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