Notes 23.1: Optics and Reflection

Angle of incidence = angle of reflection Law of reflection – Incident rays and reflected rays make equal angles with a line perpendicular to the surface (normal) Angle of incidence = angle of reflection

Image Formation by a Plane (Flat) Mirror: This type of image is called a virtual image because it is formed at a place where there is no light from the object. The mirror simply makes the light appear to be coming from behind it.

Virtual image – image that is perceived to be coming from where the light does not actually originate Plane mirrors always produce virtual images. They are the same distance behind the mirror as the object is in front and the same size.

Two types of reflection: Diffuse reflection – light reflected from a rough surface which scatters in many directions If the surface of the body is irregular, the reflection will be irregular or diffuse. The reflected beam of light therefore carries information about the nature (color, texture) of the surface but you cannot see any image or reflection from that surface.

Specular reflection – reflection that occurs off smooth surfaces A highly polished, smooth surface (for example, a mirror) reflects light in a more regular way. The light reflected by a plane (flat) mirror does not contain information about the mirror itself: when you look into a mirror you see an image of "yourself".

Notes 23.2: Refraction

Refraction : - the change in direction of a light wave as it crosses into new material. - this is a result of the wave traveling a different speed in the new material.

So how does light bend? When light enters a medium where it slows down, the ray is bent toward the normal. When light enters a medium where it speeds up, the ray is bent away from the normal.

Think of this analogy: The front axle of a wagon is rolling on concrete. It comes to a sandy beach. One wheel enters the sand first and slows down. The other wheel is still moving at faster speed on the concrete. This causes the wagon to turn toward the normal. Reverse the direction and it bends away from the normal.

Index of refraction - ratio of the speed of light in a vacuum to the speed of light in the new material. Represented by “n” n=speed of light in vacuum / speed of light in medium The greater the index of refraction, the more the light is bent and the slower it travels. Vacuum n = 1.0 Air n = 1.0001 Water n = 1.33 Glass n = 1.52 Diamond n = 2.42

Spear-fishing is made more difficult by the bending of light Spear-fishing is made more difficult by the bending of light. To spear the fish in the figure, one must aim at a spot in front of the fish

TOTAL INTERNAL REFLECTION When light is traveling into a new medium where it’s speed increases (from water into air) the angle it makes in the air is larger than the angle in the water.

Eventually the angle in the water will get large enough that the light will not travel into the air but will completely reflect back down. (The light will not travel out of the water.) Critical angle: Angle where the light totally reflects Water = 49o Glass =42O

Application of Total Internal Reflection : Fiber Optics – long thin strands of glass or plastic that can carry very large amounts of data encoded in light rays

Prisms Dispersion- the separation of light into its different colors due to refraction. Blue light travels slower in glass than red light. The blue light is bent more than the red.

Rainbows are caused as light is both reflected and refracted in a raindrop

Notes 23.3: Lenses and Images

Ray Diagrams for Pinholes: Object- real physical thing that gives off or reflects light Pinhole and Screen Light ray – straight line arrow that represents a single “beam” of light. (need 2 rays) Image – “picture” of the object that is formed where the rays land on the screen

Two types of Images: Virtual Image- image is perceived by the brain and eye as being some place that the light does not come from. Ex: image in a plane mirror is behind the mirror Real image – images are formed where the light rays actually come together; can be projected on a screen. Ex: pinhole, camera

Two Types of Lenses: 1. Converging lens – bends the light rays together so they meet at a single point. Ex: magnifying glass Also called convex, they are thicker in the middle than at the edges

Focal point – the point where parallel light meets when in travels through a lens. Focal Length – distance from center of the lens to the focal point. (measured in cm, m)

2. Diverging lens – bends light outward away from the optic axis. Ex: Some eye glasses Also called a concave lens, it is thinner in the middle than at the edges.

Ray diagrams for lenses will have: An object At least two light rays (we will draw three) Optic axis – imaginary line going through the center of the lens Two focal points on each side of the lens. An image (may be real or virtual)