Light, Reflection, and Refraction Chapters 14 and 15

Light, Reflection, and Refraction

Chapters 14 and 15

Electromagnetic Waves• An electromagnetic wave is composed of a magnetic

field wave perpendicular to an electric field wave• All objects that are not at absolute zero emit EMWs. • The hotter the object the more waves they emit.• The electromagnetic spectrum is composed of a range

of wavelengths and frequencies that range from radio waves to gamma waves.

• Visible light is a very small portion of that entire spectrum.

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c

• The speed of an electromagnetic wave in a vacuum is 3.00 x 108m/s.

• It is equal to the product of the wavelength and the frequency

• c = ƒ

• Sample Problem 14A

Visible Light

• Visible Light is the part of the EMS that we can see

• Ranges from the color red with a wavelength of 700nm (x10-9m) to the color purple with a wavelength of 400nm.

Reflection

• Light waves usually travel in straight paths.

• When a light wave encounters a different substance it changes direction.

• When it encounters a substance that does not permit light to travel through it, opaque, some of the light will be reflected.

• Usually a portion of the light is absorbed.

Reflection (cont)

• The texture of the opaque object’s surface affects how it reflects light.

• A rough object reflects light in many different directions, diffuse reflection

• A smooth object reflects light in only one direction, specular reflection

• A surface is considered smooth if variations are smaller than the size of the wavelengths being reflected.

• It is difficult to make objects smooth enough to reflect X-rays and Gamma Rays.

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Mirrors

• Mirrors are smooth surfaces that reflect nearly all of the light they encounter.

• Light that strikes a mirror at an angle from the normal line reflects at the same angle away from the normal line

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Flat Mirrors

• Flat mirrors are the simplest form of mirror where the objects distance to the mirror, p, is equal to the distance from the mirror to the image, q.

• The image appears to be located behind the mirror and is considered to be a virtual image as the object would not appear on a screen.

Ray Diagrams

• Ray diagrams are used to predict the location of the image of an object.

• To make a ray diagram for a flat mirror choose a point on the object and draw a ray toward the mirror at a perpendicular angle. This ray would reflect back on itself.

• Then draw a ray at an angle toward the mirror and draw the reflection of that ray.

• Trace back both of the reflected rays through the mirror, where they intersect, place the image.

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Concave Spherical Mirrors

• Concave spherical mirrors are those who reflective surface is on the interior of a curved surface that has a radius R to the center of curvature C.

• The optical axis is any line that passes through C and is usually oriented with an object.

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Concave Spherical Mirror Rules

• A ray traveling through C will reflect back through C. (only if object is beyond C)

• A ray traveling through the focal point f, halfway between C and the surface of the mirror, will reflect parallel to the OA

• A ray traveling to the intersection of the OA and the mirror will reflect at the same angle below the OA.

• A ray traveling parallel to OA will reflect through the focal point

Ray Diagrams

• Using any of the two rules you must draw two rays, the object occurs at the point of intersection.

• We will draw several ray diagrams to determine the image produced by an object that is

– Beyond C

– Between C and f

– Between f and mirror

Convex Spherical Mirrors

• Convex spherical mirrors are those where the reflective surface is on the outside of the curve.

• The points f and C are located behind the mirror

• Convex spherical mirrors have rules as well.

Rules• A ray parallel to the OA will reflect directly away

from f.• A ray heading towards f will reflect parallel to the OA• A ray heading towards C will reflect directly away

from C.• A ray heading toward intersection of OA and mirror

will reflect at the same angle below the OA.• Trace the 3 diverging lines back through the mirror to

reveal the location of the image which is always virtual

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Equations

• While ray tracing gives us a good idea of the location of an object it is always best to verify with math.

• If p is the object’s distance and q is the image distance then…

• 1/p + 1/q = 1/f• The magnification of the object can been calculated

using the equation…• M = -(p/q)• Sample Problem 14C

Parabolic Mirrors

• Rays that hit spherical mirrors far away from the OA often reflect though other points causing fuzzy images, spherical aberration.

• Telescopes use parabolic mirrors as they ALWAYS focus the rays to a single point.

Refraction

• Substances that are transparent or translucent allow light to pass though them.

• When light passes from one transparent/translucent substance to another it changes direction.

• This change is due to the slight differences in speed that light travels in the new substance.

• This is called refraction.

Analogy

• A good analogy for refracting light is a lawnmower traveling from the sidewalk onto grass.

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Index of Refraction

• The ratio of the speed of light in a vacuum to the speed of light in a medium is that medium’s index of refraction. (n)

• The higher the index of refraction, the slower light travels through a medium.

• Refraction causes objects to appear at locations they are not at.

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Snell’s Law

• Snell’s Law relates the indices of refraction as well as the angle away from the normal line (angle of incidence) to determine the angle of refraction.

• n1(sini) = n2(sinr)

r = sin-1{(n1/ n2)(sini)}

• Sample Problem 15A

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Total Internal Reflection

• If the angle of incidence of a ray is very large(close to 90º) the ray will reflect rather than refract.

• This principal is responsible for the properties of fiber optic cables.

• Remember the lawn mower analogy…

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Thin Lenses

• Refraction is the property that allows us to manipulate an object’s image using a lens.

• We will be working with converging and diverging lenses.

• Just like with mirrors, we will need to follow rules to draw ray diagrams to predict the location of an image.

• Thin lenses also have focal points, these points are determined not only by the curve of the lens but the index of refraction of the lens as well.

• A lens has two focal points, one on either side.

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Converging Lens Diagram

• Draw one ray parallel to OA, refracts through focal point.

• Draw one ray through center of lens, continues straight through.

• Draw one ray through focal point, refracts through lens, travels parallel to OA.

• Image located at intersection of rays.• Treat lens as though it were a flat plane.

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Diverging Lens Diagram

• Because the rays that enter a diverging lens do not intersect a virtual image is formed by tracing back the refracted rays.

• Ray 1 - parallel to OA, refracts away from f, trace back to f.

• Ray 2 - ray toward f, refracts parallel to OA, trace back parallel to OA

• Ray 3 - ray through center, continues straight, trace back toward object

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Equations

• You can use the same equations for curved mirrors with lenses

• If p is the objects distance and q is the image distance then…

• 1/p + 1/q = 1/f• The magnification of the object can been

calculated using the equation…• M = -(p/q)• Sample problem 15B