Unit 10: Light Reflection and Refraction I. Refraction
Demo: Straw in beaker full of water, half full of water, and half water/half oil A. Circumstances and Outcomes
1. As light passes from one medium to the next, it will bend (i.e. change directions) because light travels at different velocities in different media.
2. When the first part of the beam of light enters the water it slows down because light travels slower in water than it does in air.
3. Imagine driving in a car at an angle along a paved road into a patch of mud.
When the first tire hits the mud it will slow that side of the car down, while the other side of the car remains moving fast along the pavement. The outcome will be the car turning inward, or “pivoting” toward the first tire to hit the mud. It change its direction toward the normal.
4. If a car is travelling at an angle in the mud onto the paved road, the first tire that will hit the pavement will be able to travel faster, thus it will change its direction away from the normal. In other words, the car will “pivot” about the tire still in the mud.
Website: Science lesson plans: Science Light and colour lesson plans: Key stage 3 & 4 lesson plans: Secondary school lessons: Schoolsnet
5. The index of refraction, n, is a ratio of the speed of light in a vacuum versus the speed of light in various media.
a. v
n c
b. the velocity that light travels in a medium is measured by experimentation Sample Problems: Index of Refraction Sample Problems 1-2.ppt
C. Snell’s Law of Refraction
1. Snell’s Law compares the direction light travels as it is incident in one medium to the direction it takes when it refracts in a different medium.
2. nisinѲi = nR sinѲR
Sample Problems: Snell’s Law Sample Problems 1-2.ppt D. Apparent Depth
1. Although an object is at a certain distance from the surface of a medium in which the light is refracting, it will appear to be a different distance from the surface.
2. Because the light bends and we see light in the straight line that is travelling at, we perceive the object to be at a different location that it actually is.
3.
i R
n d n
d ' , where d’ is the apparent depth and d is the actual depth Sample Problem: Apparent Depth Sample Problem.ppt
E. Dispersion of Light 1. Prisms
a. When a beam of light strikes a different medium at an angle, then the white light will be dispersed into a spectrum.
b. As the light enters a new medium, its wavelength will decrease because the velocity will decrease (v = fλ) … frequency is constant.
c. The wavelength of red light is greater than the wavelength of purple light, thus the velocity of the red light will be greater than the velocity of purple light in the new medium. The purple light is “stuck in the mud”.
d. Remember, although the speed of light is considered a constant 3.0 E8 m/s, that is only in a vacuum. Light will travel different velocities in other
mediums.
Demo: Light through a prism and projected on an opaque surface.
2. Rainbows
a. If there is a combination of water droplets and sun light, then it is possible for a rainbow to form (just before or after a storm, near waterfalls, misty areas).
b. When the incident light ray strikes the water droplet, the water droplet acts as a prism. The light disperses, then reflects off the back surface of the water droplet. It then refracts again as it leaves the water droplet.
c. In order for the rainbow to form, the suns rays have to be incident at an angle of less than 42˚ above the horizon
Website: Rainbow - Wikipedia, the free encyclopedia Lab: Color of the Sky Lab.doc
Red Sky Proverb: Myth or Fact? Weather Proverbs 3. Mirages
a. When a surface is hot, then the air just above the surface will be warmer than the air farther above the surface.
b. Light travels faster in warm air than it does in cooler air because it is less dense – thus less resistance.
c. The light reflecting off the object toward the surface will bend because the beam will get “stuck in the mud” in the cooler air.
d. Because we perceive light to travel in a straight line, we see an image of the object on the surface, rather than rising above the surface.
II. Internal Reflection
A. Circumstances and Outcomes B. Critical Angle
1. The critical angle is the maximum angle of incidence that light can have in order for refraction to occur.
2. If the angle of incidence is greater than the critical angle, then reflection will occur.
3. If the angle of incidence is less than the critical angle, then refraction will occur.
4.
i R
c n
n
sin
Video: CD Pg. 489 CD Demo
Video: What’s Physics Got to Do With It? : Can You Hear Me?
