SNC2D/2P Light and Geometric Optics/Light and Applications of Optics

SNC2D/2P Light and Geometric Optics/Light and Applications of Optics

Student Activity: Refraction of Light—Reappearing Penny

Topics

refraction of light
index of refraction /

Timing

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 differentmedia

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.Organize your class into groups of 2 to 3 students.

2.Each group should place a penny at the bottom of an opaque plastic cup on the desk.

3.Predict/Explain

Ask thegroups 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.

4.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.

5.Pour some water slowly and continuously into the dish.

6.As you conduct the activity provide time for students to record their observations.

7.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.2The repositioned eye is unable to see the penny. / Fig.3Adding 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.This demonstration can be used as an introduction to or a review of refraction.

2.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 clearlake is not the same as the actual depth. Due to the refraction of light, a swimming pool or clearlake appears to be shallower than it actually is. As well, if one were ever trying to spear a fish from land, they one shoulddefinitely 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.

Ashort student activity that needs no equipment is available onSTAO’s ScienceWorks Grade 10 Optics Survival CD and online.

Additional Resources

1.The following website shows this demonstration -

2.STAO’s ScienceWorks Grade 10 Optics Survival CD: Refraction activity -