# Reflection and Refraction

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Reflection and Refraction
Chapter 28 Chapter 28 Reflection and Refraction

1. REFLECTION Most objects we see reflect light.

2. PRINCIPLE OF LEAST TIME
Fermat's principle: Light travels in straight lines and will take the path of least time.

Fermat says take this path.

3. LAW OF REFLECTION Using Fermat's principle one can show the law of reflection. A B 1 5 2 4 3 Mirror

The law of reflection states that
the angle of incidence (qi) equals the angle of reflection (qr). This is true for specular reflection. (Specular reflection is “mirror-like” reflection.)

Normal Incident Ray Reflected Ray qi qr Specular Reflecting Surface Included in the law of reflection is the fact that the incident ray, the normal, and the reflected ray all lie in the same plane.

Demo - Candle Burning Under Water
Video - Candle Burning Under Water Video - Water Wave Reflections Demo - Mirror Track Tracing – Take home exercise for PQ Points

Plane Mirror Ray Diagramming Plane Mirror

Plane Mirror Ray Diagramming Plane Mirror

Plane Mirrors Using ray diagramming one finds that the image is
1. Upright 2. Same size as the object 3. Virtual

Diffuse Reflection Video - Diffuse Reflection
If successive elevations of a surface are no more than l/8 apart, then the surface is said to be polished at that wavelength. Most objects are seen by diffuse reflection.

Diffuse Reflection Magnification of a surface
Colored lines are for the purpose of distinguishing reflected rays from incident rays.

Something that is barely polished for red light would not be polished for blue light.
Consider microwave satellite dishes. They are polished for microwaves but not for visible. The metal screen in a microwave oven serves what purpose?

Reflection from Curved Surfaces (Concave shown here)
Ray Diagramming f Principal axis Reflecting Surface The law of specular reflection is still obeyed.

Reflection from Curved Surfaces (Concave shown here)
Ray Diagramming f Principal axis Reflecting Surface The law of specular reflection is still obeyed.

Demo - Coin Mirage Demo – Deep Concave Mirror Demo - Make-up Mirror Images Concave mirrors can produce real inverted images that are magnified, diminished, or the same size as the object. Concave mirrors can produce virtual upright images that are magnified.

More on Mirrors Convex mirrors always produce a diminished, upright, virtual image. Demo - Convex Mirror Demo - Two Perpendicular Mirrors Demo - Three Perpendicular Mirrors Demo – Road Reflectors Question - Why is it hard to see at night after or during a rain?

Chapter 28 Review Questions

What type of mirror would you use to produce a magnified image of your face?
(a) flat (b) concave (c) convex (d) you could use a concave or a convex mirror

What is (are) the purpose(s) of the wire screen in the door of a microwave oven?
(a) to absorb microwaves (b) to allow you to see what's cooking (c) to reflect microwaves (d) all of the above (e) only (b) and (c)

4. REFRACTION Fermat's principle can also explain refraction (remember the beach). Refraction of light is the bending of light as it passes obliquely from one medium to another. It is due to the different speeds of light in the two different media.

Index of Refraction Index of refraction of a material equals
the speed of light in a vacuum divided by the speed of light in the material.

Slide - Twinkling Cartoon
Demo – Green Laser Refraction in an Aquarium Demo - Twinkling with Laser Slide - Twinkling Cartoon

Video - Water Wave Refractions
Video - "Broken" Pencil Refraction

Because of atmospheric refraction,
we have lingering, elliptical sunsets. Sun Earth Sun

Mirage Cool air Warm air Surface of water? Highway Mirage

Looming Warm air Cool air

Chapter 28 Review Question

An oar partially immersed in water appears "broken" because of
(a) refraction (b) diffraction (c) polarization (d) interference (e) absorption

5. CAUSE OF REFRACTION When light passes from one medium to another, its speed changes which in turn causes a bending of the light. Examples: Car running onto shoulder of road Light passing from air into water

Wave fronts of light qi Air Water qR

This bending produces illusions.
Example: Objects in water appear closer and nearer to the surface. Eye Air Water

Demo – Bow Fishing

Fish can see everything above the surface of water within a 960 cone.

