 Mirrors & Reflection.

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Mirrors & Reflection

Light We see objects because of reflected light
Travels far and fast (3 x 108 m/s) Travels in a straight lines, called rays

Reflection Review Law 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 – Concave Mirror – Convex 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 Object Image

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 1/f = 1/di + 1/do
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 Object Distance: Image Distance
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 - Lenses
Incident light rays parallel to the principal axis of a lens are refracted through the focal point (F)* (ray 1) Incident rays that pass through the secondary focal point (F’) are refracted parallel to the principal axis (ray 2) Incident rays passing through the center of lens are not refracted (ray 3)

F’ F

F’ F

F’ F

F’ F