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  1. 1. LIGHT = life G K K M Optom – CU Asst Prof MLUH
  2. 2. Light is a form of energy whose interaction with retina gives the sensation of sight. It is visible portion of the electromagnetic radiation spectrum. Light enables us to see. -luminous objects -non- luminous objects It lies between ultraviolet and infrared portions, from 400 nm at the violet end of the spectrum to 700 nm at the red end . The white light consists of seven colours denoted by VIBGYOR 1. Violet 2. Indigo 3. Blue 4. Green 5. Yellow 6. Orange 7. Red
  4. 4. Colour Wavelength (order) Red Orange Yellow Green Blue violet 620-780nm 590-620nm 570-590nm 500-570nm 450-500nm 380-450nm
  5. 5. Properties of light • Propagated as electromagnetic wave, i.e., it does not require medium for its propagation. • Speed of light in free space (vacuum) is 3 × 108 m/s (186,000 miles/s). • Speed in a medium is less than in the vacuum. • Transverse in nature, so can be polarized. • Monochromatic light refers to light of a single wavelength. •White light is heterochromatic. • Not deflected by electric and magnetic field.
  6. 6. Nature of Light Wave theory of light ( Huygen’s theory). Particle theory of light (Newton’s theory- corpuscles) Modern theory of light --- Quantam Theory (depending on the situation explaining both theories)
  7. 7. Visible light and the eye • Media of the eye are uniformly permeable to the visible rays between 600 nm and 390 nm. • Cornea absorbs rays shorter than 295 nm. Therefore, rays between 600 nm and 295 nm only can reach the lens. • Lens absorbs rays shorter than 350 nm. Therefore, rays between 600 nm and 350 nm can reach the retina in phakic eyes; and those between 600 nm and 295 nm in aphakic eyes. • Eye is most sensitive to yellow-green light (wave length 550 nm). •The sensitivity decreases on both sides of this wavelength, so it is minimum for violet and red light
  8. 8. Sensitivity of eye to visible spectrum of light
  9. 9. 1. Physical Optics Physical character and behavior of light and its interaction with matter. Physical optics takes into consideration the basic dual nature of light, the waveform and the particle (photon or quantum) form, and thus includes: A. Wave optics. B. Quantum optics. 2. Geometrical Optics Image forming properties of lenses, mirrors and prisms A. Reflection. B. Refraction.
  10. 10. Light is a transverse wave One transverse wave has a crest and a trough Wave optics
  11. 11. Longitudinal Waves Compression Rarefaction
  12. 12. Transverse Wave Phase Any point on the wave that we want to discuss. Amplitude Maximum displacement of a wave Wavelength Distance between 2 symmetrical part of wave motion Phase Difference It refers to fraction of cycle
  13. 13. Time period Time taken for a complete vibration is called time period
  14. 14. Frequency Number of oscillations/cycles per unit time is called frequency Unit = hertz Never changes when light moves from one medium to another (5.45x1014 Hz) Intensity Energy delivered or carried by the wave per unit area per unit time I = (energy delivered)/[(area/time)]
  15. 15. Phenomena based on wave optics are: • Interference • Diffraction • Polarization
  16. 16. Interference Superposition principle •When two or more low energy waves are superposed on the same space or medium •The resultant displacement at each position is the algebraic sum of the displacements due to each wave. Constructive Interference In Phase Crests of both the component waves line up Resultant amplitude is equal to the sum of the two component amplitudes Ar = (A1+A2)
  17. 17. Destructive Interference Out of Phase Crest of one wave lines up with the trough of another component wave a. b. Resultant amplitude is equal to the subtraction of two component waves Ar = (A1 – A2)
  18. 18. Applications • Holography • Laser Interferometry • Anti Reflection Coatings • Excitation filter and Barrier filter used in Fundus Fluorescein Angiography(FFA) • Optical Coherence Tomography (OCT)
  19. 19. Diffraction The ability of a wave to propagate around corners is called diffraction
  20. 20. Applications •Limits the visual acuity when the pupil size is less than 2.5mm. •Resolution of an image
  21. 21. Polarization The phenomenon of conforming the vibrations of a wave in specific direction perpendicular to the direction of wave motion is called polarization
  22. 22. Applications •Haidinger Brushes (Orthoptic Exercises) • Slit lamp (↓unwanted reflections) •Polarizing projection charts (malingering patients) Uses • Titmus Fly Test (stereopsis) • Recording and Reproducing 3D pictures • Polaroid Sunglasses
  23. 23. Quantum optics Quantum optics treats light as a particle (localized energy pocket) called photon. Phenomena based on quantum optics are: • Transmission and absorption of light • Scattering of light • LASER (Light amplification of stimulated emission of radiation) • Fluorescence.
