4HistoryIsaac Newton knew that light traveled in straight lines and cast shadows.Other scientists discovered that light was made from a stream of tiny particles or traveled as waves.Olaus Roemer estimated the speed of light in the late 1600’s, but his value was low.William Herschel, 1799, discovered that different colors of light have different temperatures. He discovered that red was the hottest color while violet was the coolest.
5Ultraviolet was discovered by Wilhelm Ritter in 1801. In 1972, the National Institute of Standards and Technology discovered that the actual speed of light was 299,792,458 meters per second.Albert Einstein proved that light moves fastest in a vacuum. However, the speed of light varies depending on the medium through which it travels.
7PolarizationPolarization results when we separate electromagnetic waves by the plane in which they are vibrating.By taking only those waves which vibrate in one plane, we cut down the amount of energy.
9Geometrical OpticsWe assume that light follows straight-line paths (rays)Changes occur when a ray hits a boundaryrays may bounce off (reflection)rays may bend into the other medium (refraction)rays may be absorbed (light energy thermal energy)Diffraction and interference are phenomena best described by light waves (not rays)
13Incident vs Reflected Rays Incident Ray – leaves the object and strikes the mirrorReflected Ray – leaves mirror and strikes your eyeThe reflected ray is on the line of sight from the image to your eye.
14Angles are Measured from the Normal In geometrical optics, angles are always measured with respect to the normal of the interfacenormalIncident ray60o30ointerface
15ReflectionThe law of reflection is always valid! The angle of incidence is always equal to the angle of reflection. This is why “rough” surfaces reflect light in all directions
19Refraction of Light Beam Part of the incident ray is reflected.
20Refraction Electromagnetic waves travel at 3 x 108 m/s (c) in a vacuum Refraction occurs when a wave changes speed as it passes from one medium to another with different characteristics.Electromagnetic waves travel at 3 x 108 m/s (c) in a vacuum
21Transmission Across a Boundary The only time a wave can be transmitted across a boundary, change its speed, and still not refract is when the wave approaches boundary in a direction which is perpendicular to the boundary.
22Consider a light ray which traverses a thick slab ray bends towards the normal upon entering the glassray bends away from the normal when it exits from the glassexiting light ray is at same angle as original ray, but is shifted over to one side
23Why refraction happens? Because speed of light changes.Because light is a wave.Because its wavelength changes in different medium.
24Fata MorganaFata Morgana, also known as Morgan le Fay, was a fairy enchantress skilled in the art of changing shape. In one traditional story she was King Arthur's sister and learned many of her skills from Merlin the Magician.A special type of complex mirage, one that sometimes gives the impression of a castle half in the air and half in the sea, is named after Fata Morgana. She was known to live in a marvelous castle under the sea. Sometimes the enchantress made this castle appear reflected up in the air, causing seamen who mistook it for a safe harbor to be lured to their deaths.The fate morgana mirage is one that can occur only where there are alternating warm and cold layers of air near the ground or water surface. Instead of traveling straight through these layers, light is bent towards the colder, hence denser, air. The result can be a rather complicated light path and a strange image of a distant object. A fate morgana actually is a superposition of several images of one object. Typically one image is upright more or less above two inverted images that may be mingled together. The images may undergo rapid changes as the air layers move slightly up and down relative to the observer.In Alaska the best chance of seeing the relatively rare fate morgana is in winter when temperature inversions develop in the larger valleys. When seeing a complex mountain image out across a valley or bay one can attempt to sort out in the mind the paths that the light rays must have taken. Perhaps it's best just to acknowledge that it is Morgan le Fay beckoning.
26Speed ChangeA ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal.A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal
28speed of light in vacuum speed of light in medium n = Index of RefractionEvery substance has an optical density called the substance's index of refraction (n).The index of refraction indicates is how well light passes through the substance (vs ) compared to how fast light travels through a vacuum (c).cvsspeed of light in vacuumspeed of light in mediumn=
29Index of RefractionIndex of Refraction is a measure of optical densityRepresented by nThe higher n is, the more optically dense the material and the slower light travels in the material
30Laws of refraction Index of refraction depends on: The medium The wavelength (or color of the light)
35ni sin qi = nr sin qr Snell’s Law Snell’s Law relates the index of refraction to the angles of incidence and refraction.ni sin qi = nr sin qrni is the refractive index of the medium the light is leaving,i is the incident angle between the light rayand the normal to the medium to medium interface,nr is the refractive index of the medium the light is entering,r is the refractive angle between the light rayand the normal to the medium to medium interface.
36Question 1 To shoot a fish with a gun, should you aim directly at the image, slightly above, or slightly below?(1) aim directly at the image(2) aim slightly above(3) aim slightly below[CORRECT 5 ANSWER]
37Answer 1(1) aim directly at the image(2) aim slightly above(3) aim slightly belowDue to refraction, the image will appear higher than the actual fish, so you have to aim lower to compensate.
38Question 2To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below?(1) aim directly at the image(2) aim slightly above(3) aim slightly below[CORRECT 5 ANSWER]
39Answer 2 (1) aim directly at the image (2) aim slightly above (3) aim slightly belowThe light from the laser beam will also bend when it hits the air-water interface, so aim directly at the fish.light from fishlaser beam
41Internal ReflectionAll rays reflect internally, but the top three rays reflect only a small percentage internally; most energy leaves the prism.The fourth and fifth rays are reflected 100 % internally
42Total Internal Reflection Total internal reflection is a special case of reflection created when when a light passes from a more optically dense medium to a less dense one at an angle such that there is no refracted ray.
43Fiber OpticsTotal internal reflection is used to send signals through fiber optic cables.
44Total internal reflection There is a critical angle for every boundaryWhen light hits the boundary at this angle it is no longer refracted, but reflected!Remember !! this only works when the incident medium has the higher index of refraction.WHY?
45n1sin qc = n2 sin 90o Critical Angle The critical angle is the angle of incidence which causes the angle of refraction to be 90°, so Snell’s law can be written asn1sin qc = n2 sin 90oThe higher the index of refraction the lower qc
46Critical AngleWhen you exceed the critical angle, you have total internal reflection
48Diffraction is the change in direction of a wave as it encounters the edge of a barrier.
49Wavelength vs Diffraction Shorter wavelengths bend the least when encountering the edge of a barrier.They are in contact with the edge for a shorter period of time.
50Double-Slit Interference BrightfringesThomas Young (1802) used double-slit interference to prove the wave nature of light.
51Remember that white light contains all the colors of the The Visible SpectrumRemember that white light contains all the colors of thes p e c t r u meach color in the spectrum has a different wavelength and so bends at a different rate.