Geometric Optics Geometric Optics: The process of using light rays to determine how light behaves when it strikes an object. Light travels in a STRAIGHT LINE

ANOTHER WAY TO THINK OF LIGHT Light leaving the candle travels in all directions, but we only SEE the light that travels in to our Eye-Brain

ANOTHER WAY TO THINK OF LIGHT (CONT’D) We don’t see light that doesn’t enter our eyes.

ANOTHER WAY TO THINK OF LIGHT (CONT’D) If the light going away from eyes is reflected in to our Eye-Brain, then we will see it.

SIDE NOTE When we look at diagrams, we will ignore light that doesn’t enter our Eye-Brain. Don’t forget that the other light exists, we are just ignoring it to simplify our lives!

RAYS & DIAGRAMS Ray Diagrams show how light moves from the object to the eye. Light Ray – a line on a diagram representing the direction and path that light is travelling.

LUMINOUS SOURCES Atoms in LUMINOUS OBJECTS emit light rays in all directions produced from other energy sources. Atoms in NON-LUMINOUS OBJECTS scatter the light rays from luminous objects in all directions.

NOTE Atoms in all objects produce or scatter light rays. This diagram only shows light rays from atoms at the top and bottom of the objects.

KEEP IT SIMPLE To make diagrams simpler, we only show one ray of light from the top and bottom of objects. We simplify it even more and just show only the rays coming from the top of the object.

MEDIUMS A TRANSPARENT MEDIUM allows nearly all rays to pass straight through unaltered. (Ex. Air) An OPAQUE MEDIUM absorbs or scatters all the rays. (Ex. Textbook, wall, etc.)

MEDIUMS (CONT’D) A TRANSLUCENT MEDIUM transmits and scatters the rays. Wax paper causes light rays to bounce off and scatter, so you can see light coming through but no clear image.

The Laws of Reflection

Reflection - Images in Plane Mirrors

Where can we see an image of ourselves?  a mirror.  calm water surface.  a piece of shiny metal wall.  a piece of window glass.

What are mirrors? A mirror is any polished surface that allows an image to be produced through reflection. An image is a reproduction of the original object in front of the mirror that is produced through the use of light.

Mirror Normal Angle of incidence Angle of reflection Reflected ray Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

Properties of Light that Lead to Reflection Light reflects off of ALL surfaces A beam of light can be thought of as a group of individual rays of light all travelling together Make sure to stress that light reflects off all surfaces – it is a common student misconception that light only reflects off shiny surfaces

Laws of Reflection Investigation Page 482

The Two Laws of Reflection

Angle of incidence = Angle of reflection LAW #1 Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at the same angle it hits it. normal incident ray reflected ray mirror

LAW #2 The incident ray, the reflected ray, and the normal all lie in the same plane.

Question 1 The angle between an incident ray and the mirror is 20. The angles of incidence and reflection are ____ and ____. 20

Question 1 The angle between an incident ray and the mirror is 20. 70 70 The angles of incidence and reflection are ____ and ____. 20

P.O.E Predict! Will there be a difference if I shine light off of a smooth piece of aluminum foil compared to if I shine light off a crumpled piece of aluminum foil? VS

P.O.E Observe!

P.O.E Explain! This can be explained with the terms specular reflection and diffuse reflection!

Specular Reflection Light reflects off a smooth, shiny surface Examples: plane mirror, still water, smooth piece of aluminum foil Produces a clear reflection parallel incident rays parallel reflected rays

Diffuse Reflection Light reflects off a rough, not perfectly smooth surface Examples: cover of a book, waves, a crumpled piece of aluminum foil Produces a diffuse (unclear) reflection parallel incident rays reflected rays in different directions

Does the angle of incidence = angle of reflection? True or False? Reflection of light by a rough surface does not obey the 2 laws of reflection. Does the angle of incidence = angle of reflection? Does the incident ray, normal, and reflected ray lie in the same plane?

Does the angle of incidence = angle of reflection? True or False? Reflection of light by a rough surface does not obey the 2 laws of reflection. Does the angle of incidence = angle of reflection? Does the incident ray, normal, and reflected ray lie in the same plane?

