Longwood PHYS 103 - How do we see objects?

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How do we see objects?Most of the light that we see is reflected from surfaces and originates for some light sourceSun light, lamplight, light from a fire, glowing hot objectsObjects are visible because light that is reflected from the surfaces of the objects reaches our eyesWe can not see objects clearly in a dark roomLight RaysIn everyday situations, we model light with light raysA ray represents a very narrow beam of lightLight rays travel in straight linesMany light rays leave each point of an objectYou see the rays that enter your eyeYour brain extends the rays backward to "locate" the objectLaw of ReflectionThe angle of incidence is equal to the angleof reflectionThe angles are measured with respect to a line perpendicular to the surface (normal)at the point of contactHow are images formed?Infant number of light rays leave each pointof an objectRays meet the surface and reflect according to the law of reflectionEyes see the reflected raysEyes don't know that the rays were reflectedBrain interprets the light as coming from a location behind the "mirror"Types of ImagesA real image is formed when light rays passthrough and diverge from the image pointReal images can be displayed on screens(mirror)A virtual image is formed when light rays do not pass through the image point but only appear to diverge from that pointVirtual images cannot be displayed on screensPlain MirrorsImages are always formed behind the mirrorImages are "virtual" - light rays do not actually emanate from the imageImage is located the same distance behindthe mirror as object is in front of the mirrorImage is the same size as the objectReversals in a Flat MirrorA flat mirror produces an image that has anapparent left-right reversalFor example, if you raise your right hand the image you see raises its left handThe reversal is not actually a left-right reversal, but a front-back reversalProperties of the Image Formed by a Flat MirrorThe image is as far behind the mirror as theobject is in the frontThe image is unmagnified The image is virtualThe image is uprightIt has the same orientation as the objectThere is a front-back reversal in the imageMagnification: ratio of the image height to the object heightHow tall does a full length mirror need to be to be able to see your full bodyDoes it mater where you place the mirror?No, as you get further from the mirror, the image in the mirror gets further away. Might have to move it up or down.Spherical MirrorsA spherical mirror has the shape of a segment of a sphereThe mirror focuses incoming parallel rays toa pointA concave spherical mirror has the light reflected from the inner, or concave, side ofthe curveA convex spherical mirror has the light reflected from the outer, or convex, side of the curveConcave Mirror - Focal PointThe laser beams are traveling parallel to the principal axisThe mirror reflects all the beams to the focal point The focal point is where all the beams intersectImage formation depends on the position ofthe object relative to the center of curvature of the mirrorConvex MirrorsA convex mirror is sometimes called a diverging mirrorThe light reflects from the outer, convex sideThe rays from any point on the object diverge after reflection as though they werecoming from some point behind the mirrorThe image is virtual because the reflected rays only appear to originate at the image pointImage Formed by a Convex MirrorThe object is in front of a convex mirrorThe image is virtualThe image is uprightThe image is smaller than the object (reduced)Spherical MirrorsJurassic Park "objects in the mirror are closer than they appear"Mirrors in convenient storesDiffuse ReflectionLight rays are reflected in many different directionsOccurs because the surface is roughSpecular ReflectionLight rays are reflected in the same directionYou can only see the rays if your eyes are inthe right positionSpecular is Latin for "mirror"What constitutes a smooth surface?Smoothness of the surface depends on the wavelength of light being reflectedThe longer the wavelength of light, the smoother the surface appears for reflectionVisible light has a small wavelength, most surfaces do no appear "smooth", diffuse reflection is most commonIf the wavelength of light is large comparedto imperfections in the surface then the surface is considered to be smoothQuick QuestionA surface is considered smooth for visible lightWhat other types of electromagnetic waves would consider the surface smooth?Anything with a longer wavelength, ex.radio waves, microwaves, infraredWhat other types of electromagnetic waves would consider the surface rough?Anything with a shorter wavelength, ex. x-rays, gamma rays, ultraviolet raysSections 14.2-14.4RefractionLight slows down when it enters matterBecause of the change in light speed the light waves bendAnalogy of a cart moving from a sidewalk tograssSpeed = frequency x wavelengthIn medium, speed and wavelength decrease, frequency stays the sameRefractive IndexIndex of refraction, n, of a materialIndicates how much the speed of light differs from its speed in a vacuumIndicates the extent of bending of raysRatio of speed of light in a vacuum to thespeed in a materialEvery material has an index of refraction(index of refraction) = (speed of light in a vacuum) {300,000,000 m/s}(speed of light in the material)n = c/v or v = c/nn does not have a unit (speed/speed)Speed of Light & Index of RefractionIn a vacuum - c (300,000,000 m/s)n = 1In the atmosphere - slightly less than c (butrounded to c)n = 1In water - 0.75cn(water) = 1.33In glass - 0.67cn(glass) = 1.5 (about)In diamond - 0.41cn(diamond) = 2.44Lower index of refraction, slower speedThe Law of RefractionShine light on a water surface:Part of the incident ray is reflectedPart of the incident ray is transmitted.. but it's bent or refractedThis happens because the speed of light decreases when it enters a mediumThe Symmetry of RefractionThe Law of RefractionThe larger the difference between the indexof refraction in the regions, the more the light bendsIf light is incident from air into each of the following material at the same angle I, in which case will the light bend the most?In which case will the angle of refraction, R,be the smallest? (most bending)n(water)n(glass)n(diamond)Snell's Lawn(1) sin I = n(2) sin Rn(1) = index of refraction where beam of light originates (incident)n(2) = index of refraction where beam of enters and bends (refraction region)I = angle beam is incident


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