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. 

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Sound waves represent what type of wave?

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The absorption coefficient corresponds to which of the following definitions.

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Energy is removed from a wave as it passes through a medium with a given thickness. This scenario corresponds to what type of absorption?

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Fill in the BlankWhen waves break on the shoreline, their energy is ____   by the sand. 

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Light waves represent what type of wave?

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High absorption coefficients indicate a material absorbs less in comparison to materials with smaller coefficients. 

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Waves of a certain frequency strike an object whose electrons have the same frequency. This scenario corresponds to what type of absorption?

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The definition "a form of energy produced by the vibrational motion of molecules" corresponds to which of the following terms?

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Sound waves represent what type of wave?

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The absorption coefficient corresponds to which of the following definitions.

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Energy is removed from a wave as it passes through a medium with a given thickness. This scenario corresponds to what type of absorption?

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What speed corresponds to a sound wave traveling through air?

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Fill in the BlankWhen waves break on the shoreline, their energy is ____   by the sand. 

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Light waves represent what type of wave?

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High absorption coefficients indicate a material absorbs less in comparison to materials with smaller coefficients. 

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Waves of a certain frequency strike an object whose electrons have the same frequency. This scenario corresponds to what type of absorption?

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The definition "a form of energy produced by the vibrational motion of molecules" corresponds to which of the following terms?

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    Absorption Definition

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

    Absorption describes the process in which wave energy is transferred to a medium.

    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 describes how far a wave penetrates a material of a specific thickness before it is absorbed.

    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 is a form of energy produced by the vibrational motion of molecules.

    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.

    The absorption of sound waves refers to the amount of energy removed from the wave as it passed through a medium with a given thickness.

    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.

    Absorption Sound Waves StudySmarterFig. 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.

    Visible light is wavelengths of electromagnetic radiation visible to the human eye.

    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.

    The absorption of light waves describes electromagnetic radiation that is absorbed and converted into a different type of energy.

    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.

    Absorption Light Waves StudySmarterFig. 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.

    Absorption Color Spectrum StudySmarterFig. 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.

    Do leaves use light absorption to get their green color?

    Solution:

    The answer is yes. Leaves use the absorption of light to complete the process of photosynthesis. Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to produce oxygen and energy (i.e. sugar). Leaves are green because of a pigment called chlorophyll, pigments absorb light. Chlorophyll absorbs red and blue light but reflects green light, which is why leaves turn green.

    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.

    Sound Waves Light Waves
    A sound wave is a longitudinal mechanical wave.A light wave is an electromagnetic wave.
    Sound waves travel \( 330\,\mathrm{\frac{m}{s}} \) in air.Light waves travel \( 3\,\times 10^8\,\mathrm{\frac{m}{s}} \) in air.
    Sound travels faster in solids compared to air.Light slows down in solids compared to air.
    The frequency of a sound wave determines pitch.The frequency of a light wave determines the type of wave and color.
    Humans detect sound waves with their ears.Humans detect light waves with their eyes.

    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.

    Determine the frequency of a \( 1.8\times 10^{-3}\,\mathrm{m} \) sound wave.

    Solution:

    To calculate the frequency of the given wave, we must rearrange our equation in terms of \( f \) as follows,

    \begin{align}\text{speed}&=f\lambda,\\f&=\frac{\text{speed}}{\lambda}.\\\end{align}

    Now we can plug in our values and solve.

    \begin{align}\text{speed}&=f\lambda,\\f&=\frac{\text{speed}}{\lambda},\\f&=\frac{330\,\mathrm{\frac{m}{s}}}{ 1.8\times 10^{-3}\,\mathrm{m}},\\f&=1.8\times10^5\,\mathrm{Hz}. \\\end{align}

    The frequency of this particular sound wave is \( 1.8\times10^5\,\mathrm{Hz}. \)

    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.

    Absorption - Key takeaways

    • Absorption describes the process in which wave energy is transferred to a medium.
    • The absorption coefficient describes how far a wave penetrates a material of a specific thickness before it is absorbed.
    • Sound travels as a longitudinal mechanical wave.
    • Sound absorption occurs when sound travels through a medium and is captured by the molecules of the medium.
    • Light travels as an electromagnetic wave.
    • For the absorption of light to occur, the frequency of an electromagnetic wave must be very similar to the natural frequency of an object.
    • The likeness between light and sound waves ends at the fact that both are waves.
    • To calculate the speed, frequency, or wavelength of a light or sound wave, the equation, \( \text{speed}=f\lambda, \) is used.
    • 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.

    References

    1. Fig. 1- Absorption of sound waves, StudySmarter Originals.
    2. Fig. 2- Absorption of light waves, StudySmarter Originals
    3. Fig. 3- Color Spectrum (https://www.pexels.com/photo/optical-glass-triangular-prism-3845161/) by Dobromir Hristov (https://www.pexels.com/@dobromir-hristov-119509/) licensed by Public Domain.
    Frequently Asked Questions about Absorption

    How does absorption work?

    Absorption works because wave energy is transferred to a medium. When a light or sound wave strikes a medium, its energy is converted into a different type, thus allowing the medium to absorb it.

    What is the absorption of radiation in physics?

    The absorption of radiation in physics refers to the absorption of light. The absorption of light waves describes electromagnetic radiation that is absorbed and converted into a different type of energy.

    What is the absorption spectrum?

    When atoms absorb energy, a color spectrum is produced. 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.

    What is an example of absorption in waves?

    Examples of wave absorption surround us in our everyday lives from feeling the sunlight on our skin while at the beach to singing at the top of our lungs in the privacy of our homes. 

    What is the absorption process?

    The absorption process varies depending on light or sound waves. However, in both cases, wave energy is transformed into another type of energy and is absorbed by the medium.

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