Refraction of light

Refraction Why does this straw look broken? When light crosses a boundary between materials, it may change direction through refraction. Light reflecting off the straw above the water goes straight to your eye. 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.

Reflection and refraction 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. When light hits glass, it can do both.

Reflection and refraction In 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. 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.

What causes refraction? 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.

Visualizing 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. Angle of incidence Angle of refraction

The direction of refraction When light slows down it bends towards the normal. When light speeds up it bends away from the normal line. Point out that the path of the light ray is reversible.

The effects of refraction Light 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 effects of refraction The observer thinks the fish is located farther to the left.

The amount 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 Air to glass produces greater refraction. The incident ray has a greater deflection.

The 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 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. Diamond’s high index of refraction is what gives it its brilliance and “fire”. There are materials with an even higher index of refraction than diamond, such as silicon crystals.

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

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

Assessment 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?

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

Assessment 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? Glass has a higher index of refraction than air so the beam bends away from the normal. The difference in the index of refraction is greater so the beam deflects more.

Snell’s law and the critical angle

Snell’s law 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 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

Engaging with the concepts A light beam travels from glass into air. Set ni = 1.52 (glass) Set nr = 1 (air) If θi = 40º what is θr? 1.52 40 1 Angle of refraction The students should discover that at 42°, the refracted ray lies along the interface between the two mediums.

Engaging with the concepts 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 1.52 40 1 Angle of refraction

The 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 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.

The 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?

Total internal reflection If you exceed the critical angle the refracted ray disappears. The result: total internal reflection. This can ONLY happen if the incident material has the higher index of refraction.

Calculate the critical angle θc How do you calculate the critical angle? Start with Snell’s law:

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:

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: The sine of 90º is 1.

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: The sine of 90º is 1. Rearrange to get the critical angle formula:

Engaging with the concepts What is the critical angle for light passing from glass (n = 1.52) into air (n = 1.0)? 1.0 Set: ni = 1.52 (glass) nr = 1 (air) 1.52

Engaging with the concepts What is the critical angle for light passing from glass (n = 1.52) into air (n = 1.0)? 1.0 41.14 Set: ni = 1.52 (glass) nr = 1 (air) 1.52 The critical angle is 41°

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

Assessment Calculate the critical angle between acrylic (n = 1.49) and water (n = 1.33). Asked: θc Given: ni = 1.49, nr = 1.33 Relationships: Solution: use the critical angle formula. so