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Electromagnetic Waves ‘LIGHT’ Nature of Light Is light a wave or a particle? There is a fair amount of evidence to support both theories! Electromagnetic vs. Mechanical Electromagnetic waves : Transverse only Does not require a medium Light Mechanical waves: Transverse and longitudinal Requires a medium Sound, earthquake, water Electromagnetic (E-M) Waves Electromagnetic (E-M) waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3 x 108 m/s (a speed value commonly represented by the symbol c). In 1931, Michelson found c = 2.99774×108m/s The modern value is c = 2.997925×108m/s Since E-M waves are indeed transverse waves our wave equation holds true for all types of E-M waves v = fλ As with any wave, light waves transfer ENERGY ! All wave phenomena that we previously discussed hold true for E-M waves Reflection Interference Constructive Destructive Doppler Effect Diffraction As well as some new ones that will be discussed in this unit Refraction Polarization E-M Spectrum Electromagnetic waves exist with an enormous range of frequencies. This continuous range of frequencies is known as the electromagnetic spectrum. The entire range of the spectrum is often broken into specific regions. The subdividing of the entire spectrum into smaller spectra is done mostly on the basis of how each region of electromagnetic waves interacts with matter. Spectrum The diagram below depicts the electromagnetic spectrum and its various regions. The longer wavelength, lower frequency regions are located on the far left of the spectrum and the shorter wavelength, higher frequency regions are on the far right. Two very narrow regions within the spectrum are the visible light region and the X-ray region. Electromagnetic Spectrum Found on your Reference Table About the Spectrum AM/FM Waves: They are found when you listen to the car radio Microwaves: Yes they are used in Microwave Ovens Use a frequency of 2.45 GHz This frequency is readily absorbed by water molecules (it makes them resonate) Excites the water molecules within the material to create heat Infrared Waves: Associated with heat. Hot lamps for keeping food warm Living things give off IR radiation Night Goggles are used to detect this type of wave Visible Light: The only part of the spectrum our eyes are sensitive to frequencies are in the range of 1014 Hz Ultraviolet Waves: Visible Good: Ultraviolet Bad Responsible for sunburn, skin cancer “Black lights” are UV lights- certain materials give off visible light when exposed to UV light Ozone layer of atmosphere blocks UV light X-Rays: Called x-rays because no one knew what they were when first discovered Pass readily through skin but absorbed by bone Anyone see a use for this? Gamma Rays: Nearly unstoppable, pass right through concrete walls Visible Light Visible light region - the very narrow band of wavelengths located to the right of the infrared region and to the left of the ultraviolet region. Normally when we use the term "light," we are referring to a type of electromagnetic wave that stimulates the retina of our eyes. Visible Light Each individual wavelength within the spectrum of visible light wavelengths is representative of a particular color. That is, when light of that particular wavelength strikes the retina of our eye, we perceive that specific color sensation. Transparent vs. Opaque Transparent Heat Infrared waves Visible light Visible Light Ultraviolet waves Glass Allows light to pass through. It depends on the frequency of the incident electromagnetic radiation Materials such as water and glass are transparent to visible light Incident Light Heat Opaque Does not allow light to transmit through Most materials are opaque to visible light The energy is simply converted into internal energy Color? You perceive color…it is simply our minds “take” on certain frequency electromagnetic waves. So, why do objects appear to be of a certain color? Color by Reflection: Objects appear a certain color because they absorb all the other colors (the frequencies of those colors causes resonance or near resonance in the molecules of the material-the energy is absorbed as heat) but “reflect” (absorbed then reemitted) that particular color. White Objects: “Reflects” all colors, absorbs little Wearing white clothes in the summer Black Objects: Absorb all colors, reflect little Wearing dark colors in the winter Color by Transmission: The color of the transparent object depends on the color of the light it re-emits (transmits) Why is the sky blue? Sun Earth Not Drawn to Scale Why are Sunsets Red? The blue light gets scattered out- there is no more left Only red light is left Sun Earth Representing Light Waves in Diagrams Ray: A line drawn to represent the direction of energy flow of a wave (corresponds to the direction of wave propagation) Sun ray Wavefront: A surface passing through those