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Physics 120 1

Geometric Optics (Ch 23)

Ray Model assume light travels in straight line uses rays to understand and predict reflection &

refraction

Physics 120 2

Reflection

Law of reflection the angle of incidence equals angle of reflection angles measured from normal

Physics 120 3

Reflection

Diffuse reflection Specular reflection

Physics 120 4

Ch 23: Question 1

What would be the appearance of the Moon ifit has (a) a rough surface? (b) a polished surface?

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Physics 120 5

Plane Mirrors

Physics 120 6

Spherical Mirrors

Physics 120 7

Focal Point & Focal Length

Parallel rays striking a concave mirror cometogether at focal point

f = r/2

Physics 120 8

Image Formation

Real Image

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Physics 120 9

Mirror Equation & Magnification

Physics 120 10

Ch 23: Problem 8

How far from a concave mirror (radius 23.0cm) must an object be placed if its image is tobe at infinity?

Physics 120 11

Convex Mirrors

Virtual Image can’t detect on paper or

screen Sign Conventions

object, image, or focal pointon reflecting side of mirrorhas a positive distance

Anything behind mirror hasnegative distance

image height is positive ifupright, negative if inverted(relative to object)

Physics 120 12

Refraction

index of refraction, n

nair = 1 nglass = 1.5

Snell’s Law

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Physics 120 13

Indices of Refraction

Physics 120 14

Total Internal Reflection Incident angle where refracted angle (θ2) is 90 is the

critical angle

at incident angles greater than critical angle, light istotally internally reflected

Physics 120 15

Ch 23: Problem 31

In searching the bottom of apool at night, a watchmanshines a narrow beam of lightfrom his flashlight, 1.3 mabove the water level, onto thesurface of the water at a point2.7m from the edge of thepool. Where does the spot oflight hit the bottom of the pool,measured from the wallbeneath his foot, if the pool is2.1m deep?

Physics 120 16

Ch 23: Problem 32 Light is incident on an equilateral glass prism at a 45

degrees to one face. Calculate the angle at which lightemerges from the opposite face. Assume n=1.58.

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Physics 120 17

Ch 23: Question 7

What is the focal length of a plane mirror?What is the magnification of a plane mirror?

Physics 120 18

Thin Lenses

Physics 120 19

Focal Length, Focal Plane and Power

f = focal length Power

inverse of focal length P = 1/f measured in diopter (D) 1 D = 1 m-1

Physics 120 20

Ray Tracing

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Physics 120 21

Thin Lens Equation

Thin Lens Equation Magnification

Physics 120 22

Sign Conventions focal length

positive for converging lenses negative for diverging lenses

object distance positive if the object is on the side of the lens from which the

light is coming (this is usually not the case) otherwise, it is negative.

image distance positive if the image is on the opposite side of lens from where

light is coming positive for real images, negative for virtual images

image height positive if image is upright, negative for inverted images h0 is always positive

Physics 120 23

Ch 23: Problem 43

A sharp image is located 78.0 mm behind a65.0-mm focal-length converging lens. Findthe object distance (a) using a ray diagram,(b) by calculation.

Physics 120 24

Ch 23: Problem 44

Sunlight is observed to focus at a point 18.5cm behind a lens. (a) What kind of lens is it?(b) What is its power in diopters?

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Physics 120 25

Ch 23: Problem 45

A certain lens focuses light from an object2.75 m away as an image 48.3 cm on theother side of the lens. What type of lens is itand what is its focal length? Is the image realor virtual?

Physics 120 26

Ch 23: Problem 53

(a) How far from a converging lens with a50.0-mm-focal-length lens must an object beplace if its image is to be magnified 2.00times and be real?

(b) What if the image is to be virtual andmagnified 2.00 times?