Mirrors & Reflection

LightWe see objects because of reflected light

Travels far and fast (3 x 108 m/s) Travels in a straight lines, called rays

Reflection ReviewLaw of reflection: θi = θr Light rays bounce off a

mirror at the same angle at which they arrive

We always define angles relative to the normal (the line perpendicular to the mirror (or lens)

Types of Mirrors Plane Mirror –

A flat mirror that reflects light in a regular way Concave Mirror –

Reflects light from inner surface Light rays are reflected so that they “come together” at a

point; a converging mirror Convex Mirror –

Reflects light from outer surface Light rays are reflected so that they “go apart” and never

come to a point; a diverging mirror

Object vs. Image Object – the source of light rays

Ex: you when you look in the bathroom mirror Image – reproduction of object formed with

lenses or mirrors. The image is formed at the intersect of the

reflected rays.

Kinds of Images Real images

Formed by converging light rays Can be projected on a screen Inverted orientation

Virtual images Formed by diverging light rays Cannot be seen on a screen Erect orientation

Reflection & Mirrors A mirror reflects rays of light so that they change

their path Mirrors can create a virtual image

Image appears behind the mirror and is reversed The light rays are reflected back to your eye at an equal

but opposite angles Incident ray – the ray that comes from the object

and hits the mirror Reflected ray – the ray that bounces off the mirror

Objects, Images & Plane Mirrors Plane mirror – flat, smooth surface that

reflects light in a regular way Ex: your bathroom mirror ObjectObject ImageImage

Things to know about Plane Mirrors Object size = Image size Object distance = Image distance Orientation = Upright Forms a virtual image Image is reversed (left to right)

Drawing Ray Diagrams – Plane A ray striking perpendicular to the surface

(parallel to normal) will reflect perpendicular to the surface; the reflected ray is extended behind the mirror

A ray striking at any angle will reflect so that θi = θr; the reflected ray is extended behind the mirror to form the image

Refraction & Lenses A lens uses refraction to cause light to come

together or spread apart Refraction – The bending of light as a result

of light crossing a boundary between two different media EX. Glass, Plastic, Water

Lens – a transparent optical device that is used to converge or diverge light rays (bend light)

Lens Types

Convex Lens (converging lens) – bend parallel light rays passing through them inward toward the focal point Thicker in the center than the edges Light travels slower in the thick center Focal length (f) is always positive

Things to know about Convex lenses… If the object is outside the focal point, it is

real & inverted If the object is at AT the focal point, no

image is formed If the object is inside the focal point, the

image is upright & virtual. Let’s take a look…

The lens equation Mathematical prediction of image location

1/f = 1/di + 1/do

f = focal length di = image distance

do = object distance

Conventions to Know: Focal Length

f is positive for convex lenses Object Distance:

do is positive for REAL objects do is negative for a virtual object*

Image Distance di is positive for a real image (image on other side

of lens) di is negative for a virtual image (image on the

same side of the lens)

Example 1 An object is placed 35 cm from a convex lens

with a focal length of 20 cm. How far is the image from the lens? What type of image is formed?

Example 2 A virtual image is formed 20 cm from a

convex lens having a focal length of 20 cm. How far is the object from the lens? What is the orientation of the image?

Drawing Ray Diagrams - Lenses1) Incident light rays parallel to the principal axis

of a lens are refracted through the focal point (F)* (ray 1)

2) Incident rays that pass through the secondary focal point (F’) are refracted parallel to the principal axis (ray 2)

3) Incident rays passing through the center of lens are not refracted (ray 3)

F’ F

F’ F

F’ F

F’ F