Chapter 29Chapter 29

Reflection Reflection and and

RefractionRefraction

ReflectionReflection• Most objects we see reflect light rather

than emit their own light.

• Common Misconception: The moon is a light emitter.

• Reality: The moon is simply reflecting the sun’s rays.

Principle of Least TimePrinciple of Least Time

• Fermat's principle - light travels in straight lines and will take the path of least time

MIRROR

A B

Wrong PathTrue Path

Law of ReflectionLaw of Reflection• “The angle of incidence equals the

angle of reflection.”

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Missing Light?Missing Light?• The law of reflection holds for both plane mirrors

and curved mirrors.

• Approximately 99% of the incoming light is reflected, depending on how highly polished the surface is. Where did that other 1% go?

Specular ReflectionSpecular Reflection• Specular Reflection - images seen on

smooth surfaces (e.g. plane mirrors)

More on Virtual ImagesMore on Virtual Images

• Virtual Image – where the divergent light rays appear to originate from. The word “virtual” is used b/c the light does not actually originate from that spot.

• Our eyes and the differentiation problem.

• Do = Di

• So = Si

Concave vs. ConvexConcave vs. Convex• Cave Raaaooorrrrwwwcurved in relative

to the observer• Vexto annoycurved out towards observer• The Gap story from ~2001 and Fun House

Mirrors

ConvexConvex

• Virtual image is smaller and closer to the mirror than the object is.

• Do > Di

• So > Si

• You’ve seen these on driveways and in the corners at WaWa. (What the heck is a WaWa anyway?)

ConcaveConcave• Virtual image is larger and further from the

mirror than the object is.• Do < Di

• So < Si

• You’ve seen these in makeup mirrors and spoons.

Diffuse ReflectionDiffuse Reflection

• Magazines vs. Hardback books

• Zoomed in view of paper

Diffuse ReflectionDiffuse Reflection• Occurs when light is scattered in different

directions.• The law of reflection occurs for each ray, but

it does not appear to b/c of the many different angles it encounters.

• Specular Diffuse

New Car LookNew Car Look• What makes a car look older?

• Explain in terms of diffuse vs. specular refelction.

• Rainy night driving.

RefractionRefraction• Who still mows their lawn? Who gets

groceries for their mom?

• What happens when one wheel hits a new surface before the other?

RefractionRefraction

Refraction of SoundRefraction of Sound

• All waves exhibit refraction. • Refraction is the bending of a

wave when it enters a new medium.

• The bending is caused by the differences in the speed of the wave in the media.

• Diagrams and Hewitt clip• Large tube walk

Refraction of SoundRefraction of Sound• Let’s explore sound.

• Recall that sound speeds up when entering water while light decreases in speed.

• What students have said: “Curves toward the region that the wave is slower in.” OR “Curves towards cool.”

Refraction of SoundRefraction of Sound

Refraction of LightRefraction of Light• Refraction is the bending of light when it

passes from one transparent medium to another.

• This bending is caused by differences in the speed of light in the media.

Refraction-Fast to Refraction-Fast to SlowSlow

WATER

AIR

Normal LineFast

Slow

Light Beam

The ray will bend towards the normal when the speedof light is slower in the new medium.

Refraction-Slow to Refraction-Slow to FastFast

WATER

AIR

Normal Line

Fast

Slow

Light Beam

The ray will bend away from the normal when the speedof light is faster in the new medium.

What has Changed?What has Changed?

• If light enters water from air, the color of the light is the same! What must that mean about the frequency?

• It remains constant!• But the ray decreased in speed,

so what must have occurred?• The wavelength decreased!

Wavelength ChangeWavelength Change

The Key to RefractionThe Key to Refraction

• The key to refraction is the fact that a part of the wave front encounters the new medium before another part.

• What if the whole wave front encounters the new medium at the same time? That is, what if the light ray approaches the new medium in a perpendicular fashion?

• No refraction occurs!• Eagles Gone Wild!

Penny in the CupPenny in the Cup

Refraction ExamplesRefraction Examples

• Light slows down when it goes from air into water and bends toward the normal.

• An Analogy: A car slows down when it goes from pavement onto gravel and turns toward the normal.

• An Illusion : Fish in the water appear closer and nearer the surface. (See Figure 27.27)

RefractionRefraction

WATER

AIR

Observer

True Fish

False Fish

Index of Refraction

• The greater the density of the media that light enters, the greater the degree of bending that occurs.

• Example: Air to water will produce a small degree of bending. Air to diamond will produce a larger degree of bending.

Index of Refraction

• The ratio of the speed of light in a vacuum to the speed in a given material is called the index of refraction.

• n = speed of light in a vacuum speed of light in material

• n = cv

Sample IndicesMedium n = c/v

Vacuum 1

Air 1.0003

Water 1.33

Ethyl Alcohol 1.36

Quartz Glass 1.46

Crown Glass 1.52

Flint Glass 1.58

Plexiglass 1.51

Sodium Chloride 1.53

Diamond 2.42

Atmospheric RefractionAtmospheric Refraction• Our atmosphere can bend light

and create distorted images called mirages.

Diagrams of Mirages

• Desert Mirage

• Urban Mirage

• Extra 5 minutes of Sunlight???

Pumpkin Sun

• The bottom of the sun or moon will flare out more than normal b/c that part is going through more atmosphere.

• More atmosphere = more refraction.

The Moon Illusion

• Distorted, but the overall size is the same. • The appearance that the moon is larger

when it is coming up on the horizon.• For future reference, the moon stays low

on the horizon during the month with the summer solstice.

• Ponzo Illusion – the main cause of the illusion: When our brain interprets things as being bigger if we think it is far away.

The Moon Illusion

• So when the Moon is on the horizon, your brain thinks it's farther away than when it's overhead. The Ponzo illusion kicks in, and your brain gets fooled into thinking the Moon is HUGE. As it gets higher, the illusion vanishes. If you actually observe the Moon with binoculars or with a 'scope, you can see it is no bigger on the horizon. In fact, it should look smaller because it's a few thousand miles farther away than when it's overhead.

The Ponzo Illusion

Strongest…..Ponzo….Ever….

More Illusions

More Illusions

Dispersion...Dispersion...

• …is the separation of white light into pure colors (ROYGBV).

• Dispersion Examples: • Prisms• Diffraction Gratings• CD’s• Raindrops

RainbowsRainbows

• Raindrops refract, reflect and disperse sunlight.

• Rainbows will always appear opposite of the Sun in the sky.

• You cannot run from or run to a rainbow!

Total Internal Reflection...Total Internal Reflection...

• …is the total reflection of light traveling in a medium when it strikes a surface of a less dense medium

WATER

AIR

Light Source

Critical Angle

Total Internal Reflection

• Total Internal Reflection occurs when the angle of incidence is greater than the critical angle.

• Fish can see everything above the water’s surface in a 96o cone of vision.

• Demo - Laser and light pipe