Introduction to Light and Color

1 3 Optics: An Educator s Guide With Activities in Science and Mathematics EG-2000-10-64-MSFCI ntroduction to Light and ColorIntroduction to LightLight is a form of radiant energyor energy that travels in waves. SinceGreek times, scientists have debatedthe nature of Light . Physicists nowrecognize that Light sometimesbehaves like waves and, at other times,like particles. When moving from placeto place, Light acts like a system ofwaves. In empty space, Light has afixed speed and the wavelength can bemeasured. In the past 300 years,scientists have improved the way theymeasure the speed of Light , and theyhave determined that it travels atnearly 299,792 kilometers, or 186,281miles, per we talk about Light , we usuallymean any radiation that we can wavelengths range from about16/1,000,000 of an inchto 32/1,000,000 of an inch.

2 There areother kinds of radiation such asultraviolet Light and infrared Light , buttheir wavelengths are shorteror longer than the visible Light hits some form ofmatter, it behaves in different it strikes an opaque object, itmakes a shadow, but Light does bendaround obstacles. The bending of lightaround edges or around small slits iscalled diffraction and makes patternsof bands or Light can be traced to certainenergy sources, like the Sun, anelectric bulb, or a match, but mostof what hits the eye is reflected Light strikes some materials,it is bounced off or reflected.

3 If thematerial is not opaque, the Light goesthrough it at a slower speed, and itis bent or refracted. Some Light isabsorbed into the material andchanged into other forms of energy,usually heat energy. The Light wavesmake the electrons in the materialsvibrate and this kinetic energy ormovement energy makes heat. Frictionof the moving electrons makes , Color , and Their Uses4 Light , Color , and Their UsesOptics: An Educator s Guide With Activities in Science and Mathematics EG-2000-10-64-MSFCI ntroduction to ColorColor is a part of the electro-magnetic spectrum and has alwaysexisted, but the first explanation ofcolor was provided by Sir IsaacNewton in passed a narrow beam ofsunlight through a prism located ina dark room.

4 Of course all the visiblespectrum (red, orange, yellow, green,blue, indigo, and violet) was displayedon the white screen. People alreadyknew that Light passed through aprism would show a rainbow or visiblespectrum, but Newton s experimentsshowed that different colors are bentthrough different angles. Newton alsothought all colors can be found inwhite Light , so he passed the lightthrough a second prism. All the visiblecolors changed back to white is the only source of Color of an object is seen becausethe object merely reflects, absorbs, andtransmits one or more colors thatmake up Light .

5 The endless variety ofcolor is caused by the interrelationshipof three elements: Light , the source ofcolor; the material and its response tocolor; and the eye, the perceiver of made by combining blue,yellow, and red Light are calledadditive; and they are formed byadding varying degrees of intensityand amounts of these three primary colors of Light arecalled cyan (blue-green), yellow, andmagenta (blue-red).Pigment Color found in paint, dyes,or ink is formed by pigment moleculespresent in flowers, trees, and Color is made by absorbing, orsubtracting, certain parts of thespectrum and reflecting or transmittingthe parts that remain.

6 Each pigmentmolecule seems to have its owndistinct characteristic way of reflecting,absorbing, or transmitting certainwavelengths. Natural and manmadecolors all follow the same natural : An Educator s Guide With Activities in Science and Mathematics EG-2000-10-64-MSFCI ntroduction to MirrorsAs we look around the room, wesee most objects by the Light that isdiffusely reflected from reflection of Light takesplace when the surface of the objectis not smooth. The reflected rays froma diffusely reflecting surface leave thesurface in many different to Mirrors and LensesWhen the surface is smooth, suchas the surface of glass or a mirror,then it can be easily demonstratedhow reflected rays always obey thelaw of reflection as illustrated , Color , and Their Uses6 Light , Color , and Their UsesOptics.

7 An Educator s Guide With Activities in Science and Mathematics EG-2000-10-64-MSFCM irrorImage of Object(Virtual Image)ObjectLaw of ReflectionThe angle of incidence is equal tothe angle of Image Formed byReflection in a Flat MirrorEvery object we see has many raysof Light coming from it either byreflection or because it is a lightsource such as a Light bulb, the Sun,a star, etc. Each point on that objectis a source of Light rays. In theillustration below, the tip of the arrowis used as an example of a point on theobject from which rays of Light wouldbe coming.

8 As the rays from the objectSmoothReflectingSurfacei = Angle of Incidencer = Angle of Reflectionr = iir(See Glossary,page 63.)are reflected by the mirror, thereflected rays appear to come from theimage located behind the mirror at adistance equal to the object's distancefrom the mirror. The image is called avirtual image since the rays do notactually pass through or come fromthe image; they just appear to comefrom the image as illustrated : An Educator s Guide With Activities in Science and Mathematics EG-2000-10-64-MSFCThe Image Formed by aConcave MirrorA concave mirror that is part of aball or hollow sphere (that is, it hasa circular cross section) is a sphericalmirror.

9 The focal length is approximatelyone-half the radius of curvature. A raythat is both parallel and very close tothe optical axis will be reflected by themirror so that it will cross the opticalaxis at the paraxial focal point. Theparaxial focal point is located adistance of one-half the radius ofcurvature from the point on the mirrorwhere the optical axis intersects themirror. The word paraxial comes fromthe Greek para or par meaning atthe side of, or beside, and axial. Thusparaxial means beside the ray that is parallel to theoptical axis, but not close to the axis,will be reflected by the mirror so thatit crosses the optical axis, not at theparaxial focus, but a small distanceConcaveMirrorRealImageObject(2)( 1)cRadius ofCircle(3)fOpticalAxiscloser to the mirror.

10 This difference inthe axis cross-over points is calledspherical the mirror has a cross sectionthat is a parabola instead of a circle,all of the rays that are parallel to theoptical axis will cross at the samepoint. Thus, a paraboloidal mirror doesnot produce spherical aberration. Thisis why the astronomical telescopeknown as the Newtonian (invented byIsaac Newton) uses a paraboloidalprimary demonstration purposes inthe classroom, it works out that we canmake the approximation that sphericalmirrors behave almost like paraboloidalmirrors and determine that the focallength of a spherical mirror is aboutone-half the radius of curvature ofthe , Color , and Their Uses8 Light , Color , and Their UsesOptics.