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What is light? For the purposes of this class, light will refer to visible light - light that can be seen by the human eye. Like all electromagnetic waves, light has a particlewave duality: it exhibits features of both a particle and a wave. Wave features Light has the same properties of any other wave: Absorption Reflection Interference Diffraction Refraction We know that light behaves as a transverse wave, moving at the speed of light in the vacuum of space, and slowing down once it encounters a medium. Particle features Light consists of little packets of energy, called quanta, or photons According to the particle view of light: the brightness of the light is the number of photons, the color of the light is the energy contained in each photon. Take the example of a beam of light striking a metal plate, which causes the metal atoms to release electrons. If light is a wave: the energy in the wave should depend only on its amplitude (the intensity of the light). Based on Max Planck's work, Einstein proposed: light delivers its energy as photons; each photon has an energy of Planck's constant (6.626 x 10-34 J·s) times its frequency. Higher-frequency photons have more energy and make the electrons come flying out faster: same light intensity but a higher frequency = electrons released have maximum KE. If the frequency is low enough, none of the photons will have enough energy to knock an electron out of an atom. Very low-frequency light = NO electrons, no matter how high the intensity. In 1913-1914, Millikan's experiments involving the photoelectric effect agreed exactly with Einstein's predictions about photons, not with the wave theory. Einstein actually won the Nobel Prize for his work on the photoelectric effect, (not for the theory of relativity). Ultimately, light is seen as both a particle and a wave, because it has features of both! Visible Light Light ranges from wavelengths of 400 nm (violet) to 700 nm (red). Violet light has the shortest wavelength and highest frequency of visible light; red has the longest wavelength and the lowest frequency. Humans see best in the yellow-green range of the visible spectrum; this is why many fire companies have switched to a yellow-green color for their vehicles in recent years. There are two basic types of light sources: Incandescent and Luminescent Incandescence involves the vibration of entire atoms. Incandescent light: • produced when atoms are heated and release some of their thermal vibration as electromagnetic radiation. • the most common type of light that you see everyday: Sunlight, light bulbs (not fluorescent ) and fires also known as "black body radiation." In luminescence, only the electrons vibrate. Luminescent light: • Occurs at lower temperatures • Creates light when electrons rise to a higher level, then fall back down, releasing energy • Requires electrical current or chemical reactions to boost the electrons to the higher energy level to keep the cycle going: Electrical current: fluorescent lights , neon light , mercury-vapor street lights, light emitting diodes, television screens and computer monitors ; Chemical reactions: light sticks and fire-flies The Inverse Square Law The total amount of light a source emits is called its luminosity. The intensity of light observed from a source of constant luminosity is reduced as the square of the distance from the object. This is known as the inverse square law for light intensity. The inverse-square law in action. A certain amount of light passes through the hole at a distance of 1 foot from the light-bulb. At distances of 2 feet, 3 feet, and 4 feet from the bulb, the same amount of light spreads out to cover 4, 9, and 16 times the hole's area, respectively. What is a laser? The acronym LASER stands for: Light Amplification by Stimulated Emission of Radiation Stimulated Emission An atom that is struck by a photon of light becomes "excited". When another photon strikes that atom, it releases a new photon that is completely identical to the incoming photon; same color, going in the same direction. The atom is stimulated to emit the new photon. Candelas and Intensity of Light The candle (candela) is the unit used to measure the intensity of light. The measurement refers to the intensity of the light emitted from a luminous source. Originally the candle was defined in terms of the intensity of a standard spermaceti candle. Spermaceti is a waxy substance produced in the head of a sperm whale. Illuminated Objects reflect light rays. Luminous Objects emit light. Luminous Flux, P The rate at which luminous energy is being emitted, transmitted, or received from a source Unit: lumen, lm Illuminance, E Measures the illumination of a surface. E = (Luminous flux, P) (area around source) may also be shown in terms E= P of power (4πd2) (Watts) d : distance from source Unit = Lumens/m2 = lux or lx Named for one of its discoverers (Willebrord Snel van Royen), Snell's law states that the ratio of the sines of the angles of incidence and refraction is equal to the ratio of velocities in the two media, or equivalently to the inverse ratio of the indices of refraction: Into a faster medium: the light bends away from normal Into a slower medium: light bends toward the normal Normal: imaginary line perpendicular to the surface of the medium (the boundary between mediums). index of refraction for the original (incident) medium incident angle index of refraction for the second medium angle of refraction Use ray diagrams to illustrate the path of light and to find the location and size of the image as it passes through convex and concave lenses. A converging lens (or convex lens) bends light so that the light rays come together to a point. The human eye has a single converging lens. A diverging lens (or concave lens) bends light so it spreads light apart instead of coming together. An object viewed through a diverging lens appears smaller than it would look without the lens. A mirror reflects light and allows you to see the image reflection. Flat mirrors show a true-size image. Consider a ray of light coming from a light bulb and striking a mirror. The incident ray is the light ray that strikes the mirror. The reflected ray is the light ray that bounces off the mirror Narcissus, by Michelangelo Caravaggio, ca. 1598 The law of reflection The law of reflection says the angle of incidence equals the angle of reflection. Light rays reflect from a mirror at the same angle at which they arrive. Angles are always measured relative to the normal line. Θi = Θr The normal line Between the incident and reflected rays, there is an imaginary line called the normal line which is perpendicular to the surface of the mirror. The angle between the incident ray and the normal line is called the angle of incidence. The angle of reflection is the angle between the normal line and the reflected ray. A prism is another optical device. It is made of a solid piece of glass with flat polished surfaces. May also be made of acrylic. Prisms can both refract and reflect light. A clear cloud-less daytime sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. Molecules of oxygen and nitrogen in the air scatter the light. In 1911, Einstein calculated the detailed formula for the scattering of light from molecules; this was found to be in agreement with experiments. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules. What causes rainbows? Rainbows: Refraction and Reflection Refraction: each time light crosses a boundary from one substance to another, the rays bend (change direction) depending on the wavelength (color) of light. In a rainbow white sunlight enters a raindrop and is broken into different colors heading in slightly different directions. The light is then reflected (and magnified) off the back of the raindrop and passes back into the air again, in the process being further refracted.