UNIT 11, Section 1 Properties of Light Chapter 16 CCSD Objectives: 9.7 THE STUDENT WILL SOLVE PROBLEMS USING THE WAVE EQUATION (FOR LIGHT). 10.1 THE STUDENT WILL DEVELOP A MODEL FOR LIGHT. 10.2 THE STUDENT WILL DIFFERENTIATE BETWEEN THE FUNDAMENTAL PROPERTIES OF LIGHT. 10.3 THE STUDENT WILL ANALYZE THE DEVELOPMENT OF THE CURRENTLY ACCEPTED MODEL OF THE ATOM. 10.5 THE STUDENT WILL DISCUSS THE ELECTROMAGNETIC SPECTRUM AS IT RELATES TO THE STRUCTURE OF THE ATOM. 10.6 THE STUDENT WILL CALCULATE LIGHT INTENSITY, POWER / AREA. (E = P / (4 Π D2)
A. Light - an electromagnetic wave 1. History a. Newton proposed that light was a particle, explained reflection, uses rays. b. Christen Huygens proposed that light was a wave, and that wave theory explained reflection and refraction as well as diffraction i. particle theory cannot explain all of these ii. Uses wave fronts>
2. Visible Light is only part of the electromagnetic spectrum a. above the radio and infrared sections b. spectrum refers to frequency as the independent variable. How something changes with frequency 3. wavelengths from 3.0x10-7 to 7.6x10-7 m 4. the shorter wavelengths are violet, longer are red - ROY G BIV 5. Discreet verses Continuous a. Excited electrons emit discreet wavelengths b. Hot atoms emit a continuous spectrum
DIAGRAM OF ELECTROMAGNETIC SPECTRUM
6. Light (particles) travels in a straight line, which provides the basis for ray optics arrows represent the path the light is traveling, called rays b. beam of light - rays traveling in the same direction 7. Light does not require a medium for transmission
B. Dual Nature of Light - can be looked as B. Dual Nature of Light - can be looked as carrying energy as a wave or as a particle 1. As a wave - look at Huygen’s Principle: all points on a given wave-front are taken as point sources for the production of spherical secondary waves, called wavelets.
2. Quantum Theory of Light - light is emitted from a source in discrete bundles (quantized) called photons (Einstein) a. photoelectric effect - ejection of electrons from a metal surface when exposed to light b. kinetic energy of an ejected electron is independent of the intensity of the light iii. E = h f (E- energy, h- Planck’s constant, f- frequency)
3. Atomic Structure a. Bohr Model- electrons occupy energy levels with specific energies. i. absorbing a photon with just the right amount of energy moves electron higher. ii. Dropping down to a lower energy level emits a photon of the difference in energy. b. energy is released in the form of light, photons, of specific energies or wavelengths (colors) for each atom.
C. Speed of Light 1. Today it is a defined value and not possible to measure, c = 299,792,458 m/s. Can be used to measure an object’s length by the time of travel of light. 2. First determined in the “lab” by Albert Michelson. Used a rotating octagonal mirror as a timer and light traveled over a measured path (less than 1 cm in error) between two mountain tops. 3. c = f
D. Transmission and Absorption of Light 1. Transparent - objects through which we can see clearly. Transmits light 2. Translucent - objects transmit light but cannot be seen clearly. 3. Opaque - transmit no light. Absorbs or reflects all light that falls on them.
E. Illumination by a Point Source 1. Luminous-emits light waves, like the sun 2. Illuminated - reflects light waves, like the moon. 3. Luminous Intensity (I) a. Amount of light a source gives out, depends on energy put into the source and the efficiency of the source. b. Incandescent less efficient than fluorescent, by a 4:1 ratio c. Measured using candela (cd), 1/60th of the luminous intensity of a square centimeter of fused thorium maintained at 2046 K.
4. Luminous Flux (P) a. Rate at which light energy is emitted from a source b. Measured using lumens (measures the rate of energy, therefore a power unit) c. Looking at light energy radiated as if from a point source in the center of a sphere. Surface area of the sphere is 4r2. d. Luminous flux is directly proportional to the intensity. P = 4 I
5. Illuminance (E) [light intensity] a. Rate at which light energy falls on a unit area some distance from a source. b. Measured using lumens/m2 or lux (lx). One lux is the illuminance of a surface located one meter from a one candela source. c. Illuminance, E, varies inversely with the square of the distance, r, directly with the luminous flux, P, and directly with the source intensity, I.
F. Light and Color 1. White light is light composed of all colors, whereas black is the absence of any color or of reflected light. In electronics we talk of white noise to indicate all background noise present in a system 2. Spectrum is the name given by Newton to the ordered arrangement of colors that results from white light being dispersed in a prism
3. Primary Additive Colors - red, blue, green a. added correctly you get white light b. process used in color TVs c. all of the colors in the visible spectrum can be generated by mixing together proper proportions of these three colors. d. Note: we are talking of mixing light of different wavelengths - art talks of mixing pigments
4. Secondary Light Colors - formed by mixing two of the primary colors. a. yellow (red & green) b. cyan (blue & green) c. magenta (blue & red) 5. Complementary color - the secondary color that when added to a primary color produces white light. Cyan and red = white 6. Dye - a molecule that absorbs wavelengths of light and reflects/transmits others. Explains why an object has a particular color.
7. Pigment - a particle larger than a molecule and it can be seen with a microscope. a. absorbs certain colors and reflects others b. pigments mix to form suspensions c. primary pigment - absorbs only one color from white light, they are the secondary light colors d. secondary pigment - absorbs two primary colors and reflects one from white light, they are the primary light colors
G. Polarization of Light 1. Light is comprised on many electromagnetic waves moving in all directions. This is because light is emitted by atoms oscillating in many different directions. Such light is unpolarized. waves oriented to a particular plane are said to be plane polarized. b. although light contains beams vibrating in every possible plane, all can be resolved into vertical and horizontal components c. average-1/2 vertical and 1/2 horizontal.
2. When light is reflected it becomes polarized a. one angle at which maximum polarization occurs. b. Light reflected off of a horizontal surface is horizontally polarized, as surface absorbs oscillations perpendicular to it. This reflected light is called glare.
H. Interference in Thin Films 3. A polarizer only allows light to pass through in one direction, those parallel to the plane. 4. Polarization shows light is a transverve wave. Sound cannot be polarized as it is longitudinal. H. Interference in Thin Films 1. A soap film is held vertically: weight makes it sag and become thicker at the bottom, & light passing through it will be refracted differently. 2. Part of light is reflected, part is transmitted and refracted.