points of a wave that have the same phase and amplitude Wavefront: lines represent crests Light Rays Law of Reflection: The angle of incidence is equal to the angle of reflection Incidence Reflection Law of Reflection Law of Reflection: The angle of incidence is equal to the angle of reflection. Incidence θi Normal θr Reflection θi = θr This holds true for every reflection Light Wave Phenomena: Reflection Regular Reflection Occurs off of smooth surfaces Often produces glare Diffuse Reflection Occurs off of irregular surfaces Images Forming Images: Light must reach your eye from the object to form an image of the object Light travels in straight lines Camera Obscura: Pin Hole Camera Image Characteristics Real: The image is formed by converging light rays The image can be projected on a screen Virtual: The image is formed by diverging light rays The image can not be projected on a screen Upright: The image has the same vertical orientation as the object Inverted: The image has the opposite vertical orientation as the object Reversed: The image is flipped from side to side Images formed by reflections Mirror Types: Plane Mirror: Concave Mirror (converging mirror): Convex Mirror: (diverging mirror): Ray Diagram: Plane Mirrors The image is always: Virtual, same size, upright, reversed And the object distance = image distance Ray Diagram: Concave/Converging Mirrors R, radius of curvature Principle Axis C, center of curvature f, focal length Ray Diagram: Concave/Converging Mirrors If the light ray passes parallel to the principle axis it reflects through the focal point If the light ray passes through the focal point it reflects parallel to the principle axis Convex/Diverging Mirrors If the light ray is directed parallel to the principle axis, the ray reflects as if it came from the virtual focal point f If the light ray is directed towards the virtual focal point, it is reflected parallel to the principle axis C Convex/Diverging Mirrors Images formed by diverging/convex mirrors are always virtual upright smaller Depending on where object is located image will appear differently Refraction The sudden change in direction of a light ray as the light ray passes from one medium to another. It is caused by a change in speed. Absolute Index of Refraction: The ratio of the speed of light in a vacuum to the speed of light in the substance Depends on the frequency of incident light “optical density” – how easy is it for light to travel through the material c n v *dimensionless quantity that is always equal to or greater than 1* *As n increases the speed of light decreases* Common indices of refraction for yellow light Substance Index of refraction Air 1.00 Diamond 2.42 Fused Quartz 1.46 Crown Glass 1.52 Flint Glass 1.66 Sodium Chloride 1.54 Water 1.33 Lucite 1.50 Corn Oil 1.47 Pyrex Glass 1.468 FOUND ON REFERENCE TABLE When light passes perpendicular to the surface, no refraction occurs. The light ray still changes speed, however there is no change in direction. n1 n2 n1 = n2 No Refraction Example: n1 = glass n2 = glass When light passes into a substance for which the speed of light is the same, light “passes” through unchanged, it is not refracted n2 > n1 Ray Bends TOWARD Normal Example: n1 = air n2 = water When a light ray passes into a substance in which the speed of light decreases, the refracted ray “bends” towards the normal n2 < n1 Ray Bends AWAY Normal Example: n1 = water n2 = air When a light ray passes into a substance in which the speed of light increases, the refracted ray “bends” away from the normal General Rule of Refraction, a.k.a. Snell’s Law It can be found experimentally that the ratio of the index of refraction for the substance the light is leaving to the index of refraction for the substance the light is entering is equal to the ratio of the sin of the angle of refraction to the angle of incidence n1 sin n2 sin or more commonly expressed n1sinθ1 = n2sinθ2 Snell’s Law What happens to the light wave when it enters a new medium? n1 n1= c and v1= f1λ1 v1 n2 n2 = c and v2 = f2λ2 v2 So, if v decreases, the wavelength decreases But we know that the frequency of a wave does not change when it changes mediums and the speed of light in a vacuum is constant so after some careful algebra you can get: n2 v1 1 n1 v2 2 Dispersion The process of separating light into its component colors due to the dependency of the index of refraction on wavelength/frequency. Dispersive Materials: Materials for which the index of refraction varies with wavelength (examples: water, glass, diamond) Non-Dispersive Materials: Materials for which the index of refraction does not vary with wavelength (examples: vacuum, air) For dispersive materials the index of refraction varies with wavelength, therefore the angle of refraction varies with wavelength. In general, as frequency increases, wavelength decreases, the index of refraction increases