Demo: Fiber Optics Kit
Sample Problems: Critical Angle Sample Problems 1-2.ppt Assignment: Pgs. 508-511 #’s 55-56, 60-61, 67-76, 101
Website: http://astro.unl.edu/naap/motion3/animations/sunmotions.swf (Path of the Sun in our sky)
Website: The Science of Light
Website: Physics 2000 - Table of Contents (Lasers, TV Screens, Laptop Screens) Website: PhET Color Vision - Light, Radiation, Spectrum, Photons, White Light Assignment: Refraction Worksheet 1.doc
Quiz: Refraction Quiz.doc
III. Lenses (assume the source of incident light is to the left of the lens) A. Convex/Converging
1. Convex lenses bend light so that the light rays converge at a point.
2. Focal length is positive
B. Concave/Diverging
1. Converging lenses bend light so that the light rays diverge or spread out.
2. Focal length is negative
C. Images 1. Real
a. A real image is formed when the light rays converge and create an image of the object that can be projected onto another surface.
b. A real image is always inverted and smaller than the object and will correspond with a positive image distance.
2. Virtual
a. A virtual image is formed when the light rays will appear to the observer to come from a spot on the same side of the lens as the object. Virtual images can not be projected onto another surface.
b. A virtual image is always upright and will correspond with a negative image distance.
Website: Virtual Images (sample ray diagrams)
c. Virtual images are not really there, but we can see them
Website: Reading on Refraction, Virtual Image (explains why we can see virtual images)
D. Thin Lens Equation and Magnification f
d di o
1 1
1 , where di = image distance, do = object distance, and f = focal length
o i o i
d d h M h
Refraction Sign Conventions:
+ -
do
object is to the left of the
lens
object is to the right of the lens
di
image is to the right of the lens
(real)
image is to the left of the lens
(virtual) f convex lens concave lens
M upright image inverted image
ho object is
upright object is inverted hi image is
upright image is
inverted
Website: Thin Lens Java applet written by Fu-Kwun Hwang
Website: PhET Geometric Optics - Refraction, Lens, Vision, Light, Images, Optics
E. Applications of Lenses 1. Eyes
A. Cones → Color B. Rods → Light/Dark
C. Farsighted → Hyperopia, Nearsighted → Myopia
Website: Refraction and Refractive Errors - How the Eye Sees - AllAboutVision.com Website: CNIB - How to Read Your Eyeglass Prescription
2. Telescopes
When to use each one: Refracting and Reflecting Telescopes 3. Cameras
4. Photocopy Machines 5. Microscopes
Activity: Students look at computer monitor through microscope to see pixels that can be seen with the naked eye.
Video: YouTube - Bill Nye the Science Guy Optics2of3
Video: YouTube - Bill Nye The Science Guy on The Eyeball (Full Clip) Sample Problems: Lens Refraction Sample Problems 1-4.ppt
Assignment: Pgs. 510-511 #’s 81-93 odds Assignment: Refraction Worksheet 2.doc III. Reflection
A. Plane Mirrors
1. The angle of incidence will be equal to the angle of refraction
2. The image will be a “mirror” of the object, i.e. the right side of the image will be the left side of the object.
B. Convex Mirrors
1. The light rays diverge when they reflect from the mirror.
2. The image formed is always virtual 3. Commonly used to see around corners
C. Concave Mirrors
1. The light rays converge when they reflect from the mirror.
2. The image will be real unless the object is within the focal length of the mirror (i.e. really close to the mirror)
D. Thin Lens Equation and Magnification
R f d di o
2 1 1
1 , where R = radius of curvature (f = ½R)
o i o i
d d h
M h
Reflection Sign Conventions:
+ -
do object is to the left
of the mirror object is to the right of the mirror di
image is to the left of the mirror
(real)
image is to the right of the mirror
(virtual) f concave mirror convex mirror
M upright image inverted image
ho object is upright object is inverted
hi image is upright image is inverted
Sample Problems: Light Reflection Sample Problems 1-4.ppt Assignment: Reflection Worksheet.doc
Assignment: Mirror and Lens Research Assignment.doc
Test Review: Refraction and Reflection Review.doc Test: Refraction and Reflection TestB.doc
Content Expectations:
P 4.8A P 4.8B P 4.8e P 4.8f P 4.9A P 4.9B P 4.9C