CHAPTER 22CHAPTER 22Reflection Reflection andand Refraction of Light Refraction of Light

A is A

“Something cannot be itself and something else at the same time.”

Aristotle

Light exhibits wave-like properties when studied under certain conditions.

Light exhibits particle-like properties when studied under certain conditions.

Light does not exhibit wave-like and particle-like properties simultaneously.

Properties of LightProperties of LightSpeed:Speed:

In a vacuum (c): 2.997924574x108m/s

c = 3.00x108m/s

In other mediums: Speed of light is less than c.Nothing can travel faster than the speed of light.

Direction:Direction: Light travels in a straight line path until it encounters a boundary between two different mediums.

Ray Model of LightRay Model of LightReflection:Reflection: Light rays reflect (bounce) off the surface of a

new medium that it encounters in a very predictable fashion.

The “Normal” is a line perpendicular to the surface of the second medium.

1 = 1’

Angle of Incidence (1) = Angle of Reflection (1’)Angles are measured from the Normal.

I = rAlways!!

This law is obviously true in some situations and not so obviously true in other situations.

Light rays reflect from the smooth surface in only one direction because all the Normal lines are parallel to each other. All i are the same All r are the same.Light rays reflect in many directions from a rough surface because each ray encounters the surface with a Normal not parallel to the other Normals.However, for each ray:

i = r

Ray Model of LightRay Model of Light

Refraction:Refraction: The tendency for light ray to bend when traveling from one medium into another medium. Examples:Examples: Rays traveling from:

air to waterair to glasswater to glass

Law of Reflection 1 = 1’v1 = speed of light in air (still approximately 3.0x108m/s)v2 = speed of light in glass

v1 v2

1 = Incident Angle1’ = Reflected Angle2 = Angle of Refraction

Light takes the quickest path between two points.Only 1 Medium Straight Line Path2 Mediums Encountered Bent Line Path

The bent line path allows light to travel relatively more distance in the medium in which it travels faster and less distance in the slower medium.

Draw how you would travel if you were a lifeguard at point A trying to quickly reach a person at point B.

Sand

Shoreline

Ocean

A

B

Light bends toward the Normal when passing from fast medium to a slow medium…just like the lifeguard.Light bends away from the Normal when passing from a slow to a fast medium…just like you in the ocean if you noticed someone stealing from your possessions on shore.

Your possessions

You

Normal

1

2

Law of RefractionLaw of Refraction

Index of Refraction (n)Index of Refraction (n) =speed of light in vacuumspeed of light in medium

n = cv

Index of Refraction is another physical property of a substance (medium)

n 1 because c v Always!However nair 1.00 (to 3 sig.figs.)

f1 = f2

Waves don’t “pile up” at the boundary.

v1 v2

1 2

The wavelength changes at the boundary.

Law of Refraction (Snell’s Law)Law of Refraction (Snell’s Law)

sin1

sin2

v1

v2

= = constant (for two given mediums)

sin1

sin2

c/n1

c/n2

=

v2 = cn2

v1 = cn1

sin1

sin2

n2

n1

=

n1 sin1 = n2 sin 2 Snell’s Law

ExampleExample (Snell’s Law)

Find the angle (relative to the Normal) of the ray in the water.

Strategy:Strategy:Draw the Normal and measure 1 relative to the normal.Look up n1 and n2

n1 = 1.00 (air)n2 = 1.33 (water)Plug into Snell’s Law and Solve for 2

1.00 sin60 = 1.33 sin2

sin2 = .651 2 = sin-1(.651)2 = 41

1=60air

water

30

41

ExampleExample (Snell’s Law and “Critical Angles”)

Find the “critical angle” where light travels parallel to the surface of the water upon leaving the water.

Apply Snell’s Law1.33 sinC = 1.00 sin90 sinC = .75

C = sin-1(.75)C = 49

Question:Question:What happens to the light leaving the water if the incident angle is greater than C?

air n=1.00

water n=1.33

Normal

C

Total Internal ReflectionTotal Internal Reflection

Apply Snell’s Law1.33 sin60 = 1.00 sin2

sin2 = 1.15 2 = sin-1(1.15)

ERRORThe red ray is reflected internally as if the air/water surface acted like a mirror.The law of reflection is followed: 1 = 2

Where will the red ray travel? 60

air n=1.00

water n=1.33

C=49

=60

Normal

Fiber OpticsFiber Optics(Total Internal Reflection Application)(Total Internal Reflection Application)

Core:Core: Transparent material about the diameter of a piece of spaghetti.Light travels through the core.High or Low value for ncore?? High

Cladding:Cladding: Encases the core.Light is not supposed to travel through cladding.High or Low value for ncladding??

Low

Jacket:Jacket: Protective Coating End View

Jacket Cladding

Core