Science 7–10 · Year 9
Converging lens: forming a real image on a screen and finding the focal length
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
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The idea
A converging lens brings light to a focus, and a more strongly curved lens focuses closer to the lens. An object beyond the focal point forms a real, inverted image on a screen, and as the object is brought closer to the focal point the image moves further from the lens and grows larger.
What you need
- converging lens of focal length about 10 cm in a holder, 1
- second, more strongly curved converging lens of focal length about 5 cm in a holder, 1
- illuminated object: a lamp behind a cardboard cut-out arrow or an LED torch behind a mesh, 1
- white card screen in a holder, 1
- metre rule or optical bench, 1
- darkened room
How to do it
- Point the 10 cm lens at a distant window or tree and move the screen until the image is sharp; the lens-to-screen distance is its focal length. Repeat with the more strongly curved lens and compare.
- Using the 10 cm lens, set the object 30.0 cm from the lens. Move the screen until the arrow's image is sharpest; record the image distance and whether the image is upright or inverted, larger or smaller.
- Repeat for object distances of 25.0, 20.0, 15.0 and 12.0 cm.
- Measure the height of the object and of each image, and compare each image height with the object height.
- Tabulate object distance, image distance and image height, and describe how the image moves and changes size as the object comes closer.
- Move the object to 8.0 cm and try to find an image on the screen; then look through the lens at the object.
What you should see
A window 5 m away focuses about 10.2 cm from the 10 cm lens and about 5.1 cm from the more strongly curved lens, so the more curved lens has the shorter focal length. With the 10 cm lens and the object at 30 cm the sharp image is 15.0 cm from the lens, inverted and half the size; at 20 cm the image is at 20 cm and the same size; at 15 cm it is at 30 cm and twice the size; at 12 cm it is at 60 cm and five times larger. At 8 cm no image can be caught on the screen, and through the lens the arrow appears upright and enlarged.
What changes
- What you change
- object distance u (cm), or the lens used
- What you measure
- image distance v (cm), image height (cm)
- What you keep the same
- same lens
- object, lens and screen centred on one axis
- same object brightness
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Covering half the lens removes half the image.
- The image forms on the lens.
- A lens always magnifies.
Safety card
Hazards
- hot lamp
- dark room
- focusing sunlight with the lens
Controls
- never use the sun as the object
- handle the lamp by its holder
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-WAM-01SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9U04AC9S9I03AC9S9I04AC9S9I05
Sources
The pages the author read to write this activity.