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 source Sun light lamplight light from a fire glowing hot objects Objects are visible because light that is reflected from the surfaces of the objects reaches our eyes We can not see objects clearly in a dark room Light Rays In everyday situations we model light with light rays A ray represents a very narrow beam of light Light rays travel in straight lines Many light rays leave each point of an object You see the rays that enter your eye Your brain extends the rays backward to locate the object Law of Reflection The angle of incidence is equal to the angle of reflection The angles are measured with respect to a line perpendicular to the surface normal at the point of contact How are images formed Types of Images Infant number of light rays leave each point of an object Rays meet the surface and reflect according to the law of reflection Eyes see the reflected rays Eyes don t know that the rays were reflected Brain interprets the light as coming from a location behind the mirror A real image is formed when light rays pass through and diverge from the image point Real 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 point Virtual images cannot be displayed on screens Plain Mirrors Images are always formed behind the mirror Images are virtual light rays do not actually emanate from the image Image is located the same distance behind the mirror as object is in front of the mirror Image is the same size as the object Reversals in a Flat Mirror A flat mirror produces an image that has an apparent left right reversal For example if you raise your right hand the image you see raises its left hand The reversal is not actually a left right reversal but a front back reversal Properties of the Image Formed by a Flat Mirror The image is as far behind the mirror as the object is in the front The image is unmagnified The image is virtual The image is upright It has the same orientation as the object There is a front back reversal in the image Magnification ratio of the image height to the object height How tall does a full length mirror need to be to be able to see your full body Does 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 Mirrors A spherical mirror has the shape of a segment of a sphere The mirror focuses incoming parallel rays to a point A concave spherical mirror has the light reflected from the inner or concave side of the curve A convex spherical mirror has the light reflected from the outer or convex side of the curve Concave Mirror Focal Point The laser beams are traveling parallel to the principal axis The mirror reflects all the beams to the focal point The focal point is where all the beams intersect Image formation depends on the position of the object relative to the center of curvature of the mirror Convex Mirrors A convex mirror is sometimes called a diverging mirror The light reflects from the outer convex side The rays from any point on the object diverge after reflection as though they were coming from some point behind the mirror The image is virtual because the reflected rays only appear to originate at the image point Image Formed by a Convex Mirror The object is in front of a convex mirror The image is virtual The image is upright The image is smaller than the object reduced Spherical Mirrors Jurassic Park objects in the mirror are closer than they appear Mirrors in convenient stores Diffuse Reflection Light rays are reflected in many different directions Occurs because the surface is rough Specular Reflection Light rays are reflected in the same direction You can only see the rays if your eyes are in the right position Specular is Latin for mirror What constitutes a smooth surface Smoothness of the surface depends on the wavelength of light being reflected The longer the wavelength of light the smoother the surface appears for reflection Visible light has a small wavelength most surfaces do no appear smooth diffuse reflection is most common If the wavelength of light is large compared to imperfections in the surface then the surface is considered to be smooth Quick Question A surface is considered smooth for visible light What other types of electromagnetic waves would consider the surface smooth Anything with a longer wavelength ex radio waves microwaves infrared What other types of electromagnetic waves would consider the surface rough Anything with a shorter wavelength ex x rays gamma rays ultraviolet rays Sections 14 2 14 4 Refraction Light slows down when it enters matter Because of the change in light speed the light waves bend Analogy of a cart moving from a sidewalk to grass Speed frequency x wavelength In medium speed and wavelength decrease frequency stays the same Refractive Index Index of refraction n of a material Indicates how much the speed of light differs from its speed in a vacuum Indicates the extent of bending of rays Ratio of speed of light in a vacuum to the speed in a material Every 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 n n does not have a unit speed speed Speed of Light Index of Refraction In a vacuum c 300 000 000 m s In the atmosphere slightly less than c but rounded to c n 1 n 1 In water 0 75c n water 1 33 In glass 0 67c n glass 1 5 about In diamond 0 41c n diamond 2 44 Lower index of refraction slower speed The Law of Refraction Shine light on a water surface Part of the incident ray is reflected Part of the incident ray is transmitted but it s bent or refracted This happens because the speed of light decreases when it enters a medium The Symmetry of Refraction The Law of Refraction The larger the difference between the index of refraction in the regions the more the light bends If 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 Law n 1 sin I n 2 sin R n 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 on the surface R

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# Longwood PHYS 103 - How do we see objects?

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