What do we know already?

Light always travels in a straight line When Light hits a surface it does one of two things Get absorbed by the surface Reflect off the surface Reflection – the change in direction of an EM wave at a surface that causes it to move away from the surface.

Reflection Depends on the Surface A ray of light striking a highly reflective surface such as a mirror and reflecting A ray of light striking a rough surface and being scattered and not reflected

Three Main Types of Mirrors Flat Concave Convex

Flat Mirrors θi θr normal Incident ray Reflected ray Light strikes the surface at an angle Θi = incident angle: the angle between a ray that strikes a surface and the line perpendicular to that surface at point of contact Θr = reflected angle: the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves

Θi = θr Angle of incoming light ray = angle of reflected light ray Law of Reflection θi θr Θi = θr Angle of incoming light ray = angle of reflected light ray

Why we see an image behind the mirror What is actually happening: Light is reflected off the mirror by the Law of Reflection: θi = 0° = θr What we see: Our brain cannot see light bending: It assumes that the reflected ray is traveling directly from something at the spot of the image Incident Ray Reflected Ray The “unbent” ray that our brain makes up h h’ mirror object image

You can predict Image location by use of Ray Diagrams q p h h’ mirror object image

Who Can See Who in a Flat Mirror? Make a mark where the image would be located Draw a straight line from a person to another’s image If this line passes through the mirror, they can see each other A B C A can see B B can see A But C can see neither A nor B

Curved Mirrors Concave Convex

The Set Up for Concave Mirrors Object Principle Axis C F f : focal length R: Radius p = object distance

Focal Length (and Focal Point) Positive for Concave Mirror Negative for Convex Mirror Focal Point – The point where every ray that hits the mirror will pass through There can be no image formed when an object is placed at the focal point **all rays will

How is an Image Formed? An image is formed when reflected light rays intersect at a common point If rays never intersect (parallel to each other), no image is formed

Two Rules of Note! Why so important???? Any ray that is originally traveling parallel to the principal axis will reflect through the focal point Any ray that originally travels through the focal point will reflect parallel to the principal axis Why so important????

Rules for Drawing Ray Diagrams C f 1. Draw a parallel ray from the object to the mirror and reflect through focal point 2. Draw a ray from the object through the focal point to the mirror and reflect back parallel to principal axis 3. Draw a ray from the object to the mirror through the center of curvature and reflect back along the same line (usually not used)

The point where all three lines meet is where your image will be located Try this one! An object is placed 10 cm from a concave mirror with a focal length of 6 cm **Use a ruler and draw your picture to scale**

Convex Mirror Set Up c f Focal point is NEGATIVE

Imaging with a Convex Mirror Tiny image formed 1) Parallel then through focal point c f 2) Through focal point then parallel Regardless of where an object is placed… A Convex Mirror will always produce: SMALLER, UPRIGHT image

Real vs. Virtual Image Virtual image: refers to an image that only “appears” to be behind the surface of the mirror. The image is not located at this apparent position and can’t be focused on a screen. Located on the opposite side of the mirror as the object Real Image: refers to an image that can be focused on a screen placed at a position where rays from a point on an object pass through a common point Located on same side of the mirror as the object

Mirror Equation f = focal length do = object distance di = image distance *it does not matter the units for this equation HOWEVER all distances must be in the SAME units

Magnification Equation hi = Image height ho = object height What the sign of the magnification means: negative Inverted image positive Upright Image

Practice Problem! A 4.00-cm tall light bulb is placed a distance of 45.7 cm from a concave mirror having a focal length of 15.2 cm. Determine the image distance and the image size. Info we Know:

Sign Conventions f is (+) if the mirror is a concave mirror f is (-) if the mirror is a convex mirror di is (+) if the image is a real image and located on the object's side of the mirror. di is (-) if the image is a virtual image and located behind the mirror. hi is (+) if the image is an upright image (and therefore, also virtual) hi is (-) if the image an inverted image (and therefore, also real)