Chapter 9 Optics. 2 Light waves Light generally refers to the narrow band of EM waves that can be...

Chapter 9

Optics

2

Light waves

Light generally refers to the narrow band of EM waves that can be seen by human beings. These are transverse waves. The frequencies range from 4×1014 to 7.5×1014

hertz.

3

Light waves, cont’d

We typically express the wavelengths in nanometers.

The wavelengths of visible light range from 750 nm (red light) to 400 nm (blue light).

91 nanometer 10 meter

0.000 000 001 meter

1 nm

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Keep in mind that: We perceive different frequencies of light as

different colors, and White light is typically a combination of all

frequencies of the visible spectrum.

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To describe various properties of light, we use two different methods: wavefronts, or light rays.

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Reflection

Specular reflection is reflection off of a very smooth surface. Specular reflection

occurs when the direction the light wave is traveling changes.

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Reflection, cont’d

To study reflection, we need some terminology. The normal is an imaginary line drawn

perpendicular to the mirror and touching it at the point where the incident ray strikes the mirror.

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The angle of incident is the angle between the incident ray and the normal.

The angle of reflection is the angle between the reflected ray and the normal.

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The law of reflection states that the angle of incidence equals the angle of reflection.

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Diffuse reflection occurs when light strikes a surface that is not smooth and polished but rough. The light rays

reflect off the random bumps and nicks in the surface.

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Objects have color because the light actually penetrates into the material.

Some of the light is reflected and some is absorbed.

12


A white surface takes the color of whatever light strikes the surface. Shining a red light onto a white piece of paper

makes the paper look red.

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A colored surface only reflects light of the same “color” as the surface. A red surface looks red when you shine white

light on it. It appears somewhat black if you shine blue on it.

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Diffraction

Diffraction occurs when a wave passes through a hole or a slit.

It is only observable when the opening is not too much larger than the wavelength of the wave.

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Diffraction, cont’d

Since the wavelength of visible light is so small, diffraction is not noticeable through a window.

You need a very narrow slight to see the diffraction of light.

This figure shows the result of laser light after passing through a slit 0.008 cm wide.

The screen was 10 meters from the slit.

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Interference

Interference occurs when two waves overlap. Two waves are said to be “in phase” when

their peaks overlap. When two waves are in phase, their

amplitudes add. This is called constructive interference.

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Interference, cont’d

Two waves are said to be “out of phase” when the peaks of one wave overlap the valleys of the other..

When two waves are out of phase, their amplitudes cancel.

This is called destructive interference.

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Here is a diagram illustrating a two-slit interference experiment. Light is passed through two slits. The light strikes a screen behind the slits. An interference

pattern appears on the slit.

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Bright regions on the screen indicate constructive interference.

The light from each slit constructively interfere. Dark regions on the screen indicate

destructive interference.

The light from each slit destructively interferes.

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Here is an actual photograph of two-slit interference using a laser.

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Thin-film interference occurs when light passes through a thin layer of one substance next to another substance. Here, light passes

from air, through a thin layer of oil floating on a surface of water.

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Some of the light is reflected off the top surface of the oil.

The rest of the light passes through the oil. Some of the

transmitted light is eventually reflected off the water’s surface and passes back into the air.

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The ray that passes through the oil travels a greater distance.

If the two waves emerge in step, you have constructive interference.

If the two waves emerge out of step, you have destructive interference.

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Important factors that determine whether the interference is constructive or destructive include: The wavelength of the light, The thickness of the film, and The angle at which the light strikes the film.

If a single wavelength of light is used, you obtain bright and dark areas.

If several wavelengths of light are used (white light), you obtain different colors

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Here are two examples of thin-film interference using multi-wavelength light.

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Polarization

Imagine a rope attached at one end to a wall. The other end is free for you to move. If you move your hand from side-to-side, we

would say that the wave is horizontally polarized.

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Polarization, cont’d

If you move your hand from up and down, we would say that the wave is vertically polarized.

Polarization is only possible with transverse waves.

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Since light is a transverse wave, it can be polarized.

A Polaroid filter, absorbs light passing through it unless the light is polarized in a particular direction.

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If vertically polarized light is passed through a vertical polarizer, all the light passes.

If light polarized to 45º from the vertical is passed through a vertical polarized, only some of the light passes.

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If horizontally polarized light is passed through a vertical polarizer, none of the light passes through the polarizer.

