November 6th, 2015 Katie Hellier IRIS Science Academy Week 8: Mirrors & Lenses November 6th, 2015 Katie Hellier IRIS 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 1 – light bounces off nose towards mirror 2 – hits mirror, bounces off – observer can see these rays from many angles 3 – if we trace the rays backwards, they all converge to one point – this is what the observer at the end of the light ray sees This happens at every point on the object (face), so the observer sees the full object (face) as the virtual image (ie the reflection) Since all the angles are the same, the image appears the same distance behind the mirror as the actual person is in front of the mirror self-check: Imagine that the person in figure u moves his face down quite a bit --- a couple of feet in real life, or a few inches on this scale drawing. Draw a new ray diagram. Will there still be an image? If so, where is it visible from?

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 This is a concave mirror; still symmetric. Real lights rays converge (bend towards the center); imaginary parts move outward. Curve causes the normal to be changing continuously, so the angles are smaller compared to the original normal; the imaginary portion of the ray gets pushed back (ie smaller angle  longer distance) Longer distance for the rays to converge means the object appears farther behind the mirror, even though they aren’t Since all distances to be greater, the chin appears farther away from the nose, etc  magnification!

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 Convex Lens Concave Lens Object is inside focal length Object is inside focal length Object is outside focal length Object is outside focal length

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 Diagrams Rules for drawing a Ray Diagram: Any ray that enters parallel to the axis on one side of the lens proceeds towards the focal point F on the other side. 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. 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! This equation tells us where we can expect to see things 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.