# Ch23 Geometric Optics Reflection & Refraction of Light

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• Ch23 Geometric OpticsReflection & Refractionof Light

• ReflectionObserve the reflection of a laser beam due to the mirror.The Normal is an imaginary line drawn at a 90 angle with the surface of the mirror at the point where the beam strikes the mirror.The angle between the incident ray and the normal is known as the angle of incidence (i ).The angle between the reflected ray and the normal is known as the angle of reflection (r ).MirrorIncident RayReflected RayNormal

• Mirror TypesThere are three types of mirrors: plane mirror, concave mirror, and convex mirror.Each of these three mirror types form images with different properties.A plane mirror forms an image that is the same size as the object.A convex mirror form images that are smaller than the object.A concave mirror form real images that are inverted.What type of mirror is shown in the figure to the right?Convex Mirror?????Mirror Type?????ObjectReflected Image

• Diffuse Scattering.Imagine that several parallel laser beams are fired at a mirror and a ceiling tile.The reflected beams from the mirror surface would still be parallel, and i and r would be equal.However, the reflected beams from the tile surface would not be parallel.The reflected beams from the tile surface exhibit diffuse scattering.Mirror

• Formation of Images by Spherical MirrorsWe use ray diagrams to determine where an image will be. For mirrors, we use three key rays, all of which begin on the object:

A ray parallel to the axis will be reflected through the focal pointA ray through the focal point will be reflected parallel to the axisA ray perpendicular to the mirror will be reflected back on itself

• cfConcave: Object outside of CImage is:InvertedSmallerBetween C & FReal

• cfConcave: Object is between C & FImage is:InvertedLargerOutside of C Real

• cfConcave: Object is at CImage is:InvertedSame sizeAt CReal

• cfConcave: Object is at f

• cfConcave: Inside of fImage is:Up rightLargerBehind the mirrorvirtual

• cfConvex: Image is:Up rightSmallerBehind the mirrorvirtual

• 23.3 Formation of Images by Spherical MirrorsGeometrically, we can derive an equation that relates the object distance, image distance, and focal length of the mirror:(23-2)

• 23.3 Formation of Images by Spherical MirrorsWe can also find the magnification (ratio of image height to object height).(23-3)The negative sign indicates that the image is inverted.

• Simply put, Refraction means bends.When discussing light beams, light bends when it goes from one medium (glass, water, air, etc.) to another.If it goes from a more dense medium to a less dense medium, then light speeds up and bends away from the Normal.If it goes from a less dense medium to a more dense medium, then light slows down and bends towards the Normal.Consider the wheel and axel to the right.It rolled from pavement (faster) to gravel (slower)The left wheel slowed down causing the axel to turn towards the normal.Pavement GravelRefraction

• RefractionWhen light undergoes refraction while traveling from a less dense to a more dense medium, the light bends towards the normal.This is because light travels faster in a less dense medium than in a more dense medium.Is the fish safe from the laser if we point the laser at the fish?

Normal

• RefractionWhich of the three paths (A, B, or C) is the one the beam would actually travel?Remember, lights bends towards the normal when it goes from a less dense to a more dense medium because it slows down in the more dense medium.Which is the correct path?ABCAirGlassWater

• Snells LawSnells law allows us to determine the angle of refraction given the angle of incidence of a light ray on a boundary interface.

Based upon the picture to the right, which medium is more dense: 1 or 2? Why?Because medium 2 is more dense n2>n1.

n1 sin 1 = n2 sin 2n1n2

• Index of Refraction, nThe index of refraction is a ratio of the speed of light in a vacuum (c) to the speed of light in a medium (v) like glass, oil, water, quartz, diamond,.The top figure shows a laser aimed into a evacuated cylinder (nothing is inside it).The speed of light in this cylinder is c = 3 x 108 m/s.The bottom figure shows a laser aimed into a cylinder of water.The light will travel slower in this cylinder, and its speed (v) can be measured in a lab.The ratio of these speeds gives the index of refraction (n) of the material through which the light is shined (in this case water).

• Internal ReflectionSuppose we had a laser submerged in water.If it was pointing straight up, then the beam would pass through the boundary interface without reflecting or refracting.If we rotate the laser, then some of the beam will reflect and some will refract.This phenomena occurs at the critical angle.As the laser is rotated past the critical angle, all of the beam will be reflected and none will be refracted.This phenomena is known as total internal reflection.

• Lens VocabularyA Center of CurvatureB Focal PointC Focal LengthD Principal AxisAABBCCD

• ff An object is located in front of a diverging lensWhere is the image located?

• MagnificationMagnification is the enlargement or reduction of the size of the image of an object produced by a lens.If the object is beyond twice the focal length (2f), then the image will be smaller, real, and invertedIf the object is at 2f, the it will be the same size and real.Between f and 2f, the object will be larger and real.At f the image will focus at infinity (not be visible).In between f and the lens, the image will be much larger, virtual, up right.

• Lenses and Magnification Converging LensReal or virtual?

The focal length is positive for converging lenses

• Lenses and Magnification Diverging LensReal or virtual?

The focal length is negative for diverging lenses

• 23.3 Formation of Images by Spherical MirrorsWe can also find the magnification (ratio of image height to object height).(23-3)The negative sign indicates that the image is inverted.

• Light SpectrumLight is composed of massless particles known as photons.The spectrum of electromagnetic radiation spans several wavelengths (frequencies) of the photons.Of particular interest to us is the visible spectrum.To remember the visible spectrum, use the name ROY G BIV.