Week 8: Mirrors & Lenses

November 6th, 2015Katie HellierIRIS Science Academy

Last Week

•Light – what is it?

•Absorption – takes in photons

•Reflection – photons bounce off

•Refraction – photons pass through material, but are bent

Video

• Concave and Convex Mirrors• https://youtu.be/jtTBOMVMSYM

Reflection - Mirrors

• Light bouncing off mirror creates a virtual image “behind” the mirror• Virtual image – light rays

look like they come from a point from behind the mirror to the observer (but there’s nothing behind the mirror)

• Angles on each side of the normal are equal – angle formed on other side of the mirror is the same

Curved Mirrors• Mirrors can be curved, like in a funhouse• Real light rays bend toward the center; imaginary move outward

• Changes how “far back” the image forms

• This creates a distorted shape, magnified or shrunken

• Magnification (m) is the scaling factor of the image

Real Images

• Real image is when light rays converge to one point (not actually a material thing)• In a curved mirror, if

the object is far back enough, an image can be formed (on a piece of paper, for example)• It’s also inverted!

Video

• How do Lenses Work• https://youtu.be/qbPfzOuIeSM

Refraction - Lenses

• Lenses allow light to pass through, but bend it (like glasses)• Can create real and virtual images based on curvature• Images can be larger, smaller, upright, inverted

Object is outside focal length

Object is outside focal length

Object is inside focal length

Object is inside focal length

Convex Lens Concave Lens

Prism

• Prism – transparent optical device that refracts light• Two polished, flat sides must have an angle

between them• The material used

changes how much each wavelength is bent dispersion

Magnifiers

• Use convex lenses to create a magnified, virtual image• Level of magnification depends on location

between object and eye• Can be used to concentrate light (eg magnifying

glass and the sun)

Microscopes and Telescopes• Both create images of optics• “Micro” – Greek word meaning small• “Tele” – Greek word meaning far• “Scope” – Greek word meaning to see

• Refracting microscopes & telescopes require 2 lenses to create an image

• Reflecting telescopes use 1 lens and mirrors• The largest telescope is 30m (~100ft) mirror in Mauna Kae, HI!

Video

• How telescopes work• https://youtu.be/WvLspPgC1EU

Ray DiagramsRules for drawing a Ray Diagram:1. Any ray that enters parallel to the axis on one side of the

lens proceeds towards the focal point F on the other side.2. Any ray that arrives at the lens after passing through the

focal point on the front side, comes out parallel to the axis on the other side.

3. Any ray that passes through the center of the lens will not change its direction.

The Lens Equation

• Tells us where we can expect to find the image/object• f – focal point: distance from the lens to its focus• o – object: distance from lens to the object• i – image: distance from lens to the image

• Don’t forget to pick one side of lens as positive, the other as negative distance!

Object

Image

Lab – Building an Optical Device

• Solar Ovens - use your understanding of reflection, absorption, lenses and mirrors to make a solar oven.

• Telescopes – use your understanding of lenses and the lens equation to make a refracting telescope.