Refraction of lightRefraction of light

Why does this straw look broken?

Light reflecting off the straw above the water goes straight to your eye.

When light crosses a boundary between materials, it may change direction through refraction.

RefractionRefraction

Light reflecting off the straw below the water bends—refracts—as it passes into the air, so it appears to have come from a different direction.

Light may reflect at the boundary between two materials, staying in the original medium.

Light may refract as it crosses a boundary between two materials, changing its direction.

Reflection and refractionReflection and refraction

When you look out a window at night, you can see your reflection in the glass.

Since the light also refracts, someone standing outside can see you too.

Reflection and refractionReflection and refractionIn this window you can see items inside the store AND the woman’s reflection.

This is because the light reflects and refracts at the same time.

Refraction is a property of all waves.

Refraction occurs at a boundary between two materials.

Light refracts because it travels at different speeds in different materials.

What causes refraction?What causes refraction?

Angle of incidence

Angle of refraction

Visualizing refractionVisualizing refraction

The angle of incidence lies between the incident ray and the normal.

The angle of refraction lies between the refracted ray and the normal.

When light slows down it bends towards the normal.

When light speeds up it bends away from the normal line.

The direction of refractionThe direction of refraction

The effects of refractionThe effects of refractionLight from this fish bends away from the normal as it passes from water into air.

Where does the observer THINK the fish is located?

The observer thinks the fish is located farther to the left.

The effects of refractionThe effects of refraction

The amount of refraction depends on the combination of materials.

Which combination of materials results in greater refraction?

• air to glass

• air to water

The amount of refractionThe amount of refraction

The amount of refraction depends on the combination of materials.

Which combination of materials results in greater refraction?

• air to glass

Notice the greater deflection of the refracted ray in the glass.

The amount of refractionThe amount of refraction

The index of refractionThe index of refraction

Every light medium has an index of refraction n that determines how much it will refract light.

The value of n is never less than 1, and has no units.

On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal.

AssessmentAssessment

On the diagram below, label the incident angle, incident ray, refracted angle, refracted ray, and the normal.

AssessmentAssessment

AssessmentAssessmentIf a light beam shines from glass (n = 1.5) into air (n = 1.003), does it deflect towards the normal or away from it?

If a light beam shines from glass (n = 1.5) into air (n = 1.003), does it deflect towards the normal or away from it?

If the glass is replaced with diamond (n = 2.4), does the beam deflect more or less?

AssessmentAssessment

Glass has a higher index of refraction than air so the beam bends away from the normal.

If a light beam shines from glass (n = 1.5) into air (n = 1.003), does it deflect towards the normal or away from it?

If the glass is replaced with diamond (n = 2.4), does the beam deflect more or less?

The difference in the index of refraction is greater so the beam deflects more.

AssessmentAssessment

Glass has a higher index of refraction than air so the beam bends away from the normal.

Snell’s law and the critical angle

Snell’s law and the critical angle

The refracted angle depends on:

•the angle of incidence•the index of refraction of each material

The angle of refraction can be calculated using Snell’s law.

Snell’s lawSnell’s law

ni = index of refraction of 1st material

nr = index of refraction of 2nd material

θi = angle of incidence

θr = angle of refraction

sin is the sine function

Snell’s lawSnell’s law

A light beam travels from glass into air.

Set ni = 1.52 (glass)

Set nr = 1 (air)

Engaging with the conceptsEngaging with the concepts

11.52 40

Angle of refraction

If θi = 40º what is θr?

Engaging with the conceptsEngaging with the concepts

11.52 40

Angle of refraction

A light beam travels from glass into air.

Set ni = 1.52 (glass)

Set nr = 1 (air)

If θi = 40º what is θr? 77.7º

The critical angleThe critical angle

Light passing from high to low index of refraction (ni > nr) bends

away from the normal.

As the angle of incidence increases, the angle of refraction approaches 90º.

The critical angleThe critical angle

Light passing from high to low index of refraction (ni > nr) bends

away from the normal.

As the angle of incidence increases, the angle of refraction approaches 90º.

The critical angle is the angle of incidence that results in an angle of refraction of 90º.

What do you think happens if you exceed the critical angle?

The critical angleThe critical angle

Light passing from high to low index of refraction (ni > nr) bends

away from the normal.

As the angle of incidence increases, the angle of refraction approaches 90º.

The critical angle is the angle of incidence that results in an angle of refraction of 90º.

Total internal reflectionTotal internal reflection

This can ONLY happen if the incident material has the higher index of refraction.

If you exceed the critical angle the refracted ray disappears.

The result: total internal reflection.

Calculate the critical angle θc Calculate the critical angle θc How do you calculate the critical angle?

Start with Snell’s law:

The critical angle is θi when θr = 90º:

How do you calculate the critical angle?

Start with Snell’s law:

Calculate the critical angle θc Calculate the critical angle θc

How do you calculate the critical angle?

Start with Snell’s law:

Calculate the critical angle θc Calculate the critical angle θc

The critical angle is θi when θr = 90º:

The sine of 90º is 1.

How do you calculate the critical angle?

Start with Snell’s law:

Rearrange to get the critical angle formula:

Calculate the critical angle θc Calculate the critical angle θc

The critical angle is θi when θr = 90º:

The sine of 90º is 1.

Set: ni = 1.52 (glass)

nr = 1 (air)

Engaging with the conceptsEngaging with the concepts

1.0

1.52

What is the critical angle for light passing from glass (n = 1.52) into air (n = 1.0)?

Engaging with the conceptsEngaging with the concepts

The critical angle is 41°

1.0

1.52

41.14

What is the critical angle for light passing from glass (n = 1.52) into air (n = 1.0)?

Set: ni = 1.52 (glass)

nr = 1 (air)

AssessmentAssessmentCalculate the critical angle between acrylic (n = 1.49) and water (n = 1.33).

Calculate the critical angle between acrylic (n = 1.49) and water (n = 1.33).

Asked: θc

Given: ni = 1.49, nr = 1.33

Relationships:

AssessmentAssessment

so

Solution: use the critical angle formula.