Physics 11–12 · Year 11
Image formation with converging lenses and curved mirrors on an optical bench (syllabus practical)
Module 3: Waves and Thermodynamics (Ray Model of Light)
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
Rays from each point of an object are redirected by a lens or mirror to meet at an image point whose position and size follow from the focal length.
What you need
- Optical bench with metre scale, illuminated object (crossed arrows or an LED lamp behind a cut-out), white screen
- Converging lenses of focal length 10 cm and 20 cm, concave mirror of focal length 15 cm, lens holders
- Ray box and lens sections for ray diagrams on paper
How to do it
- Find the focal length roughly by focusing a distant window onto the screen.
- Set the object at 30.0 cm from the 10 cm lens; move the screen until the image is sharp and record the image distance and the image height.
- Repeat at object distances of 25, 20, 15 and 12 cm; then place the object at 8 cm and view the enlarged virtual image through the lens.
- Plot 1/v against 1/u; the intercepts give 1/f.
- Repeat with the concave mirror using a half-screen beside the object.
What you should see
For f = 10.0 cm and u = 30.0 cm the image forms at v = 15.0 cm, real, inverted and half the object's height; at u = 15.0 cm it forms at 30.0 cm and is twice the height. The 1/v against 1/u graph is a straight line of gradient -1 with both intercepts at 0.100 per cm. Inside the focal length no image forms on the screen and the eye sees an upright enlarged virtual image.
What changes
- What you change
- object distance
- What you measure
- image distance and magnification
- What you keep the same
- same lens
- object height
- screen perpendicular to the axis
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 whole image remains, only dimmer.
- The image sits at the focal point for every object; only very distant objects image at the focus.
Safety card
Hazards
- hot lamp housing
- focused sunlight if a window is used
Controls
- use LED lamps
- never focus the Sun onto skin or paper
Note
Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.