Prisms: Dispersion Red light has the longest wavelength, therefore the smallest index of refraction: it bends the least and it travels the fastest through the substance Violet light has the shortest wavelength, therefore the highest index of refraction: it bends the most and travels the slowest Dispersion in Motion Total Internal Reflection As light passes from a substance with a higher index of refraction to a substance with a lower index of refraction, the light ray bends away from the normal. Eventually, as the incident angle increases, the angle of refraction increases as well approaching 90o, after which none of the light escapes. This is called total internal reflection (TIR) Critical Angle (θc) The angle of incidence for which the angle of refraction is equal to 90o From the general rule for refraction: n1sinθc = n2sin90 n2 sin c n1 only works when n1 > n2 When the angle of incidence exceeds the critical angle, TIR occurs As the ratio between n2 and n1 increases, the critical angle decreases (its easier to “trap” light inside the substance) Forming Converging Lenses Forming Diverging Lenses Ray Diagram: Converging lenses A light ray directed parallel to the principle axis is refracted through the focal point R=2f f A light ray directed through the focal point is refracted parallel to the principle axis f A light ray directed through optical center passes straight through R=2f Ray Diagram: Diverging Lens A light ray directed parallel to the principle axis is refracted such that is appears to originate at the virtual focal point 2f f’ A light ray directed towards optical center, “passes” straight through f’ 2f Images formed by diverging lenses are always virtual, upright, and smaller Ray Diagram Practice Problems with spherical lenses and mirrors Spherical Aberration: The focal point of light rays far from the principle axis of a spherical lens/mirror are different from the focal points of the light rays close to the principle axis. The result is a blurry image. Corrective Procedure: Minimize the amount of lens/mirror being used around the principle axis ex) Adjustable aperture on a camera Chromatic Aberration A problem only with lenses Based on the dependency of the index of refraction on frequency The focal points of different colors of light are different White light Red light focal point Violet light focal point Interference of light waves: Constructive interference Results in bright light Destructive interference Results in dim light Conditions for sustained interference patterns produced by light waves 1) The light sources must be coherent 2) The waves must have identical wavelengths Two speakers driven by a single amplifier can produce a sustained interference pattern Two light sources will not However, never you worry. I can produce “two sources” from one by way of a screen with two small holes cut into it. Double Slit interference pattern obstruction Viewing surface Laser Pattern of light and dark spots How is that possible? Diffraction Diffraction: A general wave phenomena (happens with all waves) The bending of a wave around a barrier or obstruction Some useful information about diffraction as seen on the video disc Classic diagram Wave fronts The larger the wavelength the more diffraction the wave undergoes The smaller the “opening” the more diffraction the wave undergoes Conclusions From Young’s Double Slit Experiment y = mLλ d As L increases, y increases As d decreases, y increases As λ increases, y increases where y is the distance from the central maximum to the bright fringes Diffraction Gratings Wave Fronts Viewing Screen The more openings there are, the harder (less likely) that the conditions will be right between all of the “sources” to result in an interference pattern. The result, the pattern of light and dark bands “spreads” out. Polarization Light is said to be polarized if the variances in the electric field all occur in the same plane. The fact that light can be polarized provides evidence that, not only is light a wave, but it is a transverse wave Head on view of electric fields for unpolarized light Head on view of electric fields for polarized light E E (a) So, how do you go from (a) to (b)? (b) Polaroid Filter The most common method of polarization involves the use of a Polaroid filter. Polaroid filters are made of a special material that is capable of blocking one of the two planes of vibration of an electromagnetic wave. Way to Think A picket-fence analogy is often used to explain how this dual-filter demonstration works. Polarization by reflection When a beam of light is reflected off a surface, the light can be completely polarized, partially polarized or not polarized depending on the angle of incidence For incidence angles of 0o and 90o No polarization occurs For incident angles between 0o and 90o The light will be partially polarized horizontally Hence, hitherto, and therefore… polarized