Chapters 14-16 14-1 Characteristics of Light Electromagnetic waves is a transverse wave consisting of oscillating electric and magnetic fields at right.

Chapters 14-16

Unit 9 – Waves & Optics

14-1 Characteristics of Light

• Electromagnetic waves is a transverse wave consisting of oscillating electric and magnetic fields at right angles to each other.

• The EM spectrum includes more than visible light.

• EM waves vary depending on frequency & wavelength.

Electromagnetic Spectrum f

Radio >30 cm < 1.0 x 109

Microwaves 30 cm - 1mm 1.0 x 109 – 3.0 x 10 11

Infrared 1mm – 700 nm 3.0 x 10 11 – 4.3 x 10 14

Visible light 700 nm – 400 nm 4.3 x 1014 – 7.5 x 1014

Ultraviolet 400 nm – 60 nm 7.5 x 1014 – 5.0 x 1015

X-rays 60 nm – 1 x 10-4 nm 5.0 x 1015 – 3.0 x 1021

Gamma rays 0.1 nm – 1 x 10-5 nm 3.0 x 1018 – 3.0 x 1022

Radio waves generally are utilized by antennas of appropriate size (according to the principle of resonance), with wavelengths ranging from hundreds of meters to about one millimeter.

Lowest frequencies allow communication with submarines

High frequencies are used for FM radio broadcasts, wireless LAN, radar and radio astronomy

Radio

AM – KHz•Amplitude modulation

FM – MHz•Frequency modulation

• EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is used in microwave ovens.

• Garage door openers

• Radar

• Atomic and molecular research

• Aircraft navigation

• Microwave communication

Microwaves

Infrared photography Pit vipers Infrared therapy Military applications Analyze art/relics Weather forecasting

Infrared

• Above infrared in frequency

• This is the range in which the sun and stars emit most of their radiation.

• Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.

Visible Light

Causes sunburn/cancer. The Sun emits a large amount of UV

radiation, but most of it is absorbed by the atmosphere's ozone layer before reaching the surface.

Used for:Sterilization of medical instrumentsIdentification of fluorescent minerals Detect forged documentsForensics

Ultraviolet

• Detect stars around black holes • Medicine and industry • Border control/airports

X-rays

•Food treatment•Often used to kill living organisms, in a process called irradiation.

•Sterilizing medical equipment (as an alternative to autoclaves or chemical means)

•Treatment for cancers

Gamma Rays

• All forms of EM travel at the speed of light.

c = fc = speed of light in a vacuum (3.0 x 108

m/s)l = wavelength (meters)

f = frequency (Hertz)

Brightness• Brightness decreases

the further you get from the source of the light.

• Brightness decreases by the square of the distance from the light source.

• Ex. Twice as far, 1/4 times as bright

14-4 What is color?Visible light is a small slice of the electromagnetic spectrum.

• The colors are distinguished by their frequencies.

When all of the colors of light are combined, you get white light.•White is therefore not technically a color, it is a combination of all of them.

If you remove all of the colors of light, you get black.•Black is not a color, it is the absence of color.

What about black & white?

•We have receptor cells in the retina of our eyes: rods and cones

•Rods are responsible for detecting light intensity.–Animals with good night-vision have lots of rods in their eyes.

–More sensitive than cones.

•Cones are responsible for detecting colors–There are three types: red, green, and blue–Rods & Cones

Perception of color

Cones and color vision• Each type is sensitive

to a certain range of frequencies

• As light strikes the cone cell, a chemical reaction sends an electrical signal to the brain. Your brain converts these to what you see.

• Notice that we are most sensitive to light in the green to yellow range.

• What do we see in the dark?

• About 7 percent of the male population has some degree of colorblindness.

• This does not mean that they don’t see colors, they just see them differently and have difficulty distinguishing some colors.

• It can disqualify people from some careers.• It is a recessive trait on the X chromosome•Females can be colorblind, but it would require 2 defective X chromosomes, very rare.

•Colorblind Test

Colorblindness

How colorblind people see the world

Color by addition• If our eyes only really see red,

green, and blue, then we should be able to make any color from combinations of those.

• By mixing these three colors of light, we can make any image.

• Component video inputs for televisions use these: RGB

Primary Additive ColorWhen you combine all 3 additive primary colors, you produce white light

When you combine 2 additive primary colors, you produce a secondary color.

Red + Green = Yellow

Red + Blue = Magenta

Green + Blue = Cyan

Additive Color Simulator

Subtractive Color Process

• We use this to mix paints and print pictures.

• Yellow reflects red and green and absorbs blue.

• Magenta absorbs green.

• Cyan absorbs red.

mag + cyan = blue

mag + yellow = red

cyan + yellow = green

mag + yellow + cyan = black

• Complementary colors - any two colors of light which when mixed together in equal intensities produce white.

• The complementary color of red light is cyan light.

