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Reflection & Refraction
Properties of waves and light
Reflection: Light
Light is just like any other wave: Reflection = Bouncing back when
encountering a “non absorbing surface” Ex: waves in a pond, light in a mirror Ray tracing: lines describing the direction of
propagation of the wave. They describe the direction of the wave.
Ray tracing:
How a mirror works: Light scattered or generated by objects are reflected on a mirror. Understanding how light behaves helps use it better!
Reflection
The phenomenon called reflection follows specific laws.
Reflection from a perfect surface: “specular reflection”
Angle in = angle out
Reflection
The law of reflection is always valid! The angle of incidence is always equal to the angle of reflection. This is why “rough” surfaces reflect light in all directions
Reflection
Amount of reflected light is very dependent on the object’s surface. The surface is also responsible for the color in the objects we see.
How mirrors work! Images are formed.
An image is the point in space from where all the reflected light appear to come from.
Virtual image: light does not go there
Plane Mirrors
Image characteristics Virtual image Left-right reversed Upright image Unmagnified image
Curved mirrors
Also follow the laws of reflection
Practical use: Telescopes, shaving mirrors, wide angle mirrors in hallways etc…
2 types: concave and convex
Spherical mirrors
R=Radius of curvature
F=focal point; f=focal length
C=center of curvature
Laws of reflection
Light will always follow the laws of reflection. Even in curved mirrors
Concave Convex
Concave mirror images
Virtual image? Left-right reversed? Upright image? Unmagnified image?
http://micro.magnet.fsu.edu/primer/java/mirrors/concave.html
Convex mirror images
Virtual image? Left-right reversed? Upright image? Unmagnified image?
http://micro.magnet.fsu.edu/primer/java/mirrors/convex.html
Refraction
Refraction (or bending of the light) occurs as light passes from one medium to another with different characteristics
Bending of light
Light gets bent more when traveling into a “denser” medium
Ex: Glass and water
Why refraction happens?
Because speed of light changes.
Because light is a wave.
Because its wavelength changes in different medium.
Laws of refraction
Index of refraction depends on: The medium The wavelength (or color
of the light)
n=c/vSnell’s law:
N1 x sin(1) = N2 x sin(2)
Why the speed changes?
At the atomic scale...Absorption and re-
emission of of light (photons)
Too many interactions slow down the travel time of light
Vacuum n=1.00000 Air = 1.0003 Diamond=2.42
All other properties of refraction can be derived from this fact!
v=f f=c= vn=c/vn= /
Demo of refraction
http://wigner.byu.edu/LightRefract/LightRefract.html
Total internal reflection
There is a critical angle for every boundary
When light hits the boundary at this angle it is no longer refracted, but reflected!
Total internal reflection = principle of optical fibers
References:
http://micro.magnet.fsu.edu/primer/lightandcolor/index.html http://www.glenbrook.k12.il.us/gbssci/phys/Class/BBoard.html