Light in Biology and Medicine

• ELECTROMAGNETIC WAVES 

  • Our earth’s atmosphere acts as a shield to electromagnetic waves due to the absorption of molecules in gases
• Characteristics of EM Waves

  1. Transverse
  2. Can travel through a vacuum
  3. Travel at the same speed in vacuum, c = 300,000,000 m/s
  4. Can transfer energy from one place to another
  5. Can be refracted and reflected
• Production of EM Waves

  • Generated from an accelerated charge which may be
  • a proton inside the nucleus of an atom
  • an electron in an atom
  • an accelerating electron in a piece of metal
  • ions in a plasma
  • a charged particle in a galactic magnetic field
  • flipping particles with intrinsic magnetic fields
• LIGHT POLARIZATION

  • The direction of propagation of light is constant, but the planes of vibrations are different
  • Unpolarized light can be polarized with the use of polarizers
• 1. Plane polarized light consists of waves in which the direction of vibration is the same for all waves
• 1. Circularly polarized light consists of 2 linear components in the electric field of light that have equal amplitudes but have a phase difference of π/2
• 1. Elliptically polarized light is like circularly polarized light, but the amplitude and phase difference between the 2 light components are not equal
• • Circular dichroism spectroscopy can be used to characterize proteins
• • Optical rotation dispersion can be used to distinguish between L and D enantiomers
• DIFFRACTION

  • Bending of waves around corners and other barriers
• • It is straightforward to show that light of wavelength λ which arrives perpendicular to a diffraction grating with spacing between groves of d will be diffracted to angles
• HUYGEN’S PRINCIPLE

  • Every point on a wavefront is in itself the source of spherical wavelets which spread out in the forward direction at the speed of light
  • The sum of these spherical wavelets forms the wavefront
• LIGHT ABSORPTION SPECTROSCOPY

  • An analytical spectroscopic technique that is used to measure the amount of light or wavelength that is absorbed by a sample
  • Determines how much EM energy is lost after the sample is illuminated
  • The intensity of light after it passes through a material is indicative of its extinction coefficient and amount
• LASERS

  • Light Amplification by the Stimulated Emission of Radiation
  • Stimulated emission of light that occurs due to the stimulation of excited atoms between 2 mirrors
  • The release of light to stimulate other atoms amplifies the waves that reflect back and forth the mirrors
• OPTICAL TWEEZERS

  • Laser beams focused to trap, move, and manipulate molecules
  • Used to make measurements on the mechanical force needed to stretch, bend, and twist macromolecules
• BIOLOGY OF THE EYE

  1. Light enters the eye through the cornea which refracts the light rays
  2. Light passes through pupil and reaches lens
  3. Lens further refines the light, fine-tuning its focus onto the retina
  4. Eyes are upside down on the retina, but the brain interprets the image as upright
  5. Stimulation of opsins by light cases conversion of the images to neural signals
  6. Initiating process of visual perception
• SNELL’S LAW

  • Describes the relationship between the angles of incidence and refraction when light passes through different mediums
• • Radio - Antennae
• • Microwave - Bolometers; microwave "dishes"; microwave cavities
• • Infrared - Thermistors; IR-sensitive semiconductors, heat-sensitive molecules 
• • Visible - Chemical transformations; photocells; kinescopic tubes, photomultipliers; photovoltaic cells; photoconductive cells
• • UV - Atomic excitation; crystal excitation; fluorescence; phosphorescence
• • X-rays - phosphor screens; scintillation crystals; ionization chambers; Geiger counters; photomultiplier tubes
• • Gamma-rays - Geiger counters; ionization chambers; scintillation materials; photo-nuclear reactions; nuclear absorption