  24. 24. Absorption The amount of light lost in a medium while passing through it
  25. 25. Transmission The total amount of light coming out from a medium in the incident direction
  26. 26. Scattering When a beam of light passes through a medium (gas) a part of it appears in the direction other than the incident direction (heterogeneous medium) this phenomenon is called scattering of light Depends on • Size of the particle • Distance between particles • Strength of the interaction
  27. 27. Scattering of light
  28. 28. Small Particles < λ/10 • Wavelength dependent • Small wavelength(more) • Large wavelength(less) eg: Air molecules Applications • Blue color of sky • Corneal haze in oedema • Appearence of Cataract • Cells and Flare • Glare
  29. 29. LASER Light Amplification by Stimulated Emission of Radiation Characteristics •Monochromatic •Coherent •Collimated
  30. 30. Monochromatic Same Wavelength Coherent Same phase difference
  31. 31. Collimated All the rays are exactly parallel to each other
  32. 32. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.
  33. 33. Applications of LASER •Excimer Laser (Refractive Surgery) •Nd: YAG (Neodymium Yttrium Aluminum Garnet) Dissection of Posterior Capsule, Peripheral Iridotomy •Argon and Krypton lasers for retinal photocoagulation •Carbon dioxide lasers for photo vaporization
  34. 34. 2. Geometrical Optics Image forming properties of lenses, mirrors and prisms A. Reflection. B. Refraction.
  35. 35. Reflection of light Reflection of light is a phenomenon of change in the path of light rays without any change in the medium.  The light rays falling on a reflecting surface are called incident rays and those reflected by it are reflected rays. A line drawn at right angle to the surface is called the normal. Laws of reflection 1. The incident ray, the reflected ray and the normal at the point of incident, all lie in the same plane. 2. The angle of incidence (i) is equal to the angle of reflection (r).
  36. 36. Laws of reflection
  37. 37. REGULAR REFLECTION OR SPECULAR REFLECTION mirror-like reflection of light. the reflected rays are also parallel to each other.
  38. 38. IRREGULAR REFLECTION OR DIFFUSED REFLECTION •the different portions of the surface reflect the incident •light in different directions • no definite image is formed • the surface becomes visible. • non-luminous objects visible
  39. 39. Refraction of light Refraction of light is the phenomenon of change in the path of light, when it goes from one medium to another. The basic cause of refraction is change in the velocity of light in going from one medium to the other. Laws of refraction 1. The incident (i) and refracted (r) rays are on opposite sides of the normal (N) and all the three are in the same plane. 2. The ratio of sine of angle of incidence (i) to the sine of angle of refraction (r) is constant for the part of media in contact. This constant is denoted by the letter m and is called ‘refractive index’ of the medium In which the refracted ray lies with respect to medium 1 (in which the incident ray sin i lies), i.e. sin r = m2. When the medium 1 is air (or vaccum), then n is called the refractive index of the medium This law is also called Snell’s law of refraction.
  40. 40. Laws of refraction. N1 and N2 (normals); I (incident ray); i (angle of incidence); R (refracted ray, bent towards normal); r (angle of refraction); E (emergent ray, bent from the normal)
  41. 41. Total internal reflection When a ray of light travelling from an optically denser medium to an optically-rarer medium is incident at an angle greater than the critical angle of the pair of media in contact, the ray is totally reflected back into the denser medium. This phenomenon is called total internal reflection. Critical angle refers to the angle of incidence in the denser medium, corresponding to which angle of refraction in the rare medium is 90°. It is represented by C and its value depends on the nature of media in contact. Application. 1. fibroptic lights 2. applanation tonometer 3. gonioscope.
  42. 42. Refraction of light (1-1’) path of refracted ray at critical angle, c (2-2’) Total internal reflection (3-3’)