reflection refraction and light 2010
TRANSCRIPT
INCLUDING TOTAL INTERNAL REFLECTION AND
THE CRITICAL ANGLE
Light: Reflection & Refraction
Law of Reflection
The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.
Diffuse Reflection
Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as diffuse reflection.
Why does a dry road diffuse light, when a wet road specularly reflects.
Index of Refraction
n = c/v The larger the index,
the slower the speed of light
Index of Refraction
Substance Index of Substance Index ofRefraction Refraction
Diamond 2.419 Ethyl Alcohol 1.361Cubic Zirconia 2.21 Ice 1.309
Glass (flint) 1.66 Water 1.333Glass (crown) 1.52 Air 1
Reflection and Refraction at an Interface
Refraction: The Bending of Wavefronts
Index of refraction
Refraction Rules
Light twists inward when entering mediumof higher index of refraction
Refraction Rules
Light twists outward when entering mediumof lower index of refraction
Understanding Refraction
Wheels on axle rolls along a smooth sidewalk and onto grass. Which picturepath is followed? What happens if the motion is reversed?
Understanding Refraction
One side of wave front slows down,and the entire train of fronts twists.
Illustrating Cart Analogy
Right front wheel slows down before left front
Left front wheel slows down before right front
Snell’s (Sahl’s) Law (a history)
Ibn Sahl was an Arabian Mathematician and optics engineer associated with the court of Baghdad. In 984 he wrote a treatise On Burning Mirrors and Lenses in which he set out his understanding of how curved mirrors and lenses bend and focus light. Ibn Sahl is credited with discovering the law of refraction, usually called Snell's law.
In 1621, Willebrord Snellius (Snel) derived a mathematically equivalent form, that remained unpublished during his lifetime.
Snell’s Law
Snell discovered that the ratio between the sine of the incident and refracted angles is equal to the ratio of the incident and refracted velocities.
Snell’s Law continued
Because the velocity of light is extremely difficult to measure, it is more practical to use the indices of refraction. Due to the fact that the velocity is inversely proportional to the index of refraction, you get…
Or…
Law of Refraction: Snell's Law
Right front wheel slows down first.
Snell's Law: n2 sin Q2 = n1 sin Q1
Snell's Law Example
n1 = 1.0 (air) n2 = 1.52 (glass)Q1 = 30 degrees------------------------ n2 sin Q2 = n1 sin Q1
1.52 sin Q2 = 1.0 sin 30 sin Q2 = 0.33
Put calculator inMode Degree
Q2 = sin-1 (0.33) = 19.3 degrees
Rear View Mirror
Rear View Mirror Explained
Apparent Depth in Water
Light exits into medium (air)of lower indexof refraction,and turns left.
More Apparent Depth
Spear-fishing is made moredifficult by the bending oflight.
To spear the fish in the figure,one must aim at a spot in frontof the fish
Refraction at Sunset
Why does the sun appear to beflattened at sunset?---------------------------------------------------The sun actually falls below below thehorizon, i.e., it "sets", a few secondsbefore we see it set.
A Fish-Eye's View
Water on Road Mirage
There's no water on the road; why does it appear so?
The Oasis Mirage
Displacement through a Slab of Glass
Entering and exiting rays are displaced from each other, but parallel.
Internal Reflection
All rays reflect internally, butthe top three rays reflect onlya small percentage internally;most energy leaves the prism. The fourth and fifth rays arereflected 100 % internally.
Internal Reflection
Critical angle…
An incident angle at which the refracted angle is 90o
Note: This can only happen in a case when
light travels from a higher index of
refraction to one with a lower index of
refraction.
`
Critical Angle Calculation
What must be Q1 to get Q2 = 90 deg ?
Snell's Law:
n1 sin Q1 = n2 sin Q2
= n2 sin 90 sin Q1 = n2 / n1
------------------------------Assume water to air: n1 = 1.33 n2 = 1.00q1 = sin-1 (0.752) = 48.8 degrees
Qc = critical angle = 48.8 degrees
Cone of Light
Crictical angle for water = 48.8 degrees
Light within the 48.8 degree coneis detected by fish, while nothing in theair outside that cone can be seen.
The only light reaching the fish outsidethe cone is that light (not shown) whichis reflected off the bottom of the pool.
Critical Angle of Diamond
n = 2.419
Qc = sin-1(1.00/2.419) = 24.42 degrees
90.00 - 24.42 = 65.58 degrees
Light outside of 65.58 degreecone is reflected back inside.
Virtually all light entering thetop face of the diamond isreflected internally.
Total internal reflection
When incident light is at an angle greater than the critical angle, the light will reflect instead of refract.
When there is total internal reflection, the light will obey the law of reflection.
Optical Fiber
Optical Fiber
Optical Fibers in Medicine
Broncho Scope Arthroscopic Surgery Colonoscope
Index of Refraction vs Color
v = c/n
The speed of blue light isless than the speed ofred light.
Blue light bends morethan red light.
The Spectrum of White Light
Note that the shorter wavelengths of light are bent more than the longer; blue more than red.
Refraction in a Raindrop
Forming a Rainbow
Rainbow
Index Matching