Nature of Light. What is light? Sources of light. an electromagnetic radiation capable of stimulating the retina of the eye.

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1 Nature of Light What is light? an electromagnetic radiation capable of stimulating the retina of the eye. electrons Nucleus Electron gains energy When it moves to a higher level Photon bundle (quantum) of energy traveling in waves electrons give off light energy (photons) when they move to a lower level Sources of light Sun Incandescent lamps Fluorescent lamps Flames Light waves have wavelengths from about 380 nm ( m) to 760 nm ( m). 1

2 Electromagnetic Spectrum Visible light is a SMALL portion of the entire range of electromagnetic waves. Light is one of the most important means of learning about the physical nature of our world microscopes, telescopes and human eyes are all important science tools Nature of Light Particle Theory (Newton) Wave Theory (Huyqens) Electromagnetic Theory (Faraday/ Maxwell) Quantum Theory (Planck) Modern Theory 2

3 Particle Theory Streams of tiny particles moving in all directions from source Rectilinear propagation easy to explain particles in straight lines Waves bend around corners, like sound, but light doesn't go around corners (main support of particle theory) Reflection like ball bouncing on a surface Refraction like a ball rolling down an incline plane from one surface to another (change in medium) Wave Theory (1690) Like water waves in body of water. Light travels in waves in all directions from source. Consider a stone dropped in water, stone makes initial waves, but waves persist not attributed directly to stone. Huygen s Principle each point on a wave front may be regarded as a new source of disturbance. 3

4 Light is train of waves with the wave front perpendicular to light ray paths. Energy is uniformly distributed Light ray is line of direction of waves from source. Could explain reflection and refraction but NOT rectilinear propagation. Interference and diffraction (discovered 1801&1816) supported wave but not particle theory Electromagnetic Theory Faraday principle of electric generation studied attraction and repulsion of electrically charged bodies Maxwell explored transmission of heat, light and electricity Developed equations which allowed him to predicted how all three propagated in free space at speed of it as EM disturbances Energy divided equally into electric and magnetic field 4

5 Quantum Theory Planck studied distribution of radiation from hot object Predicted continuous increase in intensity as frequency increased Experimentally, found intensity approached 0 at very high frequency Explanation increments of energy equal to frequency times a constant E=hf h= J s Light must be absorbed and radiated in indivisible packets (quanta) called photons Allows certain energy values, not others. Modern Theory Dual character of light Wave- Particle Duality Radiant energy is transported in photons that are guided along their path by a wave field E=hf=mc 2 c=fλ λ = h/mc = λ = h/mv 5

6 Straight Line Nature Light travels in a straight line It forms a beam Shadows are evidence that light travels in a straight line A beam of light consists of a very large number of individual waves traveling together in a straight line Lines representing direction of the light waves are called rays. Using ray diagrams in studying light is called ray optics. Speed of Light Before the 17 th century most people believed that light traveled instantaneously. Roemer (1695) astronomer Measured because of variations in time of eclipse of Io, moon on Jupiter in different locations of earth about sun 140,000 mi/s or 225,000 km/s 6

7 Michelson (1850) U of Chicago Physics professor Measured between Mt. Wilson & Mt. San Antonio, CA 22 mi/35.4 km Also measured in evacuated tube 1 mi long c=299, 796 km/s ±1 km/s Accepted now c= m/s ±1.2 m/s Transmission and Absorption Many materials transmit light Transparent objects can be seen clearly through glass, quartz, air Translucent transmit light but cannot be seen clearly frosted glass, lamp shades, light bulbs Opaque absorb or reflect all light Illumination by a Point Source Luminous something that emits light Natural Artificial Illuminated something that reflects light Luminous intensity- amount of light that a source gives out depends on the amount of energy being put into the source also depends on how efficiently the source converts energy input to light energy 7

8 Luminous Intensity Luminous Intensity, I, is measured by comparing it with the international unit, the candela, cd. The candela is one sixtieth of the luminous intensity of a square centimeter fused thoria (a powdery white oxide of thorium) maintained at 2046 K. Usually use incandescent lamp that was rated by comparison with the standard Incandescent 40 watts ~35 cd 100 watts ~130 cd Fluorescent 40 watts ~200 cd I depends on direction measured from. Average I in all directions is often given Luminous Flux Luminous flux (Φ = phi) rate of flow of visible radiation most radiation is not visible Luminous flux is that part of total energy radiation per unit of time from a luminous source that is capable of producing sensation of light (visible) Φ is measured in lumens, lm 8

9 Luminous Flux 1 lumen is luminous flux on a unit surface all points of which are at unit distance from a point source of 1 cd. 1m 2 1m The surface area of a sphere is 4πr 2. If the sphere has a radius of 1 m the total surface area is 4π (1m) 2 or 4π m 2. The rate light crosses the entire surface is 4π lumens. A 2 cd source would emit 8π lumens and so on. Φ= 4πI The energy flux in lumens is directly proportional to the intensity of the source. Illumination Illumination, E density of luminous flux on a surface E = illumination = Φ/A = lm/m 2 As radial distance increases, area illuminated increased by d 2. E = I/r 2 = lm/m 2 (inverse square law) 9

10 Color Newton made some of the first discoveries of the relationship between light and color at the age of 24. He observed an orderly arrangement of color passing through a prism. This arrangement of color is called a spectrum. Color Addition If the correct amounts of red light, green light, and blue light are projected onto a white screen, the screen will appear white. Red light, green light and blue light are the primary colors of light. Secondary colors are produced by combining two primary colors. Each primary color has a complementary secondary color made up of two remaining primary colors. 10

11 Color Subtraction A dye is a molecule that absorbs certain wave lengths of light and transmits or reflects others. Like a dye, a pigment is a colored material that absorbs certain colors and transmits others. The difference in a pigment particle is larger than a molecule and can be seen with a microscope. Ex. Titanium (IV) oxide (white), chromium (III) oxide (green) Pigment colors are completely different than light colors! Pigments and dyes absorb certain colors from white light. A pigment that absorbs only one primary color from white light is called a primary pigment. Yellow pigment absorbs blue light and reflects red and green light. Yellow, cyan and magenta are primary pigments. A pigment that absorbs primary colors and reflects one is a secondary pigment. Secondary pigments are red, blue and green. 11

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