5.2.2 Practical Uses of Lenses
DATE:
NAME:
1
PARTNERS:
Purpose:
To investigate the properties of images formed by a lens.
Materials:
-
Convex and concave lenses of different focal lengths
Lens holder
Metre stick
Index card
Tape
Light source and object (triangle cut-out)
Ruler
Diagram:
Object and
light source
lens
Index card
(image)
Metre stick
Procedure:
1. Take the metre stick and put it on a flat surface so it rests on its narrow edge.
2. Take an index card, and holding it length wise, fold the lower end in on itself by just 1cm to form a small
lip on the card bent at 90 degrees to the rest of the card.
3. Tape this lip down at one end of the metre stick so the index card remains vertical.
4. Place a convex (converging) lens in the lens holder, and secure the lens holder on the metre stick at
the 50 cm mark.
5. Place the light source at the other end of the metre stick. Set up for light source will depend on the light
provided – your instructor and/or TA will let you know the best way to set this up.
6. Have someone in your group hold up the object card in front of the light as shown in the diagram – the
triangle should point up.
5.2.2 Practical Uses of Lenses
Activity 1 – Focusing the Image
1. PREDICT: When the image of the triangle is focused on the image card, which direction will the
triangle be pointing? Explain your reasoning; you might want to sketch in some rays from the object
going through the lens, and forming the image on the other side.
2. DISCUSS: What do the other members of your group think? Jot down any opinions or reasons that
are different from your own. Don’ t move on to the next step until your instructor tells you to do so.
3. OBSERVE: Try it! Switch on the light at your station. Have someone in your group hold the black
object card in front of the light source, and have someone else move the image card closer to the lens
until a crisp image of the triangle is focused on the image card. Secure the image card back onto the
meter stick at this location. How is the image oriented compared to the object? Is this consistent with
your prediction?
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5.2.2 Practical Uses of Lenses
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Activity 2 – Where does the image go?
1. PREDICT: If you slide a piece of paper over the top half of the object, what will happen to the image on
the image card?
2. DISCUSS: Discuss your ideas with your group. Write down any different opinions discussed in your
group.
3. OBSERVE: Try it! Take a piece of paper and lower it part way over the object. Watch what happens to
the image on the image card, and note your observations. Did this match your predictions? Does the
result make sense?
5.2.2 Practical Uses of Lenses
4. PREDICT: What will happen to the image if you cover half of the lens?
5. DISCUSS: Discuss your ideas with your group. Write down any different opinions discussed.
6. OBSERVE: Try it! Take a piece of paper and lower it part way over the lens. Watch what happens to
the image on the image card, and note your observations. Did this match your predictions? Does the
result make sense?
7. What happens if you remove the image card? Is an image still formed by the lens? Explain. Try it and
see what happens-- can you still see the image?
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5.2.2 Practical Uses of Lenses
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Activity 3 – Object Distance vs Image Distance and Properties of the Image
1. Keep the lens you were initially given in the lens holder at the 50 cm mark (make sure it is a
converging/convex lens). Adjust the image card so the image is in focus. Either with a ruler, or by
measuring directly on the metre stick, measure the distance between the centre of the lens and the
image card. Record the image distance in the table below. Make sure you include a description of
what lens you were using. For example, “ 38 mm plano-convex lens” , or “ unknown lens #12” depending
on the lens you were given.
2. Measure the distance between the light source (the object distance, measured from the black card) to
the centre of the lens. Record the object distance in the table below. Also note the properties of the
image-- is the image right-side up or upside down (inverted)? Is it smaller or larger than the object?
3. With the same lens in place, move the lens towards or away from the light source, adjust the image
distance so the object is in focus again, and then measure these new image and object distances.
Repeat this procedure for a third set of measurements if time permits.
Lens Description
Image Distance
(mm)
Object Distance
(mm)
Image right-side
up or inverted?
Image larger or
smaller?
Lens #1
Lens #1
Lens #1
Lens #2
Lens #2
Lens #2
4. Repeat steps 1-3 with a second converging/convex lens. Make sure you add a description of the lens
in your table.
5. For each lens, what happens to the image distance as the object distance gets longer? Can you
describe the relationship between the image distance and the object distance? Do the properties of the
image depend on the object distance and image distance?
5.2.2 Practical Uses of Lenses
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6. What happens to the image if you move the lens very close to the object? (i.e. make the object distance
very small?) Can you still see an image on the other side of the lens? Is there an image on the same
side as the object?
5.2.2 Practical Uses of Lenses
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Activity 4 – Concave Lens
1. Replace the lens in the lens holder with a concave lens. Again, adjust the location of the image card
along the metre stick to get an image in focus. Does this work? If not, where do you think the image is?
Look for the image on both sides of the lens.
2. What does the term virtual image mean? (remember about images formed by plane mirrors)
3. How do convex and concave lenses compare in terms of the images they form? Why is this?
Reference:
Cunningham, James and Norman Herr, Hands-On Physics Activities With Real-Life Applications, (The Center
For Applied Research in Education, West Nyack, 1994), p. 506.
5.2.2 Practical Uses of Lenses
Summary and Suggestions for the Future:
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1. What were the important concepts of physics/science that you learned from this activity? What else did you
learn?
2. Can you think of alternative hands-on ways in which these concepts could be demonstrated? Can you
suggest any improvements to these activities?
3. What changes would you make to teach these activities in a Grade 7-8 classroom? What difficulties can you
foresee?