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Physical Optics and Properties of Light Resident Lecture Amy C. Nau, O.D., F.A.A.O The Uzumaki or 'rabbit ears' illusion: when in peripheral vision these spirals appear to move in a clockwise direction. Vision is the perception of light To understand vision, you must understand the properties of light Where does it come from?: How does it interact with objects? How does it move through the eye? How can it be used to aid diagnosis of eye disorders? Photons When an atom is in the resting state, the negative electron cloud is balanced with the positive nucleus Excited atoms have electrons that are in a higher energy state, when they go back to the relaxed state, the energy released is a photon. A photon is an energy packet that travels through space as an EM wave Wave Motion Wave propagation is represented as a sine wave that varies in time and space EM energy travels as a transverse wave (vibrates perpendicular to direction of propogation) Electromagnetic waves EM waves have both electric and magnetic fields. Wave Motion Wavelength- (l m,nm) Distance from peak to peak, trough to trough or any repeatable position. It is inversely proportional to the amount of energy the atom gives up. So, short wavelengths have high energy. Cycle- (c) completion of a regular periodic event (one peak to the next) Frequency-(f, c/s, Hz) Number of cycles that pass a reference position in a given time period. Constant for all media Period (T,P s/c) Inverse of Frequency Velocity (v, m/s) rate of travel. Light has same velocity in air as in a vaccuum (3x108m/s) Amplitude (A) maximum displacement of wave Velocity, frequency and wavelength are related by v=f l cycle wavelength peak amplitude trough Electromagnetic waves Photons travel through a vacuum at a constant speed (the speed of light, c) 3x106 m/s They will slow down outside a vacuum Index of refraction (n) of a media is the ratio of speed of light in a vacuum to speed of light in the media v=f l Key Definitions Source- any object emitting EM radiation Point source- so small or far that it acts as if infinitely small Extended source- source with measurable area Monochromatic- EM single wavelength or frequency (laser, sodium gas, etc.) Polychromatic- emits radiation of several wavelengths (white light) Wavefronts Rays Used to represent the propogation of light in geometrical optics. Rays are perpendicular to wavefronts A group of rays is a pencil Convergent, divergent, parallel Beam- sum of pencils Rays Parallel Pencil Rays emitted from a source infinitely far away. Since the wavefronts have such large radii, they are functionally parallel to each other. (20 feet) Sign convention rules Assume light travels from left to right Distances are + if they travel in the same direction of light. They are – if they travel in the opposite direction. Radii of wavefronts are measured from the wavefront to the source or image. - Interference The addition of two waves to form a new wave Constructive or Destructive http://www.colorado.edu/physics/2000/applets/fourier.html Used to measure the quality and shape of optical surfaces Used to bypass the eye’s optics and project contrast sensitivity fringes on to the retina directly Seen with lasers (speckles) Coherence The ability of two waves to interfere Same wavelength, source and temporal characteristics (lasers) Incoherent light will not interfere (incandescent light bulbs) Coherence Rectilinear Propogation Light rays travel in straight lines Diffraction is when light hits an object and becomes distorted or bends. If that distorted wave hits a surface, there is a diffraction pattern Double-slit diffraction Double-slit diffraction (red laser light) 2-slit and 5-slit diffraction http://en.wikipedia.org/wiki/Diffraction Scatter Another diffraction effect in which light interacts with a series of small particles The size and spacing of the particles determines the degree of scatter Materials scatter the light they don’t absorb Smoke, fog, edema, cataract produce glare, reduced contrast IR less prone to scatter, can penetrate to the retina/choroid http://medinfo.ufl.edu/cme/hmoa2/light_scatter_t.jpg http://www.3dvf.com/DATA/PUBLISH/404/images/scattervolumelight.jpg Fluorescence Molecules absorb photons and become excited. If they return to the original state, a photon is emitted. Fluorescein Dye- illuminated with a blue wavelength of light and emits in the green. Used for cls, ocular surface inspection and FA ICG- illuminated with IR- note IR also does not scatter! Electromagentic Spectrum The amount of energy the atom releases determines the wavelength. - Small wavelength= high energy - Long wavelength = lower energy LIGHT is EM waves that are within the visible spectrum 380nm-780nm The term “visible light” is redundant………… EM Spectrum Visible light (for humans) is 380nm to 780 nm Vision and EMR An object’s color is determined by the characteristics of the wavelengths that are reflected off of it’s surface We have 3 populations of cones that are sensitive to different wavelengths Short (S)- blue Middle (M)- green Long (L)- red Spectral Sensitivity Color is a property of the way the Visual system detects light http://www.photo.net/photo/edscott/vis00010.htm Normal Protanopia Unable to percieve red Deuteranopia Unable to percieve green Ways that light changes Wavelength (color) changes depending on where light is travelling The speed of light (velocity) also changes depending on media Wavelength decreases in any media other than a vacuum. It is directly related to velocity, so if velocity decreases, so will the wavelength. Example Problem Red light, with a wavelength of 700nm in a vacuum, enters a lens so that the wavelength reduces to 450nm. What is the velocity of the light in the block of glass? Solve for frequency 3x108m/s /700x10-9 = 4.28x1014 Use this to solve for the second veocity V= 4.28x1014 x 450 x 10-9 So, you see as the wavelength decreased, so did the speed at which light travels. v=fl Vergence The reciprocal of the radius of the wavefront When measured in meters, is referred to as a DIOPTER Divergent pencils have negative vergence Convergent pencils have positive vergence VD=1/rm Problem What is the power of the following lens? VD=1/rm l=-40cm l’=90cm Object vergence (L) = 1/l = 1/-.4 = -2.5D Image vergence (L’) = 1/l’= 1/.9 = +1.11D Total change by the lens (power, or F) is +3.61D Problem Determine the vergence of a wavefront with a radius of -.04meters. VD=1/rm So, V=1/-.04 = -25D Pinhole camera h h’ a a’ h/h’=a/a’ No matter where the object is placed, a clear image will form, but the size of the image will change. A pinhole takes the place of the lens. If the pinhole is small enough to only let one ray from each object point to pass, each point of the image will be formed by a single ray, forming an inverted image that is in focus. :: < back T h e G a l l e r y A b o u t P i n h o l e W h y P i n h o l e F o r u m s M a k i n g C a m e r a s E x p o s u r e G u i d e U s e f u l L i n k s P i n h o l e E v e n t s Small aperture concepts Depth of field- distance over which an object can be moved without affecting the image (ph, small pupils) Range of vision at near increases Prosthetic cls Depth of focus- distance over which an image screen can be moved while maintaining sharpness Field of view- maximum angular size of object imaged by system (or eye) FOV of the 20D v 28D Problem A PH camera is used to photograph an object. Where must the object be placed so the image formed on film 5cm behind the PH is .1 times the size of the object? h .1(h) ? Just set up a ratio. 5cm h/? = .1(h)/.5 solve for ? And you get 50 cm. Hermann Grid Illusion In the above illustration, black dots appear to form and vanish at the intersections of the gray horizontal and vertical lines. When focusing attention on a single white dot, some gray dots nearby and some black dots a little further away also seem to appear. More black dots seem to appear as the eye is scanned across the image (as opposed to focusing on a single point). Strangely, the effect seems to be reduced, but not eliminated, when the head is cocked at a 45° angle. The effect seems to exist only at intermediate distances; if the eye is moved very close to or very far away from the figure, the phantom black dots do not appear. The illusion is known as the scintillating grid, and was discovered by E. Lingelbach in 1994. It is a modification of the Hermann grid illusion. Begin!