Light What you see (and don’t see) is what you get.
Light Assignments 36: 16/25,26,28,29,31, 37: 16/33-36,56,59 38: 17/2,3,13-16 reflection/mirrors 39: 18/2,3,9,11,15-18 refraction/lenses 40: 19/49,52,53,58 interference
Theories of Light • Wave Theory Particle Theory • Huygens Newton • Properties that support each theory Rectilinear Propagation Reflection Refraction Interference Diffraction Photoelectric Effect
Important Info re EM Spectrum V = f l c = f l c = 3 x 10 8 m/s in vacuum or air l decreases to the right f, E increase to the right E = hf h = 6.63 x 10-34 js 1 angstrom A = 10-10 m 1 nanometer, nm = 10-9 m
The Photoelectric Effect Heinrich Hertz first observed this photoelectric effect in 1887. Hertz had observed that, under the right conditions, when light is shined on a metal, electrons are released.
In 1905 Albert Einstein provided a daring extension of Planck's quantum hypothesis and was able to explain the photoelectric effect in detail. It was officially for this explanation of the photoelectric effect that Einstein received the Nobel Prize in 1921. The figure below shows a circuit that can be used to analyze the photoelectric effect. Expanding on Planck's quantum idea, Einstein proposed that the energy in the light was not spread uniformly throughout the beam of light. Rather, the energy of the light is contained in "packets" or quanta (the plural of quantum, a single "packet") each with energy of E = h f
LASER • Light amplification by stimulated emission of radiation 1. The laser in its non-lasing state
Refraction • The bending of light as it passes from one substance into another : www.mysundial.ca/tsp/refraction_of_light.html
Index of Refraction, n n = speed of light in vacuum n = c v speed of light in substance c sub n = sin I sin f nI sinq1 = n2 sinq2
The concept of refractive index is illustrated in Figure 1 below, focusing on the case of light passing from air through both glass and water. Notice that while both beams enter the denser material through the same angle of incidence with respect to the normal (60 degrees), the refraction for glass is almost 6 degrees greater than that for water due to the higher refractive index of glass.
Problem Light travels from a vacuum into water (cw = 2.25 x 108m/s). Determine the index of refraction of water. n = c v / c w = 3x108m/s 2.25 x 108m/s n = 1.33
Problem A ray of light travels from air into water at an angle of 60.0 o with the surface. A. Find the angle of refraction. n = sin i/ sin f sin f = sin i/ n = sin30.0o/1.33 = sin f = 0.376 f = sin-1 0.376 = 22.1 o
B. Find the speed of light in water n = c v / c w c w = c v / n c w = 3 x 10 8 m/s 1.33 c w = 2.26 x 10 8 m/s
i c , critical angle - limiting angle of incidence that results in angle of refraction of 90 o (red) For an angle greater than i c, total internal reflection occurs (dark blue)
If a rod of glass is pulled to a very thin diameter, and light is shone in at one end, it cannot escape, and becomes "trapped" inside the glass rod. Even if the rod is bent or curved, the light continues to be totally internally reflected and continues it's passage along the rod from one end to the other with no loss to the outside. Great use has been made of this property of "light pipes" in recent years. A single glass fiber can carry a stream of light pulses from one end to another almost instantly, making for very rapid very efficient telephone and data connections. Also, if the fibers are bundled together correctly, images can be transmitted, even round curves and corners. www.brooklyn.cuny.edu/.../SBAM/SBAM.Prisms.html
Diffraction- spreading of light around a barrier www.ligo-wa.caltech.edu/teachers_corner/lesso...
Single Slit Diffraction n l = s sin q n, dark band number l, wavelength (m) s, slit width (m) q, angle defined by central band, slit, and dark band
Constructive interference yields bright spots of light Destructive interference yields no light, www.astrophys-assist.com/.../ses01p14.htm
Double Slit Diffraction nl = d sin q n, bright band number (n = 0 for central) l, wavelength (m) d, distance between slits (m) n, angle defined by central band, slit, and band n (order of magnitude, 0,1,2,… of bright bands) This also works for diffraction gratings consisting of many, many slits that allow the light to pass through. Each slit acts as a separate light source.
Problem Find the angle of n=3 fringe (order of image) if 2 slits 0.4 mm apart are illuminated by yellow light, l = 600 nm. Sin q = nl/d = 3(600x10-9m)/4x10-4m q = sin-1 (4.50x10-3) = sin-1 0.00450 = q = 2.58x10-1 o
Diffraction Grating Problem A grating has 4000. lines per cm. At what angles are maxima formed if it is illuminated with yellow light at 600.nm? Slit spacing is: d = 1cm/4000lines = 2.5x10-4cm= 25x103nm sinq=(ln/d)= n(600nm)/2.5x103nm=n(0.24) n=1, q=sin-1(1(0.24)=13.9o n=2, q=sin-1(2(0.24)=28.7o n=3, q=sin-1(3(0.24)=46.0o
Polarization • www.edbergphoto.com/pages/Tip-polarizers.html
Polarized Sunglasses Polarized sunglasses work by filtering out certain frequencies and orientations of light, such as ultra-violet, which is harmful to human eyes. In order to polarize a material for light, etches of scratches must be microscopically put into the material, so that only the light waves that are lined up with the scratches can pass through. This is the basis behind polarized sunglasses.