15.1 Refractionpp. 562 -567

Mr. Richter

Agenda

Warm-Up Introduction to Refraction Demo Reminder: Quarter 3 Ends Friday Notes:

Refraction Index of Refraction Why Light Bends Refracted Images Snell’s Law

Objectives: We Will Be Able To… Recognize situations in which refraction will occur. Identify which direction light will bend when it passes

from one medium to another. Solve problems using Snell’s law.

Warm-Up:

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

What is really happening?

Refraction

Refraction

When light encounters a boundary, some of the light is reflected and, depending on the boundary, some of the light is refracted.

Refraction is the bending of a ray of light as it passes from one boundary to another.

When light is absorbed, it is not refracted.

Refraction

The angle of refraction θr (how much the light bends) depends on:

The media (materials) through which the light travels

The angle at which light strikes the boundary

Refraction

Your book says: “Refraction occurs when light’s velocity changes”

WRONG! OMG! Light always travels at the speed of light, no matter

what. However, it gets absorbed and reemitted more in some

mediums than others, so its “net speed” can be lessened.

Index of Refraction

The Index of Refraction

Every material bends light a different amount. The material’s index of refraction of a material

indicates how much light will be bent passing through that material.

The index of refraction of a medium is the ratio of the speed of light in a vacuum to the “speed” of light through the medium.

The Index of Refraction

Note the indices of refraction to the right (p. 564).

Air bends light very little, while diamonds bend light very well. Higher index =

more bending!

No indices below 1.

Wavelength and Refraction

Table 15-1 is only valid for wavelengths of 589 nm in a vacuum.

Different wavelengths have different indices of refraction.

This is why white light separates in a prism. Different colors have different wavelengths.

Why Light Bends

Index of Refraction: Why Light Bends

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.

Index of Refraction: Why Light Bends

The same thing happens with waves of light.

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

Refracted Images

Refracted Images

Objects appear to be at different positions due to refraction.

If light coming from the object (incident) goes to material with a lower index of refraction, the object appears farther from the normal line.

If ni > nr, then θi < θr

Refracted Images

The waves of light leaving the fish travels at one angle.

But as the light hits the boundary (air to water), part of the wave can move faster.

This changes the angle of the light that the cat sees, making the fish appear farther from the normal line. And higher up.

Refracted Images

If light coming from the object (incident) goes to material with a higher index of refraction, the object appears closer to the normal line.

If ni < nr, then θi > θr

Snell’s Law

Snell’s Law

Snell’s law determines the angle of refraction based on: the angle of incident light the indices of refraction from the two different media

Practice Problem

A light ray traveling through air strikes a smooth slab of crown glass at an angle of 30.0° to the normal. Find the angle of refraction, θr.

Given: θi = 30.0° ni = 1.00 nr = 1.52 (Table 15-1)

Calculations: θr = 19.2°

Wrap-Up: Did we meet our objectives?

Recognize situations in which refraction will occur. Identify which direction light will bend when it passes

from one medium to another. Solve problems using Snell’s law.

Homework

p. 567 #1-4