ISNS 3371 - Phenomena of Nature The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye. The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum. ISNS 3371 - Phenomena of Nature Moonrise Earthrise On the moon, there is no atmosphere to scatter light and the sky is black Atmosphere on Mars too thin to scatter light effectively - sky is reddish from presence in the atmosphere of reddish dust from surface. On Venus, almost all blue light scattered away - atmosphere dimly lit and appears reddish orange. ISNS 3371 - Phenomena of Nature Color Additive primary colors - adding light of additive primary colors produces complementary colors - all colors produces white Red Green Blue Red + Green Yellow Red + Blue Magenta Green + Blue Cyan Red Magenta Yellow Blue Green Cyan White ISNS 3371 - Phenomena of Nature Color Subtractive primary colors - mixing pigments (that absorb light) of various subtractive primary colors produce complementary colors added all together, produce black. Yellow Magenta Yellow + Magenta Red Yellow + Cyan Green Magenta + Cyan Blue Cyan ISNS 3371 - Phenomena of Nature Only four colors are used to print color illustrations and photos: (a) magenta, (b) yellow, (c) cyan, (e) black. (d) is with all but black, (f) is with all. ISNS 3371 - Phenomena of Nature Four Ways in Which Light can Interact with Matter 1. emission – matter releases energy as light 2. absorption – matter takes energy from light 3. transmission – matter allows light to pass through it 4. reflection – matter repels light in another direction ISNS4371 3371Phenomena - Phenomena of Nature ISNS of Nature Properties of Light Law of Reflection - Angle of Incidence = Angle of reflection Law of Refraction - Light beam is bent towards the normal when passing into a medium of higher Index of Refraction. Light beam is bent away from the normal when passing into a medium of lower Index of Refraction. Index of Refraction - n Speed of light in vacuum Speed of light in a medium Inverse square law - Light intensity diminishes with square of distance from source. ISNS 3371 - Phenomena of Nature Mirror reflects light at angle equal to incoming angle - Most materials reflect light randomly scattering. Movie screen scatters light into array of beams that reaches every member of the audience ISNS 3371 - Phenomena of Nature Index of Refraction As light passes from one medium (e.g., air) to another (e.g., glass, water, plexiglass, etc…), the speed of light changes. This causes to light to be “bent” or refracted. The amount of refraction is dependent on the index of refraction of the material and the angle of incidence. ISNS 3371 - Phenomena of Nature Refraction Imagine that the axles of a car represent wave fronts. If the car crosses from a smooth to a rough surface at an angle, one tire of the axle will slow down first while the other continues at normal speed. With one tire traveling faster the other, the car will turn in the direction of the slow tire. This is how refraction works. ISNS 3371 - Phenomena of Nature The Prism A prism separates white light into its component colors - because index of refraction is dependent on wavelength. Blue light bends (is refracted) more than red light Color Violet Blue Index (glass) 1.532 1.528 Green Yellow Orange Red 1.519 1.517 1.514 Dispersion - “spreading” of a light beam dependent on: - the angle between the two surfaces - the direction of incidence on the first face - the index of refraction of the prism. 1.513 ISNS 3371 - Phenomena of Nature Rainbows Water is more dense than air - light is refracted as it enters and leaves a raindrop drop - red is bent less, blue more - the light is dispersed, just like a prism - some of the light will reflect off the back of the drop if the angle is larger than the critical angle for reflection (48° for water) - the light is then refracted again as it leaves the drop, the colors of white light have been dispersed. - blue light will leave the drop at an angle of 40° from the beam of sunlight - red light will leave the drop at an angle of 42° from the beam of sunlight This process generates the primary rainbow. Naturally the sun must be behind you and the rain in front of you. ISNS 3371 - Phenomena of Nature Rainbows However, you can not see the blue light and red light refracted from the same drop - you only see one color (indeed, only one tiny beam of light!) from each raindrop. So, many drops are involved in producing the rainbow. The raindrops lower down contribute blue and green light and the raindrops higher up contribute red and yellow light. ISNS 3371 - Phenomena of Nature Rainbows antisolar point - the point where the sun is directly opposite of us. - If we look away from the sun, the shadow of our head on the ground marks the antisolar point - If the sun is in the sky, the antisolar point is below the horizon. If the sun has set, the antisolar point is above the horizon. Tells us is the direction of the sun and where we can expect the rainbow to form. ISNS 3371 - Phenomena of Nature red light gets refracted back at 42° from antisolar line - blue light gets refracted back at about 40° from antisolar line. Every raindrop within 42° of the antisolar line is reflecting red light back to your eyes. Look at every raindrop within about 40° of the antisolar line - see a circle of raindrops centered on the line - horizon gets in the way of most of the rain, so we only get to see an arc or a bow - full rainbows can be seen from airplanes and mountain-tops. Rain is falling not in a flat sheet, but in varying distances from you - causes the "rainbow circle" to be formed at varying distances - raindrops that contribute to your rainbow all lie on a cone with its apex at your eye. Move to the left or the right - looking at new raindrops - a new rainbow. Two people see different rainbows each rainbow is your own. ISNS 3371 - Phenomena of Nature Rainbows In science speak - The rainbow is an optical phenomenon caused by the refraction and reflection of light by the locus of raindrops between 40.6 and 42° of the observer's antisolar line. ISNS 3371 - Phenomena of Nature Polarization