Transcript Goal: To understand the basics of reflection and

Goal: To understand the basics
of reflection and refraction
Objectives:
1) To understand the Propagation of light
2) To understand the following possibilities
When light hits something:
Reflection
Refraction
Critical Angle
Brewster’s Angle
3) To learn about Rainbows
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?
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).
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 (θr = θi), 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.
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.
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).
Scattering
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•
•
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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.
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
Snell’s Law
• This accordion affect will change the angle
the transmitted light is going (remember
you can have some reflected AND some
transmitted).
• n1 sin(θ1) = n2 sin(θ2)
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
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 (n=1.3).
• What is the angle of refraction (yes you
will get a really wild answer – no your
calculator is not malfunctioning)?
Critical Angle
• The most Sin(θ) 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(θ2) = 1
• (in other words Sin(θ1) = n2 /n1)
• Here θ1 (or θc) 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?
Sample
• For our example of light going from a
diamond (n=2.4) to ice (n=1.3) what is the
critical angle?
Brewster’s 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 Brewster’s angle.
Equation
• tan(θb) = n2 / n1
• In this case it does not matter which is
bigger you still have an angle.
• Sample: For the diamond (n=2.4) to ice
(n=1.3) example, what is Brewster’s
angle?
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.