OPTICS 04
 

OPENING QUESTIONS: Review:

  • What is retroreflection?
  • What shapes are used to create situations where retroreflection occurs?
  • How does retroreflection happen in the human eye?
  • Why does retroreflection result in red eye?
  • What equation did we use to calculate the speed of light in water?

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FORMULAE OBJECTUS:

1) θi = θr Angle of incidence = angle of reflection

2) c/v = n Index of refraction

LEARNING TARGET: I will be able to recognize and utilize Snell's Law after today's class.

WORDS O' THE DAY:

    • medium (material through which light is travelling)
    • Reflection ("Light waves bouncing off an object")
    • Emission ("Light is given off by an object")
    • Absorption ("Some wavelengths of light are absorbed by an object")
    • Refraction ("Light bending as it changes medium")
    • Angle of Incidence ("The angle a light ray takes as it moves toward a reflecting surface")
    • Angle of Reflection ("The angle a light ray takes as it moves away from a reflecting object")
    • Retroreflection ("incident light waves are 'double bounced' back to the observer, regardless of the angle of incidence")
    • Snell's Law:

WORK O' THE DAY:

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Let us now please take a look at another aspect of refraction-- The INDEX of refraction.

We noted yesterday that the speed of light (c) is variable in that it depends on the *medium* through which it passes.

As light passes from one medium to another, it either speeds up (when passing into a less dense medium) or slows down (when passing into a more dense medium).

The obvious caveat here is that light must be able to move through BOTH media. Visible light doesn't pass from air through wood, for example so a room made entirely of wood is dark...

But I digress...

There is a specific measurement that indicates how much light speeds up or slows down when passing from a vacuum into that medium, we call that the INDEX of refraction.

NOTE: As I noted in class yesterday, the speed of light in air is VERY NEARLY the same as the speed of light in a vacuum.

The index of refraction is the ratio of the speed of light in a vacuum compared to the speed of light in that particular medium:

c/v = n

Where c is the speed of light in a vacuum, v is the speed of light in some ofther medium (such as glass or water) and n is the index of refraction in that medium

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Let's take a break to have some fun with numbers:

What do you suppose the index of refraction of air might be? (if the index of refraction in a vacuum is 1.0000000 etc)

INDEED!

The index of refraction is just a small bit above that since air is not particularly dense.

At Standard Temperatures and Pressures (if you've had chem you know that as STP, otherwise don't worry), the index of refraction is 1.00027, so let's just KISS and keep it at 1.00

 

You know how to calculate the index of refraction

You also know about the relationship between the angles a light ray makes to the speed of light across two different mediums? medias?

Work with that information to derive an equation that relates the incident angle, the refracted angle and the index of refraction of both mediums

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x

x

x

x

x

x

Please now work with your group to rewrite refraction equation #1 in terms of n, and not v.

sin Θ2/sinΘ1 = n1/n2

Which is Snell's Law... a hugely important law of optics which is often written as:

n2sin Θ2 = n1sinΘ1

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Practice problems:

1) The index of refraction of isopropyl alcohol is 1.38. If a light ray in air is incident at 27 degrees at the surface of isopropyl alcohol, what angle of refraction results?

2) A flashlight is shown upwards towards the surface of water (n = 1.33) at an angle of 18 degrees to the normal line, what angle does it make in air?

3) A flash light is shown through air upon an unknown liquid at an incident angle of 21 degrees to the normal. The resulting angle of incidence tells you the substance is benzene with an index of refraction of 1.50. What was the angle of refraction in benzene?

4) Consider problem #3. A light ray shown upwards from benzene into air would bend...

    • towards the normal (perpindicular) line?
    • Away from the normal (perpindicular) line?
    • Be fully reflected back into the benzene? (this is kindofa trick question)

5) There was a SOURCE OF ERROR in yesterday's activity.... it's a physics thing that we could do nothing about (at least not with the materials we used). What was that?