Biomedical Optics 2 MCQ

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Questions and Answers

Which of the following statements accurately describes endogenous and exogenous absorbers?

Endogenous absorbers are artificially introduced to the body.

Both types of absorbers have the same absorption spectra.

Exogenous absorbers include naturally occurring substances like hemoglobin.

Endogenous absorbers occur naturally in the body. (correct)

What characterizes the near-infrared window in terms of tissue penetration?

It allows maximum absorption by endogenous chromophores.

Optical scattering does not influence penetration at these wavelengths.

It coincides with the wavelengths of visible light.

The absorption of molecules like hemoglobin is low at these wavelengths. (correct)

How is the absorption cross-section defined?

It measures the scattering of light in optical media.

It indicates the total energy required to excite a molecule.

It is the ratio of the power incident on a molecule to the power absorbed.

It describes the likelihood of photon transition between two energy states. (correct)

What is the significance of the molar absorption coefficient in tissue optics?

It is derived from the absorption cross-sections of all molecules in one mole. Signup and view all the answers

Which aspect of optical wavelengths is not true about the near-infrared window?

It is unaffected by the absorption levels of water. Signup and view all the answers

What does Avogadro's constant represent in the context of absorption properties?

The number of molecules in one mole of a substance. Signup and view all the answers

What is the primary role of chromophores in biological tissues?

They absorb light at specific wavelengths. Signup and view all the answers

Which of the following best describes the relationship between a chromophore's molecular structure and its absorption spectrum?

Differences in structure create unique absorption spectra. Signup and view all the answers

What forms of energy can a molecule store?

Rotational, vibrational, translational, and electronic potential energy. Signup and view all the answers

What is the significance of energy quantization in molecular energy levels?

Molecules can only absorb specific, fixed amounts of energy. Signup and view all the answers

During therapeutic applications, what happens to the light absorbed by biological tissue?

It causes kinetic energy to be converted into internal energy. Signup and view all the answers

What is the nature of the absorption spectra of different chromophores?

They vary based on the molecular energy levels of chromophores. Signup and view all the answers

Which form of energy is typically associated with a molecule when it becomes excited after absorbing light?

Internal energy of the molecule. Signup and view all the answers

Which statement about energy levels in molecules is accurate?

Energy levels are fixed and quantized. Signup and view all the answers

What role does thermal equilibrium play in molecular energy levels?

It influences how molecules can exchange energy. Signup and view all the answers

What does the emission spectrum of a gas contain?

Very narrow peaks of strong emission Signup and view all the answers

What is spectral broadening?

A phenomenon where an absorption band covers a small range of wavelengths Signup and view all the answers

What characterizes the line shape function near the resonance frequency?

It peaks at the resonance frequency and is non-zero at nearby frequencies Signup and view all the answers

In biological tissues, what is a significant reason for the absorption spectra to vary smoothly?

The high number of vibrational energy levels within biomolecules Signup and view all the answers

Why might gas absorption lines be distinct?

They occur at specific frequencies only Signup and view all the answers

Which statement about non-homogeneous biological tissues is true?

They are made up of many different types of molecules Signup and view all the answers

What is typically true about absorption at wavelengths slightly different from the resonance frequency?

Absorption occurs with a finite probability Signup and view all the answers

What can influence the shape of the absorption spectrum in biomolecules?

The size and complexity of the molecule Signup and view all the answers

How are absorption lines in gas determined?

From the frequencies corresponding to strong emission Signup and view all the answers

What is a likely characteristic of a line shape function?

It can vary based on the physical conditions Signup and view all the answers

Study Notes

Optical Absorption

Biological tissues are composed of various molecules, including water, proteins, lipids, DNA, etc.
Light absorption by tissue occurs when light interacts with specific molecules within the tissue.
These absorbing molecules are called chromophores.
Different chromophores have distinct absorption spectra, meaning they absorb different wavelengths of light to varying degrees.
Absorption spectra differences arise from variations in the molecular/atomic structure of the chromophores.

