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“Light and Reflection”. Ch.13 “Light and Reflection”: Preview. Section 1 Characteristics of Light Section 2 Flat Mirrors Section 3 Curved Mirrors Section 4 Color and Polarization. Ch. 13 “Light and Reflection” Section 1 Characteristics of Light. What is Light?.
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Ch.13 “Light and Reflection”: Preview • Section 1 Characteristics of Light • Section 2 Flat Mirrors • Section 3 Curved Mirrors • Section 4Color and Polarization
Ch. 13 “Light and Reflection”Section 1Characteristics of Light
What is Light? • Light or visible light is electromagnetic radiation that can be perceived by the eye. • Not all light is visible to the human eye. Crepuscular rays of light streaming through a canopy of tree limbs.
What is the Electromagnetic Spectrum? • The electromagnetic spectrum consists of the continuum from gamma rays, X-rays, ultraviolet, visible, infra-red, microwaves, radio, and long waves,(in order of decreasing energy from top to bottom) . • There are NO sharp divisions from one kind of wave to the next.
Wave-Particle Duality of Light • Light can be described as a wave, oras a particle (called a photon), or even both as a combination (having properties of both waves and particles). The wave model is best suited for the introductory discussion in this presentation. Upper: wave nature of light. Lower: particle nature of light Illustration: www.artofinterpretation.com
Glowing Light of Heated Objects • The wave theory of light can’t explain why metals incandesce(glowing light emitted by hot objects) when heated strongly! • Ex. : A piece of iron appears dark gray at room temperature, glows red when heated more, and appears bluish-white in color at even higher temperatures. • The wave model of light can’t explain the emission of these different wavelengths of light at different temperatures.
Photoelectric Effect • The wave theory of light can’t explain the photoelectric effect! • When light of a certain frequency shines on a metal surface, photoelectrons are emitted by the metal in a process called the photoelectric effect. • A metal will not eject photoelectrons below a certain frequency of incident light. • The photoelectric effect is used to convert the energy of incident light into electrical energy.
Electromagnetic Wave Theory • Light is considered to be a transverse wave of perpendicular oscillating electric and magnetic fields, that propagate through space without a medium.
Wave Characteristics • When examining a wave, look for it’s: wavelength, frequency, and amplitude. • Notice: • the highest point of a wave – the crest. • the lowest point of a wave – the trough.
Frequency vs. Wavelength • Wavelength and frequency are inversely related; as one increases, the other decreases.
Speed of Light • All electromagnetic waves travel at the speed of 3.00 x 108 m/s in a vacuum (this is a rounded-off value!) • The speed of light(usually denoted c) is a fundamental physical constant, the speed at which light and all electromagnetic radiation travels in aperfect vacuum, which is 299 792 458 meters per second (about 186,000 miles per second). • It takes 8.3 minutes for light to travel from the Sun to the Earth!
Wave Speed Equation • The relationship between frequency, wavelength, and speed of light can be expressed as: c = where c = speed of light = wavelength (m) = ƒ =frequency (Hz) Your Physics text uses c = ƒ
Speed of Light • The speed of light (c) is the product of its wavelength and frequency. c = λ{pronounced : “see equals lambda nu”} • Because all electromagnetic waves travel at the same speed, the formula c = λ can be used to calculate the wavelength or frequency of any electromagnetic wave. • See Physics textbook, Pg. 449, # 1-6 “Practice A – Electromagnetic Waves”
Electromagnetic Waves Differ by Frequency and Wavelength • Sunlight, an example of white light, contains a continuous range of wavelengths and frequencies. • Sunlight passing through a prism is separated into a continuous spectrum of colors.
Electromagnetic Waves Differ by Frequency and Wavelength • As while light passes through the prism, the speed of each frequency is altered slightly, resulting in a separation into the familiar spectrum red, orange, yellow, green, blue, indigo, and violet (RoyGBiv) frequencies.
Rainbow • A rainbow forms when tiny drops of water in the air disperse the white light from the sun into a continuous spectrum that arches across the sky.
Huygens’ Principle • The Dutch physicist Christian Huygens stated that each point on a wave front acts as a source for new waves. Diffraction of a plane wave when the slit width equals the wavelength. Diffraction of a plane wave at a slit whose width is several times the wavelength. Wave refraction in the manner of Huygens.
Light Rays • A light ray is an idealized narrow beam of light. • Light rays emanate in all directions from their source, traveling in straight lines.
Illumination and Light Intensity • Light intensity (called illumination) depends on the brightness of the source andthe distance from the source. • The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm). • The luminous flux divided by the surface area (called illuminance– measured in lumen/m2 [lux] decreases as the radius is squared moving away from the source. A lux meter for measuring illuminances in work places.
