1. How many between 1 & 2 ?2. How many between 1 & 4?3. How many nodes are there

below?

1 2 4

How is a standing wave formed?

• 2 waves or pulses same speed, , f, undergo superposition interfering at fixed points of constructive & destructive interference forming pattern.

Light

Light is a small part of the EM spectrum.

All EM waves have the same behavior.

What is Light/EM waves2 Models

1) Particles/Photons Packets of energy

2) Electromagnetic Wavesenergy-carrying waves emittedby vibrating charges.

Light displays both types of behavior.

Light & all EM waves are transverse, non-mechanical waves.

EM waves begin as motion of charged particles.

Electromagnetic Wave VelocityElectromagnetic Wave Velocity

• The speed c is the same for all forms of EM waves in a vacuum of space.

• It is ~ 3.0 x 108 m/s.

• No mass can go the speed of light.

• Nothing can go faster.

• The velocity of EM radiation is almost the same in air as it is in space.

• In other substances it travels more slowly.

Ex: The wavelength of a certain color of visible light is 429 nm. Using your table, determine the color of the light.

• v = f.

• f = v/• For light in air or vacuum, v = 3x108 m/s

• Find f = 3x108 m/s =________________

429 x 109m.

f = 7 x 1014 Hz = violet.

Amplitude is related to the wave energy. So is frequency.

High Amplitude means bright.

Higher f = higher energy change in color.

The Electromagnetic SpectrumThe Electromagnetic SpectrumWaves become more energetic as Waves become more energetic as the wavelength gets shorter (freq. the wavelength gets shorter (freq.

gets higher)gets higher)

• Radio Waves - communication

• Microwaves - used to cook

• Infrared - “heat waves”

• Visible Light - detected by your eyes

• Ultraviolet - causes sunburns

• X-rays - penetrates tissue

• Gamma Rays - most energetic

The Visible SpectrumThe Visible Spectrum

• A range of light waves extending in wavelength from about 400 to 700 namometers.

Questions

• Is it correct to say that radio wave is a low-frequency light wave?

• Is a radio wave also a sound wave?*

Behaviors of Light & (Other waves)

Reflection

• We see things because they reflect or transmit light into

our eyes:

Physics Rules

• Luminous objects – generate their own light (the sun)

• Illuminated objects – reflect light (the moon)

• Line of Sight – The way you locate objects. You see a line from an object or image to your eyes (light from the object travels along this line to your eyes). You sight along the line.

Reflection of Light Rayincident ray hits new material, reflected

ray bounces back.

Reflected Ray • Incident Ray

Normal – line perpendicular to the mirror surface

Law of Reflectionincident angle i = r reflection angle

i – angle between incident ray and normal

r – angle between reflected ray and normal.

Specular Reflection &Diffuse Reflection

Diffuse Reflection obeys law of reflection.

•Driving at night on a wet roadway results in an annoying glare from oncoming headlights.

Problem

• A ray of light is incident on a mirror with i = 20o. Using a protractor and ruler, sketch the incident ray, the normal to the surface, the reflected ray.

• Label the angles.• Sketch 5 wavefronts on the rays that

show a wavelength of 1 cm.

Read 14-1 and 14-2 Light & Reflection

Hwk Packet Reflection

A ray of light hits a boundary between 2 different materials,

state what behavior(s) it might exhibit at the

boundary.

Refraction

A wave entering a new material will change its velocity.

This will result in a bending of the wave either toward or away from a perpendicular to the interface.

Law of Refraction - Snell’s Law

A ray passing from a faster medium to a slowermedium bends toward normal. i greater than r.

What happens to ?

For light traveling from a slow medium to a faster one, the refracted ray bends away from the normal. The light coming off fish bends at water/air interface.

Light rays from the chest bend toward boundary btw water & air.

Refraction of sunlight allows us to see sun a few minutes after it has set below horizon.

Why do waves bend?Different Velocities between 2 medium

Amount of “Bending”

The amount of bending of light is dependent on the ratio of the speed of light in the two mediums.

The greater the v between the two materials, the greater the bend.

“Absolute index of refraction, n,” is the ratio of the speed of light in a vacuum to its speed in a medium.

n = c n= index of refractionv v = velocity in medium

c = speed of light in vacuum - 3 x 108 m/s

Light in a vacuum has an index of 1 since:

n = c/v

= c/c

n = 1

All other materials have velocity less than c, so n is greater than 1. Air is so close that n ~1.

1. The speed of yellow light in calcite is 1.97 x 108 m/s.

What is the absolute index of refraction for calcite?

n = c v

3.00 x 108 m/s = 1.52 (no units)1.97 x 108 m/s

A substance has an index of refraction n, of 0.67. What is wrong with this statement?

All indices of refraction must be ≥ 1

since no substance can allow light to pass at a faster speed than c.

1. Light travels at 1.1 x 108 m/s in material X. What is the absolute index of refraction for material X.

2. What is the speed of light in Lucite?

3. Light travels at 2.66 x 108 m/s in Popalite . What is the index of refraction for Popalite?

Hwk Wave Diagrams.

