10.3 Ray Model of light

Remember in our first talk, we discussed how images that are formed by light are created by BILLIONS of light rays coming from a source

It would be near next to impossible for us to keep track of EVERY LIGHT RAY that formed an image

Physicists have figured out that since light rays all behave the same, if we know what one is doing, the others will probably be doing the same thing

So instead of having to keep track of all the light rays – we can use a few to sketch out how the image is formed

This study of physics is known as GEOMETRIC OPTICS

LIGHT RAYS ALWAYS MOVE IN STRAIGHT LINES

Arrows are used to show the direction light is travelling in

The more rays that reach your eye, the brighter the object appears

Ray Model of Light

Transmitting light Ray diagrams are used to show what

happens when light strikes an object Transparent – transmit light freely ex. Glass Translucent – transmit some light ex. Frosted window Opaque – objects absorb and reflect light and do not

transmit it ex. cardboard

Shadows Some shadows are sharp and well defined

while others are not … why?????

A shadow occurs when an opaque object blocks the direct light from a light source

The size of a shadow depends on the size of the object blocking the light and its distance from the light source

Small light source Casts sharp shadows that are well defined

Large light source Shadows with not have a sharp edge

because the object only partly blocks the light

The wider the light source, the more blurred the shadows will be

The umbra part of the shadow is the part where all of the light rays from the source are blocked\

The partial shadow from a non-point light source is called a penumbra

Light Reflection Regular reflection – light rays strike a

smooth surface and reflect in the same direction, staying parallel to one another

All rays are reflected at the same angle

Diffuse Reflection When light rays reflect off a rough or

uneven surface, they do not remain parallel but are scattered in different directions

Homework P. 406 #1-4 P. 409 #1-10

The angle of incidence of an incident light ray is always equal to the angle of reflection of the reflected light ray, as measured from the normal as long

as the incident and reflected ray lie on the same surface

i r

normal

Where: i = r

Because light rays travel in straight lines, many of the diagrams we draw end up having regular shapes in them like triangles and rectangles

Optics is the study of light and the images formed by light

Since the we use GEOMETRY to study angles and lines in math, the type of optics we are studying is referred to as GEOMETRIC OPTICS

Ray diagrams are used in Geometric optics to outline how light rays will travel and ultimately form images in sensors like our eyes or cameras

Let’s see how ray diagrams work by looking at a pinhole camera

A pinhole camera works very much like our eyes and modern cameras in how they receive light rays

It is simply a box with only one tiny pinhole to allow light through

On the other side of the box is a tiny screen or a place where film can be attached to

In partners you will build a pinhole camera Take a dark piece of construction paper Roll it into a long tube Cover one end with aluminum foil to create the

pinhole lens, attach with tape Using a pin, prick a small hole into the

aluminum foil Cover the other end with wax paper to create

the screen, attach with tape Looking into the wax paper side, try to focus

your camera on the flame of the candle

A pinhole camera creates an image that is INVERTED (upside down), REAL (the image can be formed on a screen)

The size of the image depends on how far the candle flame is from the pinhole

Also – you should notice that sometimes the flame was out of focus – only when your camera was a certain distance to the flame could you see the image

OBJECT

Light ray from top of object can only enter into camera by projecting to the lower half of the camera

Light ray from bottom of object can only enter into camera by projecting to the upper half of the camera

Image formed is real, smaller and inverted compared to object

Can you see the similar triangles? Notice that as the changes position relative to the pinhole, the size of the image changes too

INVERTED: upside down UPRIGHT/ERECT: right side up HEIGHT OF IMAGE: hi HEIGHT OF OBJECT: ho DISTANCE OF IMAGE: di DISTANCE OF OBJECT: do

All distances are measured relative to the interface that directs the light rays

An image can be smaller, larger or same size as the object

OBJECT

IMAGE

hiho

INTERFACE (ie. Lens, pinhole, mirror)

di do

That means that cameras are built to change the size of the image based on the relationships between the similar triangles outlined by the light rays travelling into them

You should notice this about your eyes too – think about how an object’s size looks as it closer and closer to you!

Therefore, the magnification of images in a pinhole camera is defined by the relationship of how hi and ho change if di and do change

If do changes, di changes; and hi and ho change by the same factor

Therefore: M = hi = di

ho do

This relationship is similar to what you will see in curved mirrors and lenses

So how do mirrors change how an object looks when light from that object reflects off the mirror?

In a plane (flat) mirror, the image is:◦ The same height as the object◦ It is the same distance behind

the mirror as the object is in front of it

◦ The image is reversed◦ The image is VIRTUAL

Your brain, being an incredible computer, measures things like distance by calculating the time it takes for light to travel from the object you are looking at to your eyes

Light rays from an object that is further away will reach your eyes after those that come from an object that is closer

This changes the relationships between hi, ho, di and do

Based on this, you brain can judge the distance the two objects are in relation to you

The light from an object in front of a mirror has to travel to the mirror, and then bounce off it to return to your eyes

But your eyes can’t tell the difference between reflected and incident rays

Plus – your brain knows that light rays always travel in straight lines

That means that your brain thinks that the object is two times farther away than it actually is and is behind the mirror

IMAGE IS FORMED THE SAME

DISTANCE BEHIND THE MIRROR AS

THE OBJECT IS IN FRONT OF IT

Images are reversed because light rays that hit a plane mirror are reflected right back along the same path

That means a point on your left side is reflected off the right side of the mirror – therefore making it look like the image is reversed when it returns to your eyes

L R

Remember that virtual images are often described as “images that cannot be formed on a screen”

Real images are ones that “can be formed on a screen” (we’ve seen this with the pinhole camera)

But it is difficult to relate when we think about mirrors, because we look into the mirror – not the screen

It is very difficult to tell what type of image a mirror would create based on these definitions

The best way to think about it is to ask: Where is the viewer relative to the mirror/lens?

If the image/object is on the same side as the viewer – the image/object is REAL

If the image/object is on the opposite side of the viewer – the image/object is VIRTUAL

VIEWER

THIS IMAGE IS ON THE OPPOSITE SIDE OF THE MIRROR RELATIVE TO THE VIEWER – SO IT IS VIRTUAL

MIRROR