Chapter 13 Part 4 Reflection, Mirrors, images and Ray diagrams Basic Optical Devices Chapter 13 Part 4 Reflection, Mirrors, images and Ray diagrams

Basic Optical Devices are used to… Change the Direction of a Ray of Light Gather parallel rays of light and focus them at the Focus or Focal Point This focus can be… Real, that is the light rays really meet there or Virtual that is the rays only appear to meet there Disperse white light into a spectrum Create an Image which is… Real and can be Projected onto a screen (its really there) Virtual which cannot be projected (it just appears to be there!)

Two types of Reflection How light reflects off a surface depends on the texture of the surface. Rough (microscopically) surfaces reflect parallel rays in different directions (see diagram) Creating a Diffuse Reflection Paper, cloth, rough wood etc, reflect diffusely Smooth (microscopically) reflect parallel rays in the same direction, a Specular Reflection Glass, polished wood, a waxed floor, & a mirror are examples of specular reflection

Mirrors Mirrors are smooth reflecting surfaces that have nearly perfect specular reflection There are 3 basic types of mirrors by shape Flat, Convex and Concave Flat mirrors are used to change the direction of a ray or to create a virtual image of equal size Ray diagram rule for mirrors: To draw the ray diagram just follow the law of reflection!

Ray Diagrams To draw a ray diagram follow these steps: Draws the optical device as a “cut away” Draw axis perpendicular to the center of the optical device Draw a picture of the object, or an arrow, from a point at where it is, perpendicular to the axis Draw two rays from the top of the object: one parallel to the axis and the other to the center of the device Follow the reflection or refraction laws, and those specific to the device to draw the ray after it encounters Add dashed rays for where rays “seem” to have gone 7. Where the rays meet (or seem to) will be an image. A real image if they meet, virtual if they only seem to.

Math and Optics Geometry allows us to describe and predict the location and magnification of the image For mirrors: p = distance from object to Mirror always positive, that is in front q = distance from image to Mirror if q > 0 image is real and appears in front if q < 0 image is virtual and appears behind mirror f = focal length (from focus to mirror) f > 0 if focus is real, f < 0 if focus is virtual

Focal Lengths of Mirrors A Flat Mirror has no focus so Its focal length is infinite and positive A Concave Mirror has a real focus in front so f > 0 A Convex Mirror has a virtual focus in behind so f < 0 The Focal length of a Spherical Mirror is half the radius of the mirror f = ½ R

Lens and Mirror Formulae For all lens and mirrors: Magnification is: If M > 0 it is erect, If M< 0 it is inverted

Convex Mirror Convex: (Curves out) Has a virtual focus always creates a smaller virtual image it is rarely used in telescopes, but gives a wide if warped view, and so they are used as rear-view mirrors and in stores Ray Diagram Rules: The ray to the center follow law of reflection The parallel to the axis ray should reflect so that it seems to come from the virtual focus

Concave: (Curves in) Used to gather light & focus it into a real focus, Creates an erect virtual and larger image of an object placed closer than its focus Create inverted real image Of an object placed farther away than the focus These images get smaller the farther away the object is Concave mirrors are often used in telescopes