Light emitted by the sun, a lamp in the classroom, a candle flame, etc… is unpolarized light - created by electric charges which vibrate in a variety of directions - an electromagnetic wave (transverse) which vibrates in a variety of directions. Helpful to picture unpolarized light as a wave which has an average of half its vibrations in a horizontal plane and half of its vibrations in a vertical plane. Polarized light waves - light waves in which the vibrations occur in a single plane. Polarization - Process of transforming unpolarized light into polarized light. Most common method of polarization uses a Polaroid filter - made of a special material capable of blocking one of the two planes of vibration of an electromagnetic wave. When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light. ISNS 3371 - Phenomena of Nature Two filters with polarization axes perpendicular to each other will completely block the light. Light is polarized upon passage through the first filter - say, only vertical vibrations were able to pass through. These vertical vibrations are then blocked by the second filter since if its polarization filter is aligned in a horizontal direction. Like picket-fence and standing wave on a rope - vibrates in a single plane. Spaces between the pickets of the fence allow vibrations parallel to the spacings to pass through while blocking vibrations perpendicular to the spacings. Orient two picket fences such that the pickets are both aligned vertically vertical vibrations will pass through both fences align pickets of second fence horizontally - the vertical vibrations which pass through the first fence will be blocked by the second fence. ISNS 3371 - Phenomena of Nature Polarization by Reflection Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces - extent dependent upon the angle at which the light approaches the surface and upon the surface material. Metallic surfaces reflect light with variety of vibrational directions - unpolarized. Nonmetallic surfaces (asphalt, snow, water, paint on a car) reflect light such that there is a large concentration of vibrations in a plane parallel to the reflecting surface. A person viewing objects by means of light reflected off of nonmetallic surfaces will often perceive a glare if the extent of polarization is large. Which pair of glasses is best suited for automobile drivers, fishermen, snow skiers? ISNS 3371 - Phenomena of Nature Adding a third filter with between two filters polarization axis at 45º to the other two will allow light though. How? Remember, unpolarized light vibrates in all different directions. So not just the light with horizontal vibrations passes through the first filter, but all light with a vibrational component in the horizontal direction - in other words, all but the light with vertical vibrations has some component in the horizontal direction that gets through. ISNS 3371 - Phenomena of Nature Before the middle filter, the light is horizontally polarized. The component of horizontally polarized light along 45º gets through the middle filter. The component of that light in the vertical direction then gets though the last filter. ISNS 3371 - Phenomena of Nature When light passes through a transparent material such as plastic, internal (and normally invisible) stresses in the material can rotate the angle of polarization - different colors will be rotated by different amounts. Place horizontal polarizer below the object, and a vertical one above it - no light will be transmitted unless there are stresses inside the object that rotate the light. Useful in the design of engineering structures - build a model out of plastic, and view it with crossed polarizers. Put a force on the model - regions of the model that are stressed the most will show up in color. This way you can determine which parts of the structure are most likely to break, and the design can be changed (if necessary) to relieve some of that stress. ISNS 3371 - Phenomena of Nature Lenses and Mirrors (and applications to astronomy) ISNS 3371 - Phenomena of Nature Geometry of a Concave Mirror Focus Principal axis Vertex Focal length Center of curvature - the center of the circle of which the mirror represents a small arc Principal axis - a radius drawn to the mirror surface from the center of curvature of the mirror - normal to mirror surface Focus - the point where light rays parallel to principal axis converge; the focus is always found on the inner part of the "circle" of which the mirror is a small arc; the focus of a mirror is one-half the radius Vertex - the point where the mirror crosses the principal axis Focal length - the distance from the focus to the vertex of the mirror ISNS 3371 - Phenomena of Nature Geometry of a Converging (Convex) Lens Focus Optical axis Focal length Optical axis - axis normal to both sides of lens - light is not refracted along the optical axis Focus - the point where light rays parallel to optical axis converge; the focus is always found on the opposite side of the lens from the object Focal length - the distance from the focus to the centerline of the lens ISNS 3371 - Phenomena of Nature Image Magnification Using a Simple Lens L2 D1 D2 L D _1 _1 = L2 D2 Focal Plane L1 ISNS 3371 - Phenomena of Nature The image formed by a single lens is inverted. ISNS 3371 - Phenomena of Nature The Eye The eye consists of pupil that allows light into the eye - it controls the amount of light allowed in through the lens - acts like a simple glass lens which focuses the light on the retina - which consists of light sensitive cells that send signals to the brain via the optic nerve. An eye with perfect vision has its focus on the retina when the muscles controlling the shape of the lens are completely relaxed - when viewing an object far away - essentially at infinity. ISNS 3371 - Phenomena of Nature When viewing an object not at infinity, the eye muscles contract and change the shape of the lens so that the focal plane is at the retina (in an eye with perfect vision). The image is inverted as with a single lens - the brain interprets the image and rights it.