Snell’s Law – Law of Refraction

• Relates angle of incidence, angle of refraction and the 2 indices of refraction as well as velocity.

n1sin 1 = n2sin 2 .

n1 = indx refrc med 1.n2 = indx refrc med 2.1 = angle of incidence2 = angle of refraction

Variations of Snell’s Law

sin 1 = v1

sin 2 v2

also n2 = v1 = 1

n1 v2 2 .

All in ref table.

1. A ray of light strikes crown glass from air at an angle of 30o from the normal to glass. What is the angle of refraction?

Hint: Use Snell’s Law

n1 sin 1 = n2 sin 2

(1.00)(sin 30) = (1.52)(sin 2)

sin 2 = 0.5 = 0.33 1.52

2 = 19o

2. A ray of light enters diamond from glycerol at an incident angle of 40o. Make a scaled sketch showing the:

Incident rayReflected RayRefracted Ray

(use Snell’s Law to calculate).

3. A ray of light travels from water into air. The incident angle is 40o.

Make a scaled sketch showing the:

Incident rayReflected RayRefracted Ray (use Snell’s Law to calculate).

What would happen if a ray of light traveled from corn oil into glycerol?

• Nothing!

• No refraction or reflection.

• Light will not change its velocity and cannot recognize the difference between the two!

Read 15-1

• Do p 567 prac prb (top) 1-3, bottom 1,2

• And pg 587 #10, 11, 14.

When Light Rays travel from fast to slow:

• The refracted rays bend…….

• The angle of refraction:……..

• The wavelength in then new material:…..

When Light Rays travel from slow to fast:

• The refracted rays bend…….

• The angle of refraction:……..

• The wavelength in then new material:…..

Phet bending light

What happens when light passes from slow to fast & i gets very

large?.

At the boundary between 2 different materials the wave is partially:reflected AND partially refracted/transmitted.

Critical Angle

When light crosses from slow to fast medium, it bends … from normal

Critical angle is a 1 occurs when refracted ray is parallel with boundary & most of the ray is reflected internally.

As i gets larger, ray bends away from normal at greater & greater angle. Beyond critical angle = total internal reflection occurs.

WATER

AIR

Light Source

Critical Angle

Total Internal Reflection

Critical angle c is the incident angle (1) that causes 2 to=90o.

So:n1sin c = n2sin 2

n1sin c = n2sin 90

3. A certain material has an index of refraction of 1.333. What is the critical angle for a light beam entering air from this material?

ni sin i = n2 sin r

but i = c & r= 90o & n for air = 1

So (ni)sin c= (1)(1).

c= sin-1 (1/ ni) sin-1 (1/ 1.333)

c = 48.6o.

4. What is the critical angle for light entering air from diamond?

Diamond Air

2.42 sin c = 1 sin 90

c = 24.4o

Note: As wave enters new material its frequency does not change. Frequency is the rate of the vibration of causing the wave. Since the velocity changes, then the wavelength must change to satisfy

v =f.

Beyond Critical AngleTotal Internal

Reflection

Hwk Rd tx 15 – 3do 582 # 1 – 4 &

Write it all out including question, equations, etc. will be graded.

Applications & Effects of Refraction

Total Internal Reflection – light can be made to internally reflect in a tube or pipe. None escapes.

Light Pipe

Fiber Optics

Light going from cool to warm air can refract and cause mirages because you sight along a line parallel to the ray.

Road Mirage

Light from sky bends up toward you in hot air. You sight down along line.Image appears to be on ground.

Dispersion

Dispersion – polychromatic (white) light is separated into component colors.

• Special case of refraction.

• Occurs b/c different /freq’s travel at dif v in materials other than empty space.

• Different ’s will be refracted/bent more than others.

Short ’s (high f’s) change velocity & are bent more than long ’s.

Each of light travels a different speed in the prism - each is refracted a different amount.

This is dispersion.

Which color/ is least refracted? Which color/ is refracted least?

See ’s

Red Light is affected least. It is the longest , and lowest f.

Dispersive Medium’s = one’s in which waves of dif f/’s, travel at dif speeds.

Ex: glass, water are dispersivevacuum is not dispersive.

Polychromatic = many colorsex: white light

Monochromatic = one colorex: laser light.

Rainbows-dispersion in droplets

.

For ray that hits high drops you see red at the top. Other colors pass above you.

Hwk Text bookPg 587 #27-29, 36

pg 588 # 27 - 30

Mechanical Universe Optics

http://www.learner.org/resources/series42.html#

Huygen’s Principle

Model’s wave propagation as if each point on wave front is a source of new circular waves (wavelets). The new wavefront is a line sketched tangent to the curved wavelets.

Every point on a wavefront serves as the source of spherical secondary wavelets, such that the wavefront becomes the envelope of these wavelets. The wavelets have the same frequency and speed as the original wavefront.

Huygen’s Principle Predicts Refractive Angle for wavefronts.

Huygen’s Animation

http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/huygens.htm

Huygen’s Principle Predicts Refractive Angle