Today 2/3 Total Internal Reflection No HW due Today Lab: Refractive index with Pins and Blocks Start reading chapter 26.6-8 on Lenses HW: 2/3 “Refraction” Due Thursday, 2/6 Exam I Thursday, Feb 13

This Week’s Lab

Pins

“Images” will not line up when the “objects” do

This Week’s Lab

Pins

“Objects” will not line up when the “Images” do

Refraction-Bending Light

Water - index of refraction, nW = 1.33

Wave crests are closer together in higher index materials because the waves travel slower.

Air - index of refraction, nA = 1.0

Think of rows in a marching band that slow down in a mud puddle.

Refraction-Bending Light

Water - index of refraction, nW = 1.33

When light enters at an angle, it “bends.”

Air - index of refraction, nA = 1.0

Think of rows in a marching band that slow down in mud puddle.

Ray direction always perpendicular to the waves.

Ray diagram example

Water - index of refraction nW = 1.33

Air - index of refraction nA = 1.0

Refraction makes it seem as though its closer

Image

Object

Ray diagram example

Water - index of refraction nW = 1.33

Air - index of refraction nA = 1.0

Refraction makes it seem as though its farther

Image

Object

Example:

4cm

7cm tall

2

When looking into a glass of water, where (at what angle, 2) should I place my eye so that I can just barely see the red dot?

n = 1.33

n = 1.0

1

n1sin 1 = n2sin 2

tan 1 = 4/7 1 = 30°

1.33 sin 30° = 1.0 sin2 2 = 41.7 How deep does the water appear to be?

tan 2 = 4/d

d = 4/tan41.7 = 4.5cm

Note notation: light from n1 to n2

d

Total internal reflection

Water - index of refraction nW = 1.33

nAsin A = nWsin W Air - index of refraction nA = 1.0

A

W

As W increases, so does A

until A becomes 90°. C is called the “critical angle”. If W is greater that C no light will escape the water and all will be “internally reflected.”

Total internal reflection only happens when light goes from high n to low n and it depends on both n’s!

nAsin 90 = nWsin C ornA = nWsin C

C

Example:What is the critical angle for light traveling from water into air?

n2 = n1sin C

n1 = 1.33

C

n2 = 1.0sin C = n2/n1 = 0.752C = 48.8°

Note that there is no critical angle for light from low index to higher index.

Example:What happens if the angle of incidence is greater than 48.8°?

n2 = n1sin C

n1 = 1.33

1 = 50°

n2 = 1.0

50°

Example:What happens if I place a sheet of glass on the water, ng = 1.5?

n2sin 2 = n1sin 1

n1 = 1.33

1 = 50

n2 = 1.0

ng = 1.5

No internal reflection possible-low to high n!

n2 = n1sin C

1.5sin g = 1.33sin50° g = 42.8°

Now what happens? C = Arcsin1/1.5 = 41.8°

The light reflected back into the glass.

g = 42.8