the art of bending light

It’s what we see…

The Art of Bending Light

Refraction•the bending of light when passing

through a new (transparent) substance

Bending Light: Vocabulary

Incident RayAngle of

IncidenceNormalRefracted RayAngle of

Refraction

Optical DensityHow difficult it is for light

to move through a substance .

High optical density light travels through more slowly

Low optical density light travels through more quickly

Optical DensityHow difficult it is for light

to move through a substance .

High optical density light travels through more slowly

Low optical density light travels through more quickly

Low to High Optical Density

Low to much higher Optical Density

High to lower Optical Density

End Part I

Quick Review

Notice when:

Light bends in

Light bends out

Speeds of LightFast lightSlow light

Optical DensityRatio of old to new speed

n =optical densityAlso known as

index of refractionc =speed of light =speed of light

in substance

Speed of lightc = 3.00 x 108 m/sin a vacuumIn space, = c

n = 1 in vacuumFastest in space.

So…n≥1 always!

Index of RefractionsTable of indices

of refraction for various substances

Which substance has greatest n? Lowest?

Trigonometry “Review”

SOHCAHTOA

sin =

IN OS AN

Snell’s Law

Basic formCompares

angles1

2

d1

d2

Working with AirIn a vacuum,n = 1

In air,n = 1.0003

Air is treated like a vacuum

Snell’s Law from Air or Vacuum

In air, n = 1So, ignore n1Let’s solve for

n2

How to choose the Equation

Speeds

Neither substance is air

One substance is air

ex) How fast is light in Water?

1

2

3

How fast is light in water?n = 1.33c = 3.00 x 108 m/s

Example 2A ray of light passes from water to quartz (n = 1.54). The angle of incidence in the water is 35°. What is the angle of refraction?Given:

Want:

Refraction Activity!

Go to pg. 64

Example 3The photo below shows a smaller beaker inside a larger beaker. Wesson oil was poured into the larger beaker until it flowed about half way up the smaller beaker. Explain why the bottom of the beaker has disappeared.n1n2Light does not

bend between beakers

Example 4The speed of light in a certain substance is 1.90 x 108m/s. If light passes from air into the substance at an angle of incidence of 54.0°, what will be the angle of refraction? Given:

Want:Find n first!

Example 4

Given:

Want:

Why?

Imagine rows of hikers

From dry land to swamp

Hikers in swamp slow down

Others stay fastAngle Changed!

OR…

Pierre de Fermat

Fermat’s Principle

Light always takes fastest path

So…

Fastest PathIs a straight line

always the fastest?

Fastest PathIs a straight line

always the fastest?

Dashed line is in slow substance for less time and in faster substance for more time.

Bent path is faster overall.

d2

d1

d1<d2

Snell’s Law SongSingin’ n1 sine theta-sub-1, hey-hey-

hey, Equals n2 sine theta-sub-2, hip hooray

Atmospheric Refraction Effects•Colors in the sky

•Air scatters light with small wavelengths•Every color has its own wavelength (λ)

Intensity of scattered light

Draw normalsMiddle ray on

normalMiddle ray

does not refract

Light rays converge!

CONVERGING LENS

LensesFocal point

Normals againFollow Rays

Backwards!Light rays diverge!DIVERGING LENS

LensesFocal point

Locating Images of Lenses

Remember Mirror Ray Patterns?

Lenses also have patterns!

Let’s break ‘em down!

Converging Lens: Parallel RaysHoly cow! Through f !

Converging Lens: From fZOMG! Parallel!

Converging Lens: Through CenterWowzers! Straight through!

Diverging Lens: Parallel RaysKrikey! Away from f !

Diverging Lens: Towards fKa’Pow! Parallel!

Diverging Lens: Through Center

Mamma Mia! Straight Through!

Find the ImageRegion Locations:

• Same side as object

• Between f and 2f• At 2f• Beyond 2f

Time to Practice!

Go to pg. 72

Lens Movie

Thin Lens EquationsTime for a new

equation…

or

Signs and LensesFocal LengthConverging Lens

Positive f

Diverging Lens Negative f

Signs and LensesImage DistanceImage on same side as object

Negative di Virtual Image

Image on opposite side

Positive di Real Image

Magnification EquationIntroducing…

Multiple LensesImage of first lens

is the object of the next!

Galileo InquisitionEppur Si Muove!

Time to PracticeGo to pg. 85!

Total Internal Reflection

Total Internal Reflection

?

No more refraction! Light must

now reflect

Only when going from high to low n

Refracted angles go up to 90°

After that, no refraction

When r = 90°, i is called the critical angle

Total Internal Reflection

Critical Angle Behavior1. Critical angle shown2. i < ic (critical angle)3. i = ic light moves

between4. i > ic totally reflected

Calculating Critical Angle

Use Snell’s Law

or

ExampleWhat is the critical angle for glass?(n = 1.50)Given:

Want:

Time to practicego to pg. 84

White Light Dispersion Now we know

refraction but… It’s more

complicated than that.

Refraction actually varies for each color!

White Light Dispersion

The white light is separated into its colors

Time to practice

Go to pg. 89

White Light Dispersion Use Snell’s

Law draw normals