Goal: To understand the basics of reflection and refraction - PowerPoint PPT Presentation

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Goal: To understand the basics of reflection and refraction

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Title: Goal: To understand the basics of reflection and refraction


1
Goal To understand the basics of reflection and
refraction
  • Objectives
  • To understand the Propagation of light
  • To understand the following possibilities When
    light hits something
  • Reflection
  • Refraction
  • Critical Angle
  • Brewsters Angle
  • 3) To learn about Rainbows

2
Propagation of light
  • When light is emitted even a single photon it
    usually goes in all directions.
  • Imagine you are camping on a cold day.
  • You start a fire to warm yourself.
  • What happens if you take a few steps back?

3
Spreading of light
  • Since light propagates in all 3 dimensions that
    means it spreads itself out onto the surface of a
    sphere.
  • Since the area of the outside of this sphere (not
    volume) relates to the radius squared that means
    that the light you receive or energy drops as
    the radius squared.
  • So, if you get twice as far away you only get
    1/4th the energy (translation you get cold).

4
When a photon meets a woman, err a surface
  • A few things can happen, and often times you will
    get all of these in different amounts.
  • 1) reflection the light in essence bounces off
    of the surface.
  • In this case the angle which the light leaves is
    the same as it hits (?r ?i), as is the
    wavelength of light (in the perspective of the
    object it is hitting)
  • The of light that gets reflected depends on the
    material it is hitting as well as the wavelength
    of light.

5
Transmitted
  • 2) Some light will be transmitted. That is it
    will pass through the material.
  • This will also cause what is called refraction
    but more on that later.

6
Absorption
  • 3) The material can absorb some of the energy.
  • The percentage it absorbs at some wavelength is
    called the albedo.
  • This can vary greatly with wavelength.
  • A thin layer of water, for example, does not
    absorb much light in the optical.
  • However, in the infrared it absorbs a LOT of
    energy.
  • Thus water vapor is a greenhouse gas (and the
    most abundant greenhouse gas in our atmosphere).

7
Scattering
  • 4) Scattering.
  • This is similar to reflection.
  • However, most surfaces are not smooth.
  • So, when the light hits the surface, it is not
    hitting a flat surface.
  • So, different light photons at slightly different
    positions can hit at a vastly different angle to
    the surface.
  • Therefore, they will reflect at a wide range of
    angles.
  • This is scattering.
  • Also, if light hits an electron the electron can
    absorb the photon and reemit in a random
    direction thus scattering the light.

8
More on Refraction
  • A surface is defined to be a point where the
    index of refraction changes.
  • The most common form of this is when you have 2
    different types of materials such as air,
    glass, water, ect.
  • For each the speed of light is different.
  • When the light enters the material the speed
    changes but the rest of the wave which has not
    hit the surface is still going at the old pace.
  • This causes an accordion affect (like cyclists
    hitting a hill and piling up).
  • Also, the index of refraction depends on the
    wavelength of light more on this at the end of
    class

9
Snells Law
  • This accordion affect will change the angle the
    transmitted light is going (remember you can have
    some reflected AND some transmitted).
  • n1 sin(?1) n2 sin(?2)

10
Sample
  • On a winter day the light from the sun hits a
    clear calm lake at an angle of 60 degrees from
    normal (up and down).
  • If the index of refraction of the water is 1.4
    then what are
  • A) the angle of reflection
  • B) the angle of refraction

11
Try one more
  • Lets say a beam of light traveling at an angle of
    75 degrees from normal in a diamond (n 2.4)
    enters into ice (n1.3).
  • What is the angle of refraction (yes you will get
    a really wild answer no your calculator is not
    malfunctioning)?

12
Critical Angle
  • The most Sin(?) can be is 1
  • That means that if you go from high index of
    refraction to low index of refraction there is a
    point where Sin(?2) 1
  • (in other words Sin(?1) n2 /n1)
  • Here ?1 (or ?c) is called the Critical Angle.
  • If your initial angle is BIGGER than this, then
    there is NO refraction (this is called total
    internal reflection)!
  • At the critical angle the angle of refraction is
    90 degrees.
  • Why would this be important?

13
Sample
  • For our example of light going from a diamond
    (n2.4) to ice (n1.3) what is the critical angle?

14
Brewsters angle
  • Another way to polarize light is through
    reflection.
  • The fraction which is polarized depends on the
    reflecting surface and the angle.
  • At a specific angle all of the light is
    polarized.
  • This is called Brewsters angle.

15
Equation
  • tan(?b) n2 / n1
  • In this case it does not matter which is bigger
    you still have an angle.
  • Sample For the diamond (n2.4) to ice (n1.3)
    example, what is Brewsters angle?

16
Conclusion
  • We learned what can happen when light hits a spot
    where the index of refraction changes.
  • Reflecting light goes at the same angle it hits
    (from point of view of the surface)
  • Refracted light will depend on the difference of
    mediums and the angle.
  • At some angle (critical angle) the refracted
    angle is 90 degrees so you get no refraction
    bigger entry angles.
  • Also, reflections polarize light and at a special
    angle, the Brewster angle, all the reflected
    light is polarized.
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