•This is reasonable since cyan light is the combination of blue and green light

•And blue and green light when added to red light will produce white light.

Complements

• The color of an object occurs when that object absorbs all colors except the color it appears to be.

• If a leaf is green, that means that it absorbs all colors but green.

• The leaf reflects green light.

• Dyes and pigments work on this principle

Color by reflection

Reflection – 14.2 & 14.3Flat Mirrors & Curved Mirrors

Unit 9 - Day 2

Reflection – the turning back of a wave at a surface. Most substances absorb a little incoming light and reflect the rest.A good mirror reflects 90% of the incoming light. You see objects and light rays because of reflected light.

14.2 – Flat mirrors

• The texture of a surface affects how it reflects light.

• Light reflected from a rough, textured surface is called diffuse reflection.

• Light reflected from a smooth, shiny surface is called specular reflection.

• Incoming and reflected angles are equal.

• The angle of incidence for a wave reflected from a surface is equal to the angle of reflection.

• Angle of incidence – the angle between the ray that strikes a surface and the normal to that surface.

• Angle of reflection – angle formed by the line normal to a surface and the direction in which a reflected ray moves.

• These angles are measured from the normal.

• Flat mirrors create a virtual image.

• A virtual image is one that is formed by light rays that only appear to intersect.

• In flat mirrors, the right & left sides are reversed. The image is just as far behind the mirror as the object is in front of the mirror.

• Concave spherical mirrors focus light

to form real images.

• A real image is formed when light rays actually intersect at one point.

• These mirrors produce images that can be smaller than, larger than, or the same size as the original object.

14.3 – Curved Mirrors

Convex Spherical MirrorsFor a convex mirror, the image of the object is upright, reduced in size and located behind the mirror

Why would we use them?

Mirror equations

• 1 + 1 = 1

di do f

• di = distance of image

• do = distance of the object

• f = focal length

• Radius = r = 2 f or ½ r = f

• Magnification (M)

• hi = height of image

• ho = height of object

• If image is larger than the object, then M > 1

• If image is smaller than the object, then M < 1

M = hi = -di

ho do

do = positive = object in front of mirror

di = positive = real image

di = negative = virtual image

f = positive = concave mirror

f = negative = convex mirror

M = positive = upright & virtual

M = negative = inverted & real

Signs

Rules for drawing rays

From the top of the object

1. Parallel to the axis & reflect back through f

2. Through f & reflect back parallel

3. Through the center of curvature

Image where all three lines intersect.

Ray Diagrams

Refraction – 15.1

Unit 9 - Day 3

Refraction - light BENDS when it goes from one kind of substance to another (because of the change in average speed).

Using v = f, we seethat the wavelength hasto change.

The average speed of the light wave changes due to change in media, but THE FREQUENCY DOES NOT CHANGE!

Index of refraction – ratio of the speed of light in a vacuum to its speed in a transparent medium

High index of refraction (dense) – slower, so bends toward the normal

Low index of refraction (less dense) – faster, so bends away from the normalMaterial Index of

Refraction

Vacuum 1.0

Air 1.0001

Water 1.33

Ice 1.31

Glass 1.5

Diamond 2.42

Different media result in different amounts of refraction.

Example: If light travels at 1.94 x 108 m/s through salt, what is the index of refraction?

Example: A light ray of wavelength 589 nm traveling through the air strikes a smooth flat slab of crown glass at an angle of 30o to the normal. Find the angle of refraction.

Snell’s Law

Lenses – 15.2

Unit 9 - Day 4

Like mirrors, lenses form images, but lenses do so by refraction rather than reflection.

The two basic types of lenses are::

CONVERGING (convex)

DIVERGING (concave)

Ray diagrams make it possible for us topredict what kind of images each type oflens creates.

Convex Lenses• Thicker in the

center than edges.

•Lens that converges (brings together) light rays.

•Forms real and virtual images depending on position of the object.

The Magnifier

Concave Lenses• Lenses that are

thicker at the edges and thinner in the center.

•Diverges light rays •All images areupright and

smaller.

The De-Magnifier

1.Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

2.Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3.An incident ray that passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Drawing ray diagrams for converging (convex) lenses

• The focal length of a converging lens is always a positive number.

• If an object is located outside the focal point of a converging lens, the image it forms is real, inverted, and on the opposite side of the lens. Both d0 and di are positive numbers.

The converging (positive) Lens

• If an object is located inside the focal point of a converging lens, the image it forms is virtual, upright, enlarged, and on the same side as the object. In this case, do is positive and di is negative.

• If an object is at the focal point, the rays do not converge and therefore no image is formed.

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The converging (positive lens)

• The focal length of a diverging lens is always a negative number.

• The image formed by a diverging lens is always virtual, upright, reduced, and on the same side of the lens as the object. In this case do is positive and di is negative.

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The diverging (negative) lens