Even in diffuse reflections, each incident ray is reflected back at the same angle it strikes the object! Angle of incidence = Angle of reflection Additional example: Textbook page 485 Figure 5

Specular Reflection A specular reflection is reflection off a smooth surface The light bounces off in a predictable and consistent way

Specular Reflection A specular reflection is reflection off a smooth surface The light bounces off in a predictable and consistent way

Diffuse Reflection A diffuse reflection occurs off a surface that is not smooth Light rays are scattered and reflect in many different directions Even microscopically rough surfaces will produce this effect

Diffuse Reflection A diffuse reflection occurs off a surface that is not smooth Light rays are scattered and reflect in many different directions Even microscopically rough surfaces will produce this effect

Parabolic (curved) Mirrors Types of Mirrors Plane Mirrors Parabolic (curved) Mirrors

Properties of Plane Mirrors The image is right-side-up (not inverted) Left and right are reversed The image is the same size as the object (unmagnified) Light reflects with the properties of angles discovered earlier this class May choose to wait until Day 10 when you explore mirror ray diagrams to outline the remainder of the properties of plane mirrors (e.g. image is virtual, virtual image distance = object distance)

Challenge Question: Which of these appears on the front of an ambulance?

The writing on an ambulance is reversed because… The image in a plane mirror is reversed Therefore, this is what we see when we look in the rearview mirror

More examples…

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Question What letters of the alphabet will appear the same in a plane mirror? A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Question What letters of the alphabet will appear the same in a plane mirror? A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Using Light Rays to Locate an Image We know that light travels in straight lines! Our brain doesn’t understand that light reflects When our eyes detect reflected light from a plane mirror, we project these rays backwards in a straight line

Source and Apparent Source Light Source Apparent Light Source Virtual rays are shown using dotted lines

Using Equal Perpendicular Lines to Locate an Image 3 cm 3 cm Two Helpful Hints: The distance from the object to the mirror is the same as the distance from the image to the mirror 2.7 cm 2.7 cm If the tip of the pencil is 3 cm away from the mirror, the reflection of image will also be 3 cm away

Using Equal Perpendicular Lines to Locate an Image Two Helpful Hints: The object-image line is perpendicular to the mirror surface

Diagram The Ray Diagram shows that: ON = IN OI ⊥NM N I O M

So how do we draw ray diagrams? Locating Images So how do we draw ray diagrams?

Drawing the virtual image Step One: Choose several points on the object and draw a perpendicular line to the mirror

Drawing the virtual image Step Two Measure the distance of this line 1 cm 1.5 cm 2 cm

Drawing the virtual image Step Three Extend this line an equal distance behind the mirror 1 cm 1.5 cm 2 cm

Drawing the virtual image Step Four Repeat these steps until you have enough lines to accurately locate your image 1 cm 1.5 cm 2 cm Virtual image = an image in which light does not actually arrive at

Drawing the virtual image Step Five Draw in the reflected ray 1 cm 1.5 cm 2 cm Virtual image = an image in which light does not actually arrive at

Drawing the virtual image Step Five Draw in the incident ray 1 cm 1.5 cm 2 cm Virtual image = an image in which light does not actually arrive at

Practice Question 1 Mirror

Practice Question 1 Mirror

Practice Question 2 Mirror

Practice Question 2 Mirror

Going back to the beginning…

Four Characteristics of an Image But…. How are we suppose to remember this? SALT

S stands for SIZE What is the size of the object? Larger, Smaller, or the Same Size as the object? LARGER SAME SMALLER

A stands for ATTITUDE How is the image oriented? Is the image upright or inverted? UPRIGHT INVERTED

L stands for LOCATION ? Is the image in front or behind the mirror? Is the length from the object to mirror the same as the image to mirror? ?

T stands for TYPE Is the image real or virtual? A real image is formed when light is actually arriving at the image location A virtual image is formed from an apparent light source VIRTUAL REAL

Question: What is the SALT for plane mirrors? Same Size Upright Behind the Mirror (the same distance behind) Virtual