refraction: tir and dispersion ap physics: m. blachly light and optics
TRANSCRIPT
Refraction:TIR and Dispersion
AP Physics: M. Blachly
Light and Optics
Nature of Light
What is light?
• Light is a wave
• But what is the medium?
• Applet
Light is also a particle
• The particle is called a photon.
• The particle nature of light was first proposed by Einstein to explain the photoelectric effect.
Reflection
A mirror and a piece of white paper both reflect essentially all light that hits them. What is the difference?
Reflection
Mirror is example of specular reflection
m irro r s u rface
Reflection
Diffuse reflection from a rough surface
Reflection
Microscopic view of a sheet of paper
Reflections
The Law of Reflection
Reflection: Headlights
Why do oncoming headlights seem so bright?
Why don’t headlights work in the rain?
Transmission
Light can enter an optically transparent material.
The light slows down as it is absorbed and emitted by the atoms
Speed of Light
n the called the index of refraction
it can be thought of as the “slow down” factor for lightc
nv
Index of Refraction
0%20%40%60%80%
100%V
acuu
m Air
Ice
Wat
er
EtO
H
glas
s
Dia
mon
d
Index of Refraction
Material Index of RefractionVacuum 1.000
Air 1.00029Ice 1.310
Water 1.330EtOH 1.360glass 1.520
Diamond 2.417
Are you awake?
Material Index of RefractionVacuum 1.000
Air 1.00029Ice 1.310
Water 1.330EtOH 1.360glass 1.520
Diamond 2.417
What is the speed of light in Water ?
Refraction
What happens if a wave of light approached an interface where it must slow down?
Refraction
RefractionRefraction is the bending of a wave when it travels from one medium to another.
Animation showing how light bends
Refraction Examples
Slice your finger example
The bent straw
Spear fishing
Refraction Examples
Catching fish
n = 1
n = 1 .5
Refraction Terminology
Refraction
Light bends towards the normal when it enters a material with a slower speed (higher n)
n = 1
n = 1 .5
Refraction
Light bends away the normal when it enters a material with a higher speed (lower n)
n = 1
n = 1 .5
Refraction: Snell’s Law
The angle of refraction depends on the indices of refraction, and is given by Snell’s law:
Example Problem
Light, traveling in glass, strikes an optical interface at an angle of 50 as measured from the normal. What is the angle of refraction?
n = 1 .3 3
n = 1 .5 0
Refraction of Sound
Dispersion
Dispersion is the separation of light into colors by refraction
Animation
Dispersion
The index of refraction of a material varies somewhat with the wavelength of the light.
Dispersion
Dispersion
Most famous example of dispersion is the rainbow
Rainbow
Rainbow
Total Internal Reflection
Simulator
When going from a slower to a faster medium, light cannot escape if the angle is too big.
Applications
Diamonds
Applications
Light Pipes
Are you conscious still?
Light, traveling in plastic, strikes an optical interface at an angle of 65. as measured from the normal. What is the angle of refraction?
n = 1 .7 8
n = 1 .0 0
If light passes into a medium with a smaller index of refraction, the angle of refraction is larger. There is an angle of incidence for which the angle of refraction will be 90°; this is called the critical angle:
(23-5)
Total Internal Reflection
If the angle of incidence is larger than this, no transmission occurs. This is called total internal reflection.
Total Internal Reflection
Example
The index of refraction for glass is 1.33. What is the critical angle for TIR for a beam of light traveling in glass and striking an optical boundary with air ( n = 1 )?
Binoculars often use total internal reflection; this gives true 100% reflection, which even the best mirror cannot do.
Total Internal Reflection
Total internal reflection is also the principle behind fiber optics. Light will be transmitted along the fiber even if it is not straight. An image can be formed using multiple small fibers.
Total Internal Reflection
Dispersion
Wavelengths of visible light: 400 nm to 750 nm
Shorter wavelengths are ultraviolet; longer are infrared
This variation in refractive index is why a prism will split visible light into a rainbow of colors.
Dispersion
Actual rainbows are created by dispersion in tiny drops of water.
Dispersion