Today’s Topic:

Ray Diagrams – Intro to & Converging Learning Goal:

Students will be able to describe the

resulting image of light once it passes

through a converging lens. What is a focal point?

What happens at the focal point of a

converging lens?

What happens at the focal point of a diverging

lens?

Homework

Complete the Snell’s Law Worksheet

(One Day Late)

Complete The Law of Reflection packet

(Four Days Late)

Upcoming Test

Your last test will take place on Thursday,

June 4th (a week from yesterday).

Topics covered will include:

Color

The Law of Reflection

Refraction

Snell’s Law

Ray Diagrams (Converging & Diverging)

Recap A 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

Principal Axis

• F

Converging Lens

Principal Axis

Light rays that come in parallel to the principle

axis converge at the focal point.

• F

Converging Lens

Recap A diverging lens, or a concave lens, is

thinner in the middle, causing the rays of

light to appear to originate from a single

point.

F F

• F

Principal Axis

Light rays that come in parallel to the principle

axis diverge from the focal point.

Diverging Lens

Images All visible objects emit or reflect light rays in

all directions.

Images Our eyes detect these light rays.

Images We see images when light rays converge in

our eyes.

Mirrors and Lenses It is possible to see images in mirrors and

lenses as well.

object

image

Reflecting Light Light 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 However, 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.

Ray Diagrams By constructing a ray diagram, we can

determine where the image is located,

and what it will look like.

A ray diagram is a diagram showing rays

that can be drawn to determine the size

and location of an image formed by a

mirror or lens.

Ray Diagrams A ray diagram looks something like this:

Let’s take a step back and see this.

Key Features of a Lens Like a wave, lenses have some key

features we need to learn.

The principal axis of a lens is the line

joining the centers of curvature of its

surfaces.

Key Features of a Lens

The focal point (for a converging lens) is

the point where a beam of light parallel to

the principal axis converges.

Key Features of a Lens

The focal length is the distance between

the center of the lens and its focal point.

Key Features of a Lens

This is a less scary example of a completed

ray diagram.

Let’s learn the rules in constructing one of

these things.

We’re only going to focus on converging

lenses today.

Ray Tracing Rules To create a ray diagram, you will need to

draw two of the three following rays:

Ray 1: A ray parallel to the principal axis

that passes through the focal point on the

opposite side

F F

Ray Tracing Rules Ray 2: A ray that passes through the

center of the lens that is undeflected.

F F

Ray Tracing Rules Ray 3: A ray that passes through the

focal point in front of the lens that

emerges parallel to the principal axis

after refracted by the lens.

F F

Putting It All Together Draw the image where the rays intersect!

• F object

Converging

Lens

image • F

Image Descriptions Images can be described with three

classifications:

A real image is an image where the light

from the object actually converges to

form an image which can be seen on a

screen.

A virtual image is an image that appears

to be in a location where light does not

really reach.

Lens Effects Mirrors and security

mirrors produce

virtual images – the

light does not really

reach behind the

glass, it only

appears to do so.

The projector is producing a real image – it

can be seen on a screen.

Image Descriptions Images can be described with three

classifications:

An upright image has the same vertical

direction as the object.

An inverted image is flipped upside-

down.

Magnification: The image is larger (than

the object), smaller, or the same size.

More Examples

Describe the image formed above. Real

Inverted

Same Size

F F

More Examples

Describe the image formed above. Real

Inverted

Larger By moving the object closer to the lens, the

image is now magnified.

F F

More Examples

Let’s bring the object even closer! Uh oh.

In this case the rays are diverging – they do

not intersect on that side of the lens.

F F

this distance is increasing

More Examples

But if we extend the rays backwards…

The image will form here. This is a virtual

image. This configuration is how a magnifying glass works.

F F