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Most light is unpolarized. It contains components with random

polarizations. If unpolarized light is passed through a

vertical polarizer, then only those components that were originally vertically polarized pass through the filter.

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LCDs use perpendicular polarizers with a liquid crystal to either allow is block light. The liquid crystal

changes the polarization of the light.

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Plane mirrors

Most mirrors are plane mirrors. They are flat and almost perfect reflectors of

light. The reflection

appears to originate from behind the mirror.

34

One-way mirrors

A “one-way mirror” is made by partially coating glass so that it reflects some of the light and allows the rest to pass through.

This is called a half-silvered mirror.

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One-way mirrors, cont’d

The one-way mirror acts as a mirror for the side that is brightly lit.

It acts as a window on the dimly lit side. If a bright light is turned on in the dimmer

room, the “one-way” effect is lost.

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Curved mirrors

A concave mirror is a mirror that curves inward on the reflecting side.

Parallel rays that reflect off the mirror are focused at a point called the focal point.

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Curved mirrors, cont’d

A concave mirror can be used to create an image larger than that created by a plane mirror.

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A convex mirror is curved outward on the reflecting side.

It creates an image smaller than a plane mirror.

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An advantage of a convex mirror is that it has a wide field of view. Images of things spread over a wide area can

be viewed in it.

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Here is an example of the image from a convex mirror.

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Astronomical mirrors

Many large telescopes use curved mirrors. Here is a common design.

The large, concave mirror focus the object on the smaller, convex mirror, and then to the eye.

42

Astronomical mirrors, cont’d

A problem with large mirrors is spherical aberration. This occurs when not

all of the rays focus at the same point.

This can be overcome by using a parabolic mirror instead of a spherical mirror.

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The Hubble telescope originally had an aberration problem, as seen in these two images.

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An attempt to improve optical telescopes is called adaptive optics.

Here, the mirror is “deformable” so that imperfections can be electronically corrected.

45

Refraction

Imagine a light ray passing through air as it arrives at the surface of another transparent substance, e.g., glass. The boundary between the air and the glass is

called an interface.

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Refraction, cont’d

Part of the ray passes into the glass while the rest reflects.

The reflected law obeys the law of reflection. The ray that passes into the glass obeys the

law of refraction.

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The ray that passes into the glass is called the refracted ray.

The angle between the refracted ray and the normal is the angle of refraction.

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The law of refraction states that: a light ray is bent toward the normal when it

enters a transparent medium in which light travels slower.

a light ray is bent away from the normal when it enters a transparent medium in which light travels faster.

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This figure illustrates the effect as the light passes from the glass into the air. Since light travels faster in air

than glass, the ray is bent away from the normal.

The principle of reversibility states that the path of a light ray through a refracting surface is reversible.

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Why does light slow down when it passes into a transparent medium? Recall that light is an electromagnetic wave.

It is composed of oscillating electric and magnetic fields.

As this wave passes through the medium, it interacts with the electrons.

The electrons then begin to oscillate and radiate EM waves.

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These emitted waves tend to lag behind the incident waves.

Because of the lag, the speed at which the light propagates through the medium is reduced.

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The reason a change in speed causes the light to bend can be seen by examining the wavefronts as they cross the interface.

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The wavefronts have to stay parallel after they enter the glass.

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Since the light travels slower in the glass, the wavefronts must point in a different direction to remain aligned.

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This has the effect of reducing the wavelength. The speed of light is determined by the type of

medium. The frequency must remain constant.

If you shine red light into glass, it still looks like red light.

So our formula still holds.

v f

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Here is a graph of the angle of incidence passing from air into glass.

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ExampleExample 9.1The figure depicts a light ray going from air into

glass with an angle of incidence of 60º. Find the angle of refraction.

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ExampleExample 9.1ANSWER:

Looking at Figure 9.34, we can find the refracted angle since we know the incident angle.

So the refracted angle is 36º.

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Total internal reflection

Consider passing light from glass to air. The light passes from the glass into the air. As the angle of incidence increases, there

reaches a certain angle at which light no longer passes out of the glass. This means all the light is reflected back into

the glass.

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Total internal reflection, cont’d

This phenomenon is called total internal reflection.

The angle at which total internal reflection begins is called the critical angle.

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Total internal reflection, cont’d

At an angle of 43º, the angle of refraction equals 90º.

So, the critical angle is 43º. This could be

seen from Table 9.34.