sunglasses are worth the extra cash! (This is paid for by our sponsors at RayBan®) Anatomy of the Eye The eye is essentially an opaque eyeball filled with a water-like fluid. In the front of the eyeball is a transparent opening known as the cornea. The cornea is a thin membrane that has an index of refraction of approximately 1.38. The cornea has the dual purpose of protecting the eye and refracting light as it enters the eye. After light passes through the cornea, a portion of it passes through an opening known as the pupil. Rather than being an actual part of the eye's anatomy, the pupil is merely an opening. The pupil is the black portion in the middle of the eyeball. Its black appearance is attributed to the fact that the light that the pupil allows to enter the eye is absorbed on the retina (and elsewhere) and does not exit the eye. Thus, as you sight at another person's pupil opening, no light is exiting their pupil and coming to your eye; subsequently, the pupil appears black. Anatomy of the Eye Like the aperture of a camera, the size of the pupil opening can be adjusted by the dilation of the iris. The iris is the colored part of the eye - being blue for some people and brown for others (and so forth); it is a diaphragm that is capable of stretching and reducing the size of the opening. In bright-light situations, the iris adjusts its size to reduce the pupil opening and limit the amount of light that enters the eye. And in dim-light situations, the iris adjusts so as to maximize the size of the pupil opening and increase the amount of light that enters the eye. Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process. The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens. By carefully adjusting the lenses shape, the ciliary muscles assist the eye in the critical task of producing an image on the back of the eyeball. Anatomy of the Eye The inner surface of the eye is known as the retina. The retina contains the rods and cones that serve the task of detecting the intensity and the frequency of the incoming light. An adult eye is typically equipped with up to 120 million rods that detect the intensity of light and about 6 million cones that detect the frequency of light. These rods and cones send nerve impulses to the brain. The nerve impulses travel through a network of nerve cells. There are as many as one million neural pathways from the rods and cones to the brain. This network of nerve cells is bundled together to form the optic nerve on the very back of the eyeball. Image Formation In order to facilitate the ability to see, each part must enable the eye to refract light so that is produces a focused image on the retina. It is a surprise to most people to find out that the lens of the eye is not where all the refraction of incoming light rays takes place. Most of the refraction occurs at the cornea. The cornea is the outer membrane of the eyeball that has an index of refraction of 1.38. The index of refraction of the cornea is significantly greater than the index of refraction of the surrounding air. This difference in optical density between the air the corneal material combined with the fact that the cornea has the shape of a converging lens is what explains the ability of the cornea to do most of the refracting of incoming light rays. Image Formation The use of the lens equation and magnification equation can provide an idea of the quantitative relationship between the object distance, image distance and focal length. Problem The varying distance between the observer and the object poses some potential problems for the human eye. Objects located varying distances from a lens system with a fixed focal length produce images that are varying distances from the lens. Yet, the eye must always produce an image on the retina - a location that is always the same distance away from the cornea. The eye cannot afford to allow changes in the image distance. Questions So how does an eye always focus images with the same dimage regardless of the fact that the dobject is different? How can an object 100 meters away be focused the same distance from the cornea-lens system as an object that is 1 meter away? Answer The cornea-lens system is able to change its focal length. The ciliary muscles of the eye serve to contract and relax, thus changing the shape of the lens. This serves to allow the eye to change its focal length and thus appropriately focus images of objects that are both close up and far away. Farsightedness Farsightedness or hyperopia is the inability of the eye to focus on nearby objects. The farsighted eye has no difficulty viewing distant objects. Nearsightedness Nearsightedness or myopia is the inability of the eye to focus on distant objects. The nearsighted eye has no difficulty viewing nearby objects. But the ability to view distant objects requires that the light be refracted less.