960

Demo - Disappearing Pyrex

Dispersion Different frequencies are bent different amounts which causes a separation of white light into its constituent colors. This is the basic principle behind the operation of a prism. We say that a prism disperses the light. The higher frequencies interact most (slow down the most) and thus are bent the most. Demo - Aquarium Prism

Dispersion in a Prism Slit White Light Source Prism

Rainbow

Picture - Rainbow Individual drops act as dispersers. The 42o cone Demo – Rainbow Model A single eye can only see a small range of colors from a single raindrop.

Secondary Rainbow Double rainbows are dimmer, higher, and have colors reversed. Link to picture.

Green Flash Sun Earth Dispersion occurs causing
multiple images of the sun. The last to set would be blue, but most of the blue has been scattered which leaves green.

6. TOTAL INTERNAL REFLECTION
Video - Laser Under Water Critical angle is the angle where total internal reflection (TIR) begins. TIR is possible only when light is entering a medium of lesser index of refraction. Binoculars make use of TIR.

Two Prisms in a Monocular
Flex Cam – Monocular

Fiber optic devices make use of TIR.
Demo - Laser and Light Pipe Video - Woman at Edge of Pool Flex Cam – Aquarium and Finger Dip in Water

Chapter 28 Review Questions

A lingering sunset is (a) a looming effect
(b) caused by an elliptical (oval) sun (c) due to atmospheric refraction (d) caused by reflections from the upper atmosphere

When a beam of light emerges at a nonzero angle from water to air, the beam
(a) bends away from the normal (b) continues in the same direction (c) bends toward the normal (d) changes frequency (e) slows down

7. LENSES Lenses use refraction to form images. Demo - Fresnel Lens

A Convex Lens Converges Light Rays
f

Imaging with a Convex Lens
and passes through a point called the focal point. is bent upon entering the lens. Arrow as Object Upon exiting the lens it is bent again A ray parallel to the principal axis sees an image here. f Principal Axis An eye placed here Convex Lens A ray passing through the center of the lens is basically undeflected. This arrangement produces an inverted, real, diminished image.

More Imaging With a Convex Lens
is bent upon entering the lens. Upon exiting the lens it is bent again Arrow as Object and passes through a point called the focal point. A ray parallel to the principal axis f Principal Axis sees an image here. Convex Lens An eye placed here A ray passing through the center of the lens is basically undeflected. This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass. Farsighted people use lenses similar to these.

A Concave Lens Diverges Light Rays
f

Imaging with a Concave Lens
is bent upon entering the lens. Arrow as Object Upon exiting the lens it is bent again A ray parallel to the principal axis such that is appears to have come from a point called the focal point. f An eye placed here Principal Axis sees an image here. Concave Lens A ray passing through the center of the lens is basically undeflected. This arrangement produces an upright, virtual, diminished image. Nearsighted people use lenses similar to these.

Types of Lenses Convex lenses are positive converging lenses.
Plano Convex Plano Concave Double Convex Double Concave Convex Meniscus Concave Meniscus Convex lenses are positive converging lenses. Concave lenses are negative diverging lenses. Nearsighted people use lenses similar to these. Farsighted people use lenses similar to these.

8. LENS DEFECTS Spherical Aberration Cure – Diaphragm
Convex Lens Cure – Diaphragm or lens combination

Cure – Diaphragm or lens combinations (achromatic lens)
Chromatic Aberration Convex Lens Cure – Diaphragm or lens combinations (achromatic lens) Demo – Overhead Chromatic Aberration Astigmatism – due to barrel-shaped cornea

Eyeglasses Picture - Farsightedness Picture - Nearsightedness

Pinhole

Chapter 28 Review Questions

One way to reduce the problem of spherical aberration in a positive lens is to join a negative lens to it to form a compound lens. (a) True (b) False

If you wish to take a picture of your image while standing 5 ft in front of a plane mirror, for what distance should you set your camera to provide the sharpest focus? (a) 10 ft (b) 5 ft (c) 2.5 ft (d) it can't be done

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