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ExampleExample 9.2A homeowner wishes to mount a floodlight on a

wall of a swimming pool under water so as to provide the maximum illumination of the surface of the pool for use at night. At what angle with respect to the wall would the light be pointed?

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To illuminate the surface, we want the light to be refracted so that the angle of refraction is along the surface.

This corresponds to total internal reflection.

So we need the critical angle for water into air.

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Table 9.1 gives the critical angle as 48.6º.

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Fiber optics

Optical fibers use total internal reflection. They are flexible, coated strands of glass

through which you can pass light. With total internal reflection, the light passes

down the strand without passing out the sides.

So most of the light exits the strand.

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Lenses and images

A lens is a piece of glass (or other material) specially ground to alter the paths of light rays.

Imagine a block of glass round so that one end takes the shape of a segment of a sphere.

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Lenses and images, cont’d

The optical axis is the symmetry line. Assume that parallel rays are striking the

convex side of the glass. The incident rays are refracted.

Above the optical axis are refracted down, those below are refract up.

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The overall effect is that the light is focused at a point, called the focal point. One can work this out from the law of refraction by

considering each ray.

So a convex surface serves to converge incoming light.

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Next, shine light on a concave surface of glass.

The rays diverge as the light passes into the light.

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But they diverge as if they originated at a point outside of the glass.

So a concave surface serves to diverge incoming light.

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A converging lens causes parallel light rays to converge to a point, called the focal point.

The distance from the center of the lens to the focal point is called the focal length. If a source is placed

at the focal length, the rays will emerge parallel to each other.

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A diverging lens causes parallel light rays to diverge, appearing to originate from a point called the focal point.

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For both types of lenses, there are two focal points. One on each side.

As a rule, converging lenses are thicker at the center than at the ends.

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Image formation

The main use of lenses is to form images. We can draw a model of image formation

using just three rays. The particular rays we use are chosen

because they satisfy the law of refraction in a straightforward manner.

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Image formation, cont’d

The rays are: A ray initially parallel to the optical axis passes

through the focal point on the other side of the lens.

A ray that passes through the focal point (on the same side as the object) emerges parallel to the optical axis on the other side.

A ray that passes exactly through the optical axis emerges along the optical axis on the other side.

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Here is a diagram of these three rays. Where the rays intersect on the opposite side

of the lens defines the image.

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The distance from the lens to the object is called the object distance. Denoted by the symbol s.

The distance from the lens to the image is called the image distance. Denoted by the symbol p.

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If we know the object focal length and object distance, the image distance is

lens formulasf

ps f

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ExampleExample 9.3In a slide projector, a slide is positioned 0.102

meters from a converging lens that has a focal length of 0.1 meter. At what distance from the lens must the screen be placed so that the image of the slide will be in focus?

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The problem gives us:

From the lens formula:

0.102 m

0.1 m

s

f

0.102 m 0.1 m

0.102 m 0.1 m

5.1 m.

sfp

s f

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A real image is an image that can be projected onto a screen. You can see the image on the opposite side of

the lens. The image from a magnifying glass.

A virtual image is an image that cannot be projected onto a screen. You see the image by looking into the lens.

Similar to the “mirror image” seen looking into a plane mirror.

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These figures demonstrate the difference between real & virtual images.

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ExampleExample 9.4A converging lens with focal length

10 centimeters is used as a magnifying glass. When the object is a page of fine print 8 centimeters from the lens, where is the image?

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The problem gives us:

From the lens formula:

8 cm

10 cm

s

f

8 cm 10 cm

8 cm 10 cm

40 cm.

sfp

s f

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ExampleExample 9.4DISCUSSION:

The negative sign indicates that the image is on the same side of the lens as the object.

Therefore, it is a virtual image and must be viewed through the lens.

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Image magnification

Typically, a lens produces an image that is not the same height as the object.

The magnification, M, of a lens is the ratio of the image height to the object height:

image height

object heightM

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Image magnification, cont’d

The magnification of an object also depends on the object distance.

An alternative expression for magnification is:

If p is positive (image is to the right of the lens and real), M is negative — image is inverted.

If p is negative (image is to the left of the lens and virtual), M is positive — image is upright.

pM

s

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ExampleExample 9.5Compute the magnification for the slide

projector in Example 9.3 and for the magnifying glass in Example 9.4.

