SNC2D/2P Light and Geometric Optics/Light and Applications of Optics
Teacher Demo: Covered Lens
Topics
Timing
geometric optics and image formation
preparation: 20 min
demonstration: 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
A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence
supports or refutes the initial prediction or hypothesis, identifying possible sources of error,
bias, or uncertainty
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
E2.5 predict, using ray diagrams and algebraic equations, the position and characteristics of an
image produced by a converging lens, and test their predictions through inquiry [PR, AI, C]
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
E3.5 describe the characteristics and positions of images formed by converging lenses (e.g.
orientation, size, type) with the aid of ray diagrams
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
A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence
supports or refutes the initial prediction or hypothesis, identifying possible sources of error,
bias, or uncertainty
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
E3.8 explain how the properties of light or colour are applied in the operation of an optical
device (e.g., a reflecting telescope, stop lights, stage lights)
Introduction
This demonstration will help students understand what is required for an image to be formed by a
lens. As well, it will help to diagnose possible misconceptions about image formation.
Materials
9-LED flashlight
electrical tape
converging lens (e.g., magnifying glass or
Fresnel lens) and stand
large projector screen on opposite wall
cardboard or opaque tape
Safety Considerations

None
Procedure
Wear appropriate PPE: safety goggles.
Prepare the equipment before the class begins.
1.
Use small pieces of electrical tape to block out four of the flashlight’s LEDs as shown in
Fig.1. Fold back the edges of the tape.
(a)
(b)
(c)
Fig.1 Modifying the flashlight to form an arrow-shaped object
2.
3.
Set up the equipment as shown in Fig. 2, holding the flashlight in one hand.
Adjust the distance between the flashlight and the lens (close to the focal length, f) until
there is a focused image (upside down and real) of the flashlight arrow on the screen.
Fig.2 Set-up of flashlight, lens, and screen
Organize your class into groups of 2 to 3 students.
4.
Predict/Explain
Ask students to predict what will happen to the image when the lens is partially covered by
the cardboard or opaque tape. Encourage all groups to provide a rationale for their
prediction.
5.
Observe
Cover half of the lens with cardboard. Again shine the flashlight at the lens and provide
time for students to record their observations.
6.
Explain
Ask the small groups to reconvene and revise their explanations, if necessary. Challenge
them to suggest a tool or strategy to help explain their observations.
Disposal
No special concerns
What happens?
Initially, the light from the flashlight forms an image on the screen as shown in Fig.3.
Fig.3 Ray diagram of initial set-up
When half of the lens is covered up with the cardboard there is still a complete inverted real
image but the brightness of the image is about half that of the original image. Note that it does
not matter which half is covered up.
How does it work?
Students are exposed to drawing ray diagrams in order to predict where images (real or virtual)
will form. These ray diagrams are simplified and they typically use two or three strategic rays of
light. Students often think that if one of these rays is blocked then only a part of the object’s light
will reach the screen and only part of an image will appear. In reality, the flashlight (object) is
emitting a huge number of light rays that leave every point of the flashlight in all directions
(Fig.4).
Fig.4 Ray diagram of initial set-up showing “wasted rays”
Any rays that pass through the lens help to form the image. When the lens is partially covered,
the amount of light passing through the lens is less and therefore the brightness of the image, in
turn, is less. This is part of the reason why astronomical telescopes have large objective (light
collecting) lenses.
Teaching Suggestions/Hints
1.
2.
3.
This demonstration can be used as a powerful diagnostic tool for misconceptions about
image formation. It should be used with a Predict, Explain, Observe and Explain (PEOE)
framework.
A 9-LED flashlight can be purchased from many hardware and dollar stores for under $10.
It makes an excellent “luminous object” for Optics demonstrations. The adaptation shown
in Fig.1 produces a very bright object shaped like an arrow that can be used to demonstrate
the properties of images formed by a curved mirror or lens. The fact that it looks like an
arrow makes vertical inversion of many images obvious. To demonstrate horizontal
inversion, simply rotate the flashlight 90 °. The orientation of the object (the flashlight) can
be easily demonstrated by shining it toward the students. The light is bright but not
dangerous. This source is much brighter and safer than a candle. This results in an image,
which is much easier to see.
Fresnel lenses can be purchased or salvaged from an old overhead projector (Fig.5). Split
the overhead projector lens in two by sanding the edges off and then carefully separating
them. These lenses are about 30 cm × 30 cm. The larger the lens, the more light is
collected and focused, which results in a brighter image that is easier for the class to see.
Fig.5 The formation of a Fresnel lens from a regular converging lens
4.
5.
In Step 4, students will typically predict that only half of an image will form or that the
image formed will be upright instead of inverted. In Step 5 students should observe that the
entire image is still present but that the image is about half as bright.
In Step 6, a suitable tool or strategy would be a ray diagram.
Next Steps
As a follow-up, ask students to repeat this activity and this time cover up half of the light emitted
from the flashlight. In this case, they will observe a partial image. As well, you could ask the
students what would happen if the lens were removed, or if a larger Fresnel lens or smaller
magnifying glass were used
Additional Resources
1.
2.
3.
A more detailed explanation of the demonstration and related concepts - Five Easy
Lessons: Strategies for Successful Physics Teaching by Randall D. Knight.
More details on this demonstration - Goldberg, F.M. and McDermott, L. C. “An
investigation of student understanding of the real image formed by a converging lens or
concave mirror.” Am. J. Phys. 55, 108-119 (1987).
Online details on this demonstration: Optics Re-education http://science.uniserve.edu.au/disc/phys/aip/kwan.html