their tightly packed molecular orientation. Molecules can more easily collide with one another enabling sound to move faster. Our ears capture sound waves in the ear canal where the sound becomes amplified as it travels toward the eardrum. When it reaches the eardrum, it causes vibrations that enable us to hear. Typical human ears can detect sound in the range of \(20\,\mathrm{Hz}\,\text{to}\,20\,\mathrm{kHz}. \)

For absorption to occur, energy is removed from the sound wave as it is transferred from the sound wave to the medium.

Sound absorption occurs when sound travels through a medium and is captured by the molecules of the medium. Then the sound energy is converted into heat. This is done by the vibrational motion of the molecules of the medium. When sound passes through, the molecules are initially at rest. However, if the sound is strong enough to overcome the molecule's resistance to motion, the molecules begin to vibrate.

Fig. 1 - A diagram depicting the absorption of sound waves.

Factors affecting Sound Absorption

Three factors affect sound absorption.

1. Medium density: When the density of the medium increases, the absorption of low-frequency sound waves increases, while the absorption of high-frequency sound waves decreases.
2. Medium thickness: The increase in the thickness of the medium enhances the absorption of low-frequency sound waves but has no significant effect on high-frequency sound waves.
3. Pour size: Smaller pours increase sound absorption, while larger pours decrease sound absorption.

Note that high-frequency sound waves refer to waves with more oscillations and frequencies of \( 2000\,\mathrm{Hz} \) and higher. Low-frequency sound waves refer to waves with fewer oscillations and frequencies of \( 500\,\mathrm{Hz} \) and lower.

Absorption of Light Waves

Light is a colloquial word we often use to describe all light. However, the term light only refers to visible light which is one of seven types of electromagnetic radiation on the spectrum.

Our eyes take visible light and turn it into an electrical impulse that our brains interpret as an image. Typical human eyes can see light wavelengths in the range of \( 400\,\mathrm{nm}\,\text{to}\,700\,\mathrm{nm}. \) Besides visible light, electromagnetic radiation is divided into the following types:

• Radio
• Microwave
• Infrared
• Visible
• Ultraviolet
• X-rays
• Gamma Rays

where each travel at the speed of light. Note that when we say radiation, one's first instinct may be to associate the word with something terrible. However, in this context, radiation refers to something that radiates outward from a central point.

For the absorption of light to occur, the frequency of an electromagnetic wave must be very similar to the natural frequency of an object.

When light with a certain frequency strikes an object whose electrons have the same frequency, the electrons absorb the energy from the light wave and convert it into vibrational motion. As the electrons interact with surrounding atoms, vibrational energy is converted into thermal energy. Once this occurs, the light wave is absorbed by the object and will never again be released in the form of light.

Fig. 2- A diagram depicting the absorption of light waves.

Absorption Spectrum of Light

When atoms absorb energy, a color spectrum is produced. This spectrum is usually referred to as ROYGBIV. To best way to understand this is to think of a prism. When light passes through the prism, it is separated into different wavelengths, thus producing a rainbow of color.

Fig. 3- Atoms absorbing energy produce a color spectrum as seen when light strikes a prism.

Now the absorption spectrum occurs when light passes through a medium and atoms are absorbed at frequencies characteristic to that particular light ray. Due to re-emitted light not being emitted in the same direction as the absorbed light, dark lines appear on the color spectrum indicating the absence of light.

Light Waves vs. Sound Waves

As we've discussed both respective wave types, it is important to understand their similarities and differences. Light and sound waves are similar in the fact that both are waves. However, that is where their likeness ends. To distinguish between the two, let us look at the table below.

Speed, Frequency, and Wavelength

To calculate the speed, frequency, or wavelength of a light or sound wave, the equation

\begin{align}\text{speed}&=(\text{frequency})(\text{wavelength}),\\\text{speed}&=f\lambda,\\\end{align}

can be applied and rearranged in terms of all three variables. Note that \( f \) is measured in hertz denoted by \( \mathrm{Hz}\) and wavelength is measured in meters denoted by \( \mathrm{m}. \) However, it is important to understand that \( \text{speed} \) is a fixed value for light and sound waves traveling through air. Light waves move at a speed of \( 3\times10^8\,\mathrm{\frac{m}{s}} \) and sound waves move at a speed of \( 330\,\mathrm{\frac{m}{s}}. \) Therefore, we can use this equation to calculate the frequency and wavelength of a given wave. Let's try a quick example.

Examples of Wave Absorption

Examples of wave absorption surround us in our everyday lives from feeling the sunlight on our skin while at the beach to us singing at the top of our lungs in the privacy of our homes. While at the beach, we observe wave absorption from the waves crashing onto the sand. When waves break on the shoreline, they transport their energy into the sand. However, we also experience wave absorption at the beach. When sunlight hits our skin, some of its energy is absorbed by our skin and we begin "heat" up. This explains why we feel warmer in the sun compared to sitting under a beach umbrella. When we sing in our homes, we produce sound waves. The energy from the produced sound waves travels through the different materials in our homes such as curtains and sofa covers. These materials absorb sound, and therefore, convert sound energy to heat. This conversion eliminates echoes that would otherwise occur if the sound was reflected and not absorbed.