Chemical and Physical Foundations of Biological Systems Section: Content Category 4D

How light and sound interact with matter
Light is a form of electromagnetic radiation — waves of electric and magnetic fields that transmit energy. The behavior of light depends on its frequency (or wavelength). The properties of light are exploited in the optical elements of the eye to focus rays of light on sensory elements. When light interacts with matter, spectroscopic changes occur that can be used to identify the material on an atomic or molecular level. Differential absorption of electromagnetic radiation can be used to generate images useful in diagnostic medicine. Interference and diffraction of light waves are used in many analytical and diagnostic techniques. The photon model of light explains why electromagnetic radiation of different wavelengths interacts differently with matter.

When mechanical energy is transmitted through solids, liquids, and gases, oscillating pressure waves known as “sound” are generated. Sound waves are audible if the sensory elements of the ear vibrate in response to exposure to these vibrations. The detection of reflected sound waves is utilized in ultrasound imaging. This non-invasive technique readily locates dense subcutaneous structures, such as bone and cartilage, and is very useful in diagnostic medicine.

The content in this category covers the properties of both light and sound and how these energy waves interact with matter. The topics and subtopics are below.

Topic Level Key:

The abbreviations found in parentheses indicate the course(s) in which undergraduate students at many colleges and universities learn about the topics and associated subtopics. The course abbreviations are:
GC = two-semester sequence of general chemistry
OC = two-semester sequence of organic chemistry
PHY = two-semester sequence of introductory physics
Please note topics that appear on multiple content lists will be treated differently. Questions will focus on the topics as they are described in the narrative for the content category.

Sound (PHY)

  • Production of sound
  • Relative speed of sound in solids, liquids, and gases
  • Intensity of sound, decibel units, log scale
  • Attenuation (Damping)
  • Doppler Effect: moving sound source or observer, reflection of sound from a moving object
  • Pitch
  • Resonance in pipes and strings
  • Ultrasound
  • Shock waves

Light, Electromagnetic Radiation (PHY)

  • Concept of Interference; Young Double-slit Experiment
  • Thin films, diffraction grating, single-slit diffraction
  • Other diffraction phenomena, X-ray diffraction
  • Polarization of light: linear and circular
  • Properties of electromagnetic radiation
    • Velocity equals constant c, in vacuo
    • Electromagnetic radiation consists of perpendicularly oscillating electric and magnetic fields; direction of propagation is perpendicular to both
  • Classification of electromagnetic spectrum, photon energy E = hf
  • Visual spectrum, color

Molecular Structure and Absorption Spectra (OC)

  • Infrared region
    • Intramolecular vibrations and rotations
    • Recognizing common characteristic group absorptions, fingerprint region
  • Visible region (GC)
    • Absorption in visible region gives complementary color (e.g., carotene)
    • Effect of structural changes on absorption (e.g., indicators)
  • Ultraviolet region
    • π-Electron and non-bonding electron transitions
    • Conjugated systems
  • NMR spectroscopy
    • Protons in a magnetic field; equivalent protons
    • Spin-spin splitting

Geometrical Optics (PHY)

  • Reflection from plane surface: angle of incidence equals angle of reflection
  • Refraction, refractive index n; Snell’s law: n1 sin θ1 = n2 sin θ2
  • Dispersion, change of index of refraction with wavelength
  • Conditions for total internal reflection
  • Spherical mirrors
    • Center of curvature
    • Focal length
    • Real and virtual images
  • Thin lenses
    • Converging and diverging lenses
    • Use of formula 1/p + 1/q = 1/f, with sign conventions
    • Lens strength, diopters
  • Combination of lenses
  • Lens aberration
  • Optical Instruments, including the human eye

To support your studies, see the following video tutorials below from the Khan Academy MCAT Collection. The videos and associated questions were created by the Khan Academy in collaboration with the AAMC and the Robert Wood Johnson Foundation.