1. November 6th, 2015 Katie Hellier IRIS Science Academy
Week 8: Mirrors & Lenses
November 6th, 2015
Katie Hellier
IRIS Science Academy

2. Last Week Light – what is it? Absorption – takes in photons
Reflection – photons bounce off
Refraction – photons pass through material, but are bent

3. Video
Concave and Convex Mirrors

4. 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?

5. 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!

6. 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!

7. Video
How do Lenses Work

8. 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

9. 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

10. 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)

11. 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!

12. Video
How telescopes work

13. 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.

14. 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

15. 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.