PRINT AND DESIGN YOUR OWN LESSON PLAN
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Teachers ages 15-18
This lesson plan is not intended to be handed out to students but
to be used as a teacher preparation and tutorial.
This lesson is designed to help students explore the idea of
critical angle and help reinforce the idea that it only exists when
going from a slow to a large to a small index of refraction.
Students will understand why a critical angle only exists when
traveling from a large to small index of refraction.
Students will understand the correlation between index of
refraction and critical angle.
40 to 50 minutes
Index of refraction, normal, refraction, critical angle, incident ray,
refracted ray
Students should be familiar with the idea of refraction before
engaging this activity.
Start by reviewing with students what happens when light travels
from a small to a large index of refraction and vice versa (bends
towards the normal and away from the normal respectfully).
Make sure they realize that as the incident angle increases, the
refraction angle increases (according to Snell’s Law).
In class
Focusing on the example of going from a large index of refraction
to a small index of refraction, ask them what will happen when
the incident ray refracts so much that it travels across the surface
of the material.
Have them record their predictions then discuss them as a class.
After several students have expressed their ideas and thought
process, have them discover for themselves by engaging this
Algodoo activity.
CREATE A SCENE
Open the ‘Index of refraction’ scene.
MAKE A PREDICTION
Make a prediction as to what will happen when a beam of light travels from a
large to small index of refraction and refracts so much that the refracted ray
travels along the surface of the material.
RUN/INTERACT
There are six clear blocks made of different materials. Four of the six have
their printed index of refraction on the block. Starting with the water block,
place it so that it covers the bottom half of the circle (it should perfectly fit over
the yellow block). Position the laser so that it is directly at the bottom of the
circle pointing upward. Slowly rotate the laser around the circle to either side.
Notice the refracted ray should refract away from the normal. Keep rotating
the laser until the refracted ray travels across the surface of the block. Record
your observations (including what angle this happens at). Now rotate the
laser a bit more and record what happens. Repeat this with the other five
blocks.
EVALUATE
Was your prediction correct? Was there any correlation between the angle
this occurred at and the indexes of refraction? Why does this reflecting not
occur when the laser travels from the air into the block?
REVISE SCENE
Now place a block on the top half of the circle as well (so that the two blocks
overlap a tiny bit) and repeat the process looking for the angle at which the
light begins reflecting at (make sure the light is always going from a large index
of refraction to a smaller one). You may need to use the white lever to turn
the circle under the blocks.
MAKE A PREDICTION
How do you think this will affect the angle at which the light begins reflecting
at?
RUN/INTERACT
Rotate the laser with the new block configuration and record your results.