Molecular Energy Levels

Energy exists in various forms, broadly categorized as kinetic and potential energy.
Kinetic energy is related to an object's or system's motion.
Potential energy is connected to the position of an object or system.
In tissue optics, light absorption leads to an increase in the internal energy of molecules.
This internal energy can then result in thermal or chemical effects.
Molecules store energy in fixed amounts, termed energy levels, not arbitrary amounts.
A molecule stores energy in various forms, including rotational kinetic, vibrational (potential & kinetic), translational kinetic (if the molecules are free to move within a fluid), and electronic potential energy.
Energy level structures determine a molecule's absorption spectrum.
Internal energy encompasses all types of molecular energy.
The kinetic component of the internal energy is associated with thermal motion and correlates with temperature when observed macroscopically.
Rotational kinetic energy relates to the rotation of a molecule or parts of a molecule held together by a bond that allows them to rotate relative to each other.
Rotational energy transitions typically absorb low-energy photons in the microwave region of the EM spectrum.
Vibrational energy arises when different parts of a molecule vibrate with respect to each other due to the elastic nature of the bonds holding constituent atoms together.
Vibrational energy transitions absorb higher energy photons than rotational transitions, typically in the near-infrared or red end of the visible spectrum.
Electronic energy levels describe the potential energy of electrons in relation to their distance from the atomic nucleus. Transitions between electronic energy levels require much higher energy than vibrational transitions and involve UV or X-ray photons.
Excitation of electrons typically leads to photochemical reactions.

Light Absorption and Resonance Absorption

Light absorption is a typical property of vibrating systems, occurring most effectively when the light's frequency matches the system's natural frequency (resonance).
In tissue optics, the light's energy transfer into the molecule depends on the correspondence between the light's frequency and the molecule's natural frequency.
Absorption is highly probable when the light's photon energy matches the difference between the molecule's current and excited states.
Absorption processes generally occur on a very short timescale (femtoseconds).

Absorption Spectra

Emission spectra of gases display narrow peaks of strong emission corresponding to specific frequencies (wavelengths or photon energies).
Strong absorption lines often occur at the same frequencies (wavelengths or photon energies) as those in the emission spectra.
However, in practice, these lines are not infinitely narrow but exhibit a finite probability of absorbing light at wavelengths slightly different from the resonance frequency – this phenomenon is known as spectral broadening.
For large and complex biomolecules, the absorption spectra have more smoothly varying characteristics instead of distinct peaks, primarily due to various vibrational energy levels between electronic levels.
Absorption spectra reflect the presence of various chromophores(different molecules absorbing light), such as water, hemoglobin, melanin, and lipids.
Endogenous absorbers are molecules naturally present, in contrast to exogeneous absorbers which are externally introduced, like contrast agents or tattoos.

Near Infrared Window

The near-infrared range, between approximately 650–1300 nm, is often referred to as the "near-infrared window."
Within this spectral range, light penetration into tissue is more significant because endogenous absorbers, such as water and hemoglobin, display relatively low absorption.
At these wavelengths, optical scattering is still present but does not hinder light penetration to the same extent as at shorter wavelengths.

Absorption Coefficient

For a photon of a given frequency, the probability of a molecule undergoing a state transition from 'a' to 'b' is often expressed using the absorption cross-section.
Absorption cross-section is the ratio of the absorbed power to incident power and is measured in units of area.
Molar absorption coefficient is a material property.
It is calculated by combining the absorption cross-sections of all molecules in one mole of a compound.
The total absorption coefficient of a material composed of multiple chromophores is calculated as the sum of the individual absorption coefficients, weighted by their respective concentrations.

Other

Nonlinear effects, such as multiphoton absorption, depletion, saturation, and photostability, are included, but not detailed.

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Description

This quiz explores the interaction of light with biological tissues, focusing on optical absorption and the role of chromophores. It delves into how energy exists in various forms and the implications of energy absorption in biological contexts. Understand the distinct absorption spectra and energy levels relevant to molecular structures.