23.2 Refractionpp. 503 – 506, and 525 (24.1)

Mr. Richter

Agenda

Warm-Up Notes:

The Speed of Light Index of Refraction Angle of Refraction Total Internal Reflection Dispersion

Objectives: We Will Be Able To… Calculate the wavelength and frequency of light. Describe what happens when light goes from one

material to another. Understand the index of refraction. Explain total internal reflection and dispersion.

Warm-Up:

When a pencil is partially submerged, it looks like it bends at the surface of the water.

What is really happening?

The Speed of Light

The Speed of Light

Reminder: all light travels at the speed of light 300,000,000 m/s. Always.

The relationship between the speed of light, frequency and wavelength is the same as it is for all other waves.

The Speed of Light: Your Turn

The broadcast frequency of Jammin’ 94.5 is 94,500,000 Hz. What is the wavelength of the waves sent from the radio tower?

λ= 3.17 m

Index of Refraction

Index of Refraction

Light bends when it passes into a different medium.

It appears to slow down. BUT IT DOESN’T! LIGHT TRAVELS AT THE SPEED OF LIGHT.

So what happens? Different materials have different

molecular structures. Some materials take longer to absorb and re-emit light.

Index of Refraction

Every transparent material has an index of refraction.

The index of refraction (n) of a material indicates how slowly light appears to move in that material.

The index of refraction measures how much light bends.

Index of Refraction

The higher the index of refraction: The slower light appears to

move. The more light bends.

Angle of RefractionWhich way does light bend?

Angle of Refraction

Imagine your right arm as a wave. As you walk, your arm moves with

you through the air. However, if you drag your hand

along a fence, your arm drags behind, bending your body.

Angle of Refraction

The same thing happens with waves of light.

As waves encounter a new material, they will slow down (or speed up), bending the wave.

Angle of Refraction: Which Way Does Light Bend?

If light travels from a high index of refraction to a low index of refraction, light bends away from the normal line. Example: glass to air.

If light travels from a low index of refraction to a high index of refraction, light bends toward the normal line. Example: air to glass.

Angle of Refraction: Which Way Does Light Bend?

Total Internal Reflection

Total Internal Reflection Total internal reflection occurs

when the angle of refraction is greater than 90°.

This only occurs when light travels from a medium with a higher index of refraction to a lower index of refraction. Water to air, for example.

Total Internal Reflection

When traveling from high n to low n, there is a critical angle where light refracts at exactly 90°.

When light travels from water to air, the critical angle is around 49°. At angles greater than 49°

the light is reflected, not refracted.

Fiber Optics

Fiber optic cables are thin, bendable pieces of glass wound together.

The glass allows total internal reflection of light over long distances, with very little loss of light.

Allows a very quick transfer of information with very little loss of clarity.

Dispersion

Dispersion

The index of refraction depends on the wavelength of light.

Or in other words: Light bends differently depending on its wavelength, and by extension, its color.

This is how prisms separate light. Blue light has a shorter

wavelength than red, so it bends at a greater angle of refraction.

Rainbows

Dispersion is most easily observed in nature as rainbows.

Water droplets act as prisms and disperse components white light at different angles.

Wrap-Up: Did we meet our objectives?

Calculate the wavelength and frequency of light. Describe what happens when light goes from one

material to another. Understand the index of refraction. Explain total internal reflection and dispersion.

Homework

Due Thursday: p. 518 Reviewing Concepts #11, 12, 14, 15 p. 519 #5, 6