Today’s Topic:

Refraction / Snell’s Law

Learning Goal:

Students will be able to calculate the angle

of reflection of a bent light wave.

Take out your notes from yesterday as

we learn about Snell’s Law.

Homework

Complete the Snell’s Law Worksheet

(Due Monday, 6/1)

Complete The Law of Reflection

Worksheet (Two Days Late)

Upcoming Test

Your last test will take place on Friday,

June 5th (a week from tomorrow).

Topics covered will include:

Color

The Law of Reflection

Refraction

Snell’s Law

Ray Diagrams (Converging & Diverging)

Light BendingNow that we know bends as it travels

through different mediums, one might

ask “how much does the light bend?”

The amount light bends is dictated by

Snell’s Law.

Snell’s law is a formula used to describe

the relationship between the angles of

incidence and refraction when light

refracts.

Snell’s LawSnell’s Law:

n𝑖 • sin θ𝑖 = n𝑟 • sin θ𝑟

Where θ𝑖 = angle of incidence

θ𝑟 = angle of refraction

n𝑖 = refractive index of incidence medium

n𝑟 = refractive index of refraction medium

Sample ProblemA light ray traveling through air strikes a

smooth, flat slab of crown glass

(n = 1.52) at an angle of 30° to the

normal.

What is the angle of refraction?

19.2049°

Snell’s Law ExampleLight travels from air (n=1) into an optical

fiber with an index of refraction of 1.44

a) In which direction does the light bend?

b) If the angle of incidence on the end of

the fiber is 22°, what is the angle of

refraction inside the fiber?

Snell’s Law Examplea) In which direction does the light bend?

Since the light is going from a low

index of refraction (n=1) to high

(n=1.44), the light is slowing down, so

the light will bend towards normal.

Snell’s Law Exampleb) If the angle of incidence on the end of

the fiber is 22°, what is the angle of refraction

inside the fiber?

n𝑖*sin θ𝑖 = n𝑟*sin θ𝑟

(1.00)*sin(22°) = (1.44)*sin(θ2)

sin(θ2) = (1.00/1.44)*sin(22°) = 0.260

θ2 = sin−1 (0.260) = 15.0786°

Reflecting LightWe now understand that light bends

when it travels through a different

medium.

Let’s combine this with our knowledge of

reflections.

When we look at a mirror,

light from an object reflects

off of a mirror.

Reflecting LightLight from an object bounces off of a

mirror, obey the law of reflection, and our

eyes see these reflections.

However, these rays almost appear to be

coming from behind the mirror.

The object we see

“behind” the mirror

is called an image.

Reflecting Light

Reflecting LightHowever, things start

to get a little strange

once we bend the

mirror.

By bending the

mirror, the image can

appear in a different

location or different

size.

Reflecting LightWe can see this in these examples:

What is a Lens?A lens is a piece of transparent material,

such as glass, that refracts light.

A lens forms an image by bending rays

of light that pass through it.

Where are lenses in this room?

LensesLenses can be found all over the place:

LensesAll of these objects bend light in specific

ways.

You already know the fundamental

science principles that govern the

bending of these objects (refracting and

Snell’s Law).

Let’s take a closer look.

Caxis

A convex surface is called “converging” because parallel rays converge towards one another

AIR (fast) GLASS (slow)

normal

fast to slow bends towards the normal

Convex Glass Surface

Caxis

The surface is converging for both air to glass rays and glass to air rays

AIRGLASS

normal

slow to fast bends away from the normal

Convex Glass Surface

Caxis

A concave surface is called “diverging” because parallel rays diverge away from one another

AIR GLASS

Concave Glass Surface

C axis

Again, the surface is diverging for both air to glass rays and glass to air rays

AIRGLASS

Concave Glass Surface

Converging Lens

“bi-convex”

Has two convex surfaces

Diverging Lens

“bi-concave”

Has two concave

surfaces

Types of Lenses

Types of LensesA converging lens, or a convex lens, is

thicker in the middle, and causes rays of

light that are initially parallel to CONVERGE

at a single point called the focal point.

Focal point

FF

Note that a lens has a focal point on both sides of

the lens, as compared to a mirror that only has

one focal point

Converging Lens

F

Similarly to a spherical mirror, incoming parallel

rays are deflected through the focal point

Converging Lens

ExampleWhat is an example of a converging lens?

A magnifying glass

Applications of Converging Lenses

Another application is inside of a camera.

A camera uses a lens to focus an image on

photographic film.

Types of LensesA diverging lens, or a concave lens, is

thinner in the middle, causing the rays of

light to appear to originate from a single

point.

FF

FF

With a diverging lens, parallel rays are deflected such

that when extended backwards, they appear to be

coming from the focal point on the other side.

Diverging Lens

ExamplesWhat is an example of a diverging lens?

A security mirror

Short and Far SightednessWe all have converging lenses inside of

our eyes.

Near and Far SightednessFor those of you that are nearsighted, your

eyeball is too long and images focus in front

of the retina.

Near and Far SightednessTo correct the way the light lands on your

eye, a concave lens acts to expand the

focal length.

Near and Far SightednessWhen someone is farsighted, the eyeball is

too short, so the image gets focused behind

the retina.

Near and Far SightednessTo fix this, convex lenses are used to

shorten the focal length.