Key relationship
1/F=1/f₁+1/f₂−d/(f₁f₂); in contact d=0.
Physics · Practical apparatus
Combined-lens focal measurement
Compare contact and separated holders. Touching lenses add power; a gap changes the system power and principal-plane location. Equal positive and negative contact powers make an afocal pair.
Open the interactive experiment →Combined-lens focal measurement
தொடுபடும் லென்சுகளின் திறன்கள் சேரும். இடைவெளி இருந்தால் கூட்டு குவியத் தொலைவும் முதன்மைத் தளங்களும் மாறும்.
Key notes, relationships and model limits
Practical tip
Compare contact and separated holders. Touching lenses add power; a gap changes the system power and principal-plane location. Equal positive and negative contact powers make an afocal pair.
Common trap
Holder spacing is not a negligible correction for every separated lens pair.
Model limits
Thin lenses in air; signed concave second focal length. For separated lenses, equivalent focal length is relative to shifted principal planes, not necessarily a holder-to-screen distance.
What you will learn
Determine combined optical power and identify an afocal limit.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
Two+20 cm lenses touching give F10 cm;10 cm gap gives F13.33 cm.
Why this matters
The model connects combined-lens focal measurement with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
A+20 cm and−20 cm lens touching have?
- 20 cm focal length
- Zero net power
- Double positive power
Read the explanation
Their powers+5 D and−5 D cancel.
Course connection: Virtual apparatus preparation. Check exact board/exam membership in the syllabus sources. Physical practical work remains necessary.