8 cm

10 cm

-40 cm

s

f

p

0.102 m

0.1 m

5.1 m

s

f

p

Example 9.3 Example 9.4

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For the projector:

For the glass:

5.1 m

0.102 m50

pM

s

40 cm

8 cm5

pM

s

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ExampleExample 9.5DISCUSSION:

The projector image is 50 times as large as the slide and is inverted (since M is negative).

The magnifying glass image is 5 times larger and upright (since M is positive).

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Aberration

Recall that spherical aberration is the effect that rays that pass through different part of the lens are focused at different locations.

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Aberration, cont’d

Chromatic aberration occurs when a lens illuminated with white light focuses the various colors at different locations. This effect is due to dispersion (more later).

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The human eye

Light passes through the opening in the iris and forms and image on the retina. The cornea and lens acts as a single

converging lens. The eye muscles

change the thickness of the lens to change the focal length.

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The human eye, cont’d

The eye muscles change the thickness of the lens to change the focal length. A thinner lens is used for near objects. A thicker lens

is used for farther objects.

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Myopia (nearsightedness) causes the lens to focus distant objects in front of the retina.

A diverging lens corrects the problem.

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Hyperopia (farsightedness) causes the lens to focus distant objects behind of the retina.

A converging lens corrects the problem.

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Dispersion and color

Dispersion is a phenomenon in which the speed of light through a medium depends on the frequency of the light. Dispersion is why passing a

white light through a prism causes the individual colors to be separated.

The different frequencies are refracted at differing angles.

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Dispersion and color, cont’d

The different frequencies are refracted at different angles. Shorter wavelengths are refracted more than longer

wavelengths.

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Dispersion and color, cont’d

This is because (in most materials), violet light travels slower through the material than red light. In common glass, the speed of violet light is

1.95×108 m/s while for red light it is 1.97×108 m/s. This difference in speed is only about 1%.

In diamond, the difference in speeds is closer to 2%. This explains the “brilliance” of diamond.

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Atmospheric Optics

When talking about atmospheric optics, it is convenient to introduce a measure of size.

We use an angular diameter. The number of degrees the object covers.

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Atmospheric Optics, cont’d

A rainbow occurs because of the dispersion of sunlight through water droplets in the atmosphere.

Some general observations are: Rainbows are arcs of colored light, with red on

the outside of the box and violet on the inside. Rainbows are always seen against a

background of water droplets, typically with the Sun toward your back.

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Image a beam of sunlight striking a raindrop. For simplicity, we take the raindrops to be spherical.

Different rays of the beam are refracted differently because they have different angles of incidence.

Rays entering above the axis (i.e., ray 1) exit below the axis.

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The farther above the axis the ray enters, the greater its emergent angle up to a point, defined by ray 7. This ray is called the

Descartes ray. Rays above this, the exit

angles are less than that of the Descartes ray.

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This leads to a concentration of rays leaving the droplet at a maximum angle corresponding to that of the Descartes ray. This angle is about 41º

for rays 6 through 10.

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Including the effects of dispersion gives a rainbow. the amount of refraction depends on the

wavelength of the light.

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A secondary rainbow results from a similar process.

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But there is a double, internal reflection as opposed to a single internal reflection for a primary rainbow. So the order of the colors is reversed.

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A halo is a circular arc of light surrounding the Sun or full Moon in winter.

When the temperature in the upper atmosphere drops below freezing, ice crystals form.

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A common shape at high elevations is that of a hexagon. Such crystals may be considered as pieces of

60º prisms since they deviate light like them.

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The incoming, parallel rays of sunlight are bent at 22º after passing through the crystal. There is typically a

separation of colors due to dispersion by the crystal.

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The blueness of the sky is caused by air molecules scattering sunlight in all directions.

The oscillating electric field of an EM wave passing through the air causes the electrons of air molecules to oscillate.

As discussed, these electrons emit EM radiation.

This emitted light travels outward in all directions — thus the term scattered.

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The reason the sky is blue is because the air molecules are more efficient at absorbing and radiating higher frequencies of light.

The amount of radiant energy scattered per second can be expressed as

is the wavelength of the scattered light.

scattered4

1E

t

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The wavelength of blue light is about 0.7 times that of red light.

So there is about 4.2 (= 1/0.74) times as much blue in scattered sunlight as there is red.

Chapter 9 Optics. 2 Light waves Light generally refers to the narrow band of EM waves that can be...

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Transcript of Chapter 9 Optics. 2 Light waves Light generally refers to the narrow band of EM waves that can be...