Chapter 29 – Reflection & Chapter 29 – Reflection & RefractionRefraction

Chapter previewChapter previewSectionsSections

1. Reflection

2. The Law of Reflection

3. Mirrors

4. Diffuse Reflection

5. Reflection of Sound

6. Refraction

7. Refraction of Sound

8. Refraction of Light

9. Atmospheric Refraction

10.Dispersion in a Prism

11.The Rainbow

12.Total Internal Reflection

Reflection & RefractionReflection & Refraction

Section 29.1—Reflection

Reflection

Reflection – some or all of a wave bounces back into the first medium when hitting a boundary of a second medium

When all the wave energy is reflected back instead of being transmitted, it is total reflection

If some energy is transmitted and some is reflected, the wave is partially reflected

Reflection

Reflection & RefractionReflection & Refraction

Section 29.2—The Law of Reflection

The Law of Reflection The direction of incidence and reflection is best

described by straight-line rays Incident rays and reflected rays make equal angles with

a line perpendicular to the surface, called the normal Angle of Incidence – angle made by the incident ray

and the normal Angle of Reflection – angle made by the reflected ray

and the normal Law of Reflection – the angle of incidence and the

angle of reflection are equal

The Law of Reflection

Reflection & RefractionReflection & Refraction

Section 29.3—Mirrors

Mirrors

Virtual Image – the point located behind a mirror where an object appears to originate

Your eye cannot tell the difference between an object and its virtual image

The image is as far behind a mirror as the object is in front of the mirror

Mirrors The law of reflection holds for curved mirrors. a. The image formed by a Convex mirror is

smaller than the object. b. When an object is close to a concave

mirror, the image can be larger than the object.

For reflections in a plane mirror, object size equals image size and object distance equals image distance.

Reflection & RefractionReflection & Refraction

Section 29.4—Diffuse Reflection

Diffuse Reflection Diffuse Reflection – light incident on a rough surface is

reflected in many directions A surface’s roughness is dependent upon the wavelength of the

wave incident upon that surface; the longer the wavelength, the smoother the surface will appear

To a piece of paper, light is reflecting diffusely

The Law of Reflection is Always Observed (regardless of the orientation of the

surface)

Reflection & RefractionReflection & Refraction

Section 29.5—Reflection of Sound

Reflection of Sound

An echo is reflected sound Sound reflects from all surfaces of a room Acoustics is the study of the way sound

reflects off of objects in a room Reverberations – Multiple reflections of

sound within a room The walls of concert halls are designed to

make the reflection of sound diffuse

Reflection of Sound

Reflection & RefractionReflection & Refraction

Section 29.6—Refraction

Refraction

Refraction – the change in direction of a wave as it crosses the boundary between two media in which the wave travels at different speeds

Wave Fronts – lines that represent the position of different crests

At each point along a wave front, the wave is moving perpendicular to the wave front

The direction of motion is best represented by a ray

Refraction

Incident Ray

Refracted Ray

Less RigidMedium_________________

More RigidMedium

Refracted ray bends toward the normal

Refraction When one medium ends and another begins, that is called a boundary. When a wave encounters a boundary that is more dense, part of it is reflected and part of it is transmitted.

The frequency of the wave is not altered when crossing a barrier, but the speed and wavelength are. The change in speed and wavelength can cause the wave to bend, if it hits the boundary at an angle other than 90°.

Reflection & RefractionReflection & Refraction

Section 29.7—Refraction of Sound

Refraction of Sound

Sound waves are refracted when parts of a wave front travel at different speeds

This happens in uneven winds or temperatures

Sound waves tend to bend away from warm ground, since it travels faster in warmer air

On a cold night, the speed of sound is slower near the ground than above, so we can hear over larger distances

Refraction of Sound

Reflection & RefractionReflection & Refraction

Section 29.8—Refraction of Light

Refraction of Light A pond or swimming pool may appear shallower than they

actually are, a pencil in a glass of water will appear bent All of these effects are caused by changes in the speed of light

as it passes from one medium to another, or through varying temperatures and densities of the same medium – which changes the directions of light rays

Index of Refraction (n) = (speed of light in vacuum)/(speed of light in material)

Snell’s Law: n sin θ = n´ sin θ´ (where n and n´ are the indices of refraction of the media on either side of the boundary, and θ and θ´ are the respective angles of incidence and refraction)

Index of Refraction of a few substancesVacuum 1.00 Crown glass 1.52Air 1.0003 Quartz

1.52Water 1.33 Diamond 2.42Ethanol 1.36

Refraction of Light

Incident Ray

Refracted Ray

The submerged object's apparent depth equals its true depth divided by the liquid's index of refraction: d' = d(n2/n1). Note: n2 is the index of refraction of the medium above the surface and n1 is the index of refraction of the medium below the surface.

Refraction of Light

Figure 29.19There are many effects of refractiona. The apparent depth of the glass block is less than the real depth.b. The fish appears to be nearer than it actually is.c. The full glass mug appears to hold more root beer than it

actually does.

Reflection & RefractionReflection & Refraction

Section 29.9—Atmospheric Refraction

Atmospheric Refraction

On hot days there may be a layer of very hot air in contact with the ground, the light will travel faster through this air and will bend, creating a mirage

Atmospheric Refraction When you watch the sun set, you can still see the sun for several

minutes after it has sunk below the horizon, because light is refracted by Earth’s atmosphere

Reflection & RefractionReflection & Refraction

Section 29.10—Dispersion in a Prism

Dispersion in a Prism Light of frequencies closer to the natural frequency of

the electron oscillators in a medium travels more slowly in the medium

Since different frequencies of light travel at different speeds in transparent materials, they will refract differently and bend at different angles

When light is bent twice at nonparallel boundaries, as in a prism, the seperation of the different colors is apparent

Dispersion – the separation of light into colors arranged according to their frequency

Dispersion in a Prism

Reflection & RefractionReflection & Refraction

•Section 29.11—The Rainbow

The Rainbow

The rainbow takes the concept of dispersion and multiples it through the atmosphere

The sun shines on water droplets in a cloud or when it is raining

The light is dispersed by the raindrop into its spectral colors

The Rainbow Dispersion by a Raindrop:

• Each droplet acts like a prism

• •Higher drops – red is bent to the eye

• •Lower drops – violet is bent to the eye

The Rainbow Rainbows will always appear at an angle between 40ᵒ and 42ᵒ . If you are

lucky enough to see two rainbows at the same time, the second (much dimmer) one occurs above the main one, and results from sunlight creating a double reflection in the water droplets. The colors are also upside down.

Reflection & RefractionReflection & Refraction

•Section 29.12—Total Internal Reflection.

Total Internal ReflectionCritical Angle:

Figure 29.32You can observe total internal reflection in your bathtub.

a-d) Light emitted in the water at angles below the critical angle is partly refracted and partly reflected at the surface.

e) At the critical angle, the emerging beam shims the surface.

f) Past the critical angle, there is total internal reflection.

Total Internal ReflectionTotal Internal Reflection in Diamonds

The critical angle for a diamond is 24.6ᵒ, smaller than in other common substances. This small critical angle means that light inside is more likely to totally internally reflect.

Total Internal Reflection

Critical Angle – the minimum angle of incidence for which a light ray is totally reflected within a medium

Total Internal Reflection – the 100% reflection of light that strikes the boundary between two media at an angle greater than the critical angle

Optical fibers utilize the concept of total internal reflection to feed light from one location to another, these cables are very useful for communications

Total Internal Reflection

Optical Fibers:• At each contact w/ the glass air

interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber.