Download Student Activity: Refraction of Light—Reappearing Penny

SNC2D/2P Light and Geometric Optics/Light and Applications of Optics
Student Activity: Refraction of Light—Reappearing Penny
Topics
Timing
refraction of light
index of refraction
preparation: 10 min
activity: 10 min
Specific Expectations
SNC2D
A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.5 conduct inquiries, controlling some variables, adapting or extending procedures as
required, and using standard equipment and materials safely, accurately, and effectively, to
collect observations and data
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
E3.4 explain the conditions required for partial reflection/refraction and for total internal
reflection in lenses, and describe the reflection/refraction using labelled ray diagrams
E3.7 identify the factors, in qualitative and quantitative terms, that affect the refraction of light
as it passes from one medium to another
SNC2P
A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.5 conduct inquiries, controlling some variables, adapting or extending procedures as
required, and using standard equipment and materials safely, accurately, and effectively, to
collect observations and data
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
E3.4 describe qualitatively how visible light is refracted at the interface between two different
media
Introduction
This demonstration serves as a superb introduction to the refraction of light. Students will view a
penny at the bottom of an opaque plastic cup. After moving slightly so that the penny just barely
disappears from sight, water is slowly added. The penny magically reappears!
Materials
opaque plastic cup
penny
water
Safety Considerations
None
Procedure
1.
2.
3.
4.
5.
6.
7.
Organize your class into groups of 2 to 3 students.
Each group should place a penny at the bottom of an opaque plastic cup on the desk.
Predict/Explain
Ask the groups to discuss and predict what will happen if the observer moves further away
from the cup. What will the observer see, from this new position, when water is slowly
added to the cup? Encourage all groups to provide rationales for their predictions.
Observe
Have one partner (the observer) view the penny from an angle. The observer will move
down and/or away until the penny just disappears from sight. It is important that the
observer should not be able to see the penny at this point.
Pour some water slowly and continuously into the dish.
As you conduct the activity provide time for students to record their observations.
Explain
Ask the small groups to consider their observations and revise their rationales if necessary.
Invite them to suggest a model to explain what happens when they add water to the cup.
Disposal
No special concerns
What happens?
When the penny is first viewed, the light from the penny travels in a straight line to the
observers’ eyes (Fig.1). The observers then adjust their positions so that the penny disappears
from view because there are no light rays travelling from the penny to the eye (Fig.2). When
water is added to the cup, the observers now can see the penny (Fig.3). Students may report that
the penny appears to be higher in the cup, or that the cup appears to get less deep.
Fig.1 Initial set-up with no
water. The penny is visible.
Fig.2 The repositioned eye is
unable to see the penny.
Fig.3 Adding water makes the
penny visible again because rays
reaching the eye form an image of
the penny in a new position.
How does it work?
When water is added, the light from the penny travels from water (a more optically dense
medium) to air (a less optically dense) and is refracted or bent. The light bends away from the
normal as it leaves the water and enters the air. As a result of refraction, the observer perceives
the penny to be higher in the cup than it actually is.
Teaching Suggestions/Hints
1.
2.
This demonstration can be used as an introduction to or a review of refraction.
An evaporating dish or an opaque plastic cup works well. Another small, flat, coloured
object could be substituted for the penny.
Next Steps
There are many interesting applications that are related to this phenomenon. The apparent depth
of a swimming pool or clear lake is not the same as the actual depth. Due to the refraction of
light, a swimming pool or clear lake appears to be shallower than it actually is. As well, if one
were ever trying to spear a fish from land, they one should definitely take the refraction of light
into consideration.
Try repeating the procedure using different materials (such as vegetable oil) to see if there is a
different result.
A short student activity that needs no equipment is available on STAO’s ScienceWorks Grade 10
Optics Survival CD and online.
Additional Resources
1.
2.
The following website shows this demonstration http://www.youtube.com/watch?v=DN7N2hoS3wk
STAO’s ScienceWorks Grade 10 Optics Survival CD: Refraction activity http://stao.ca/VLresources/2008/RefractionEyeDemo.ppt