Illumination Decreases with Distance • The illuminance obeys the inverse square law: if you double the distance from the source to the surface illuminated, the illuminance is one-fourth. Triple the distance and the illuminance is one-ninth.
Mirrors • A mirror is an object that reflects light in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection. • The most familiar type of mirror is the plane mirror, which has a flat surface. • Curved mirrors are also used, to produce magnified or diminished images or focus light or simply distort the reflected image.
What is Reflection? • When light strikes an object, some light is absorbed, and the rest is deflected at the surface. This change of direction is called reflection. • A good mirror can reflect about 90% of the incident light, but no surface is a perfect reflector.
Diffuse Reflection • The manner in which light is reflected from a surface depends on the surface’s smoothness. • Light reflected from a rough, textured surface (such as paper, cloth, or unpolished wood) is reflected in many different directions – called diffuse reflection.
Specular Reflection • Light reflected from smooth, shiny surfaces (such as a mirror or water) is reflected in one direction only, and is called specular reflection.
Incident vs. Reflected Rays • An incident ray is a ray of light that strikes a surface. The angle between this ray and the perpendicular or normal to the surface is the angle of incidence. • The reflected ray corresponding to a given incident ray, is the ray that represents the light reflected by the surface. The angle between the surface normal and the reflected ray is known as the angle of reflection. Diagram of rays at a surface, where is the angle of incidence, is the angle of reflection, and is the angle of refraction.
How were Ancient Mirrors Made? • In classical antiquity, mirrors were made of solid metal (bronze, later silver) and were too expensive for widespread use by common people; they were also prone to corrosion. Due to the low reflectivity of polished metal, these mirrors also gave a darker image than modern ones, making them unsuitable for indoor use with the artificial lighting of the time (candles or lanterns). • The method of making mirrors out of plate glass was invented by 16th-century Venetian glassmakers on the island of Murano, who covered the back of the glass with mercury, obtaining near-perfect and undistorted reflection. For over one hundred years, Venetian mirrors installed in richly decorated frames served as luxury decorations for palaces throughout Europe, but the secret of the mercury process eventually arrived in London and Paris during the 17th century, due to industrial espionage. French workshops succeeded in large scale industrialization of the process, eventually making mirrors affordable to the masses, although mercury's toxicity remained a problem.
How are Modern Mirrors Made? • Mirrors are manufactured by applying a reflective coating to a suitable substrate. The most common substrate is glass, due to its transparency, ease of fabrication, rigidity, hardness, and ability to take a smooth finish. The reflective coating is typically applied to the back surface of the glass, so that the reflecting side of the coating is protected from corrosion and accidental damage by the glass on one side and the coating itself and optional paint for further protection on the other. • In modern times, the mirror substrate is shaped, polished and cleaned, and is then coated. Glass mirrors are most often coated with non-toxic silveror aluminum, implemented by a series of coatings:Tin(II) chloride, Silver, chemical activator, Copper, and paint. The tin(II) chloride is applied because silver will not bond with the glass. The activator causes the tin/silver to harden. Copper is added for long-term durability. The paint protects the coating on the back of the mirror from scratches and other accidental damage.
Neon Signs (Luminous Tube Lighting) • The light of a neon sign is produced by passing high voltage electricity through a tube filled with neon gas at a reduced pressure. • Neon atoms in the tube absorb energy and become excited and unstable. • The unstable excited neon atoms then lose energy (become more stable) by emitting visible light.
Atomic Emission Spectrum • The atomic emission spectrum of an element is the set of frequencies of electromagnetic waves emitted by the atoms of the element. • Neon’s atomic emission spectrum consists of several individual lines of color, not a continuous range of colors.
Atomic Emission Spectra of Helium • Each element’s atomic emission spectrum is unique, and can be used to recognize if that element is present. • The science that studies substances that are exposed to some sort of continuous exciting energy is called spectroscopy. • Helium was discovered in the light from the Sun before it was ever discovered on the Earth! (1868) (Helium’s bright line spectrum is below)
Atomic Emission Spectra Atomic emission spectrum … H Atomic emission spectrum … Fe Dark line absorption spectrum … Sun
Atomic Emission Spectrum • An atomic emission spectrum is characteristic of the element being examined and can be used to identify that element. • The fact that only certain colors appear in an element’s atomic emission spectrum means that only certain specific frequencies of light are emitted. Therefore, only photons having certain specific energies are emitted. • Scientists found atomic emission spectra puzzling because they expected to observe continuous series of colors as excited electrons lost energy. In the next section, Sec. 5.2, you will learn about the continuing development of atomic models, and how one of these models was able to account for the frequencies of light emitted by excited atoms.
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