Magnifying power of optical instruments: Class 12 practice
A simple microscope gives m = 1 + D/f (image at near point, D = 25 cm). Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Lens formula
The idea behind magnifying power of optical instruments
A simple microscope gives m = 1 + D/f (image at near point, D = 25 cm). An astronomical telescope in normal adjustment gives m = fₒ/fₑ, with tube length fₒ + fₑ.
Step-by-step method
- Pick the instrument formula.
- Substitute focal lengths in the same units.
- For telescopes, length = fₒ + fₑ.
Worked example 1 (Basic)
Find the magnifying power of a simple microscope of focal length 5 cm (image at 25 cm).
- m = 1 + 25/5
- = 6
- Answer: 6
Worked example 2 (Standard)
A telescope has fₒ = 170 cm and fₑ = 10 cm. Find its magnifying power in normal adjustment.
- m = fₒ/fₑ = 170/10
- = 17
- Answer: 17
Worked example 3 (Challenge)
A telescope (fₒ = 184 cm, fₑ = 2 cm) is in normal adjustment. Find its length and magnifying power.
- L = 184 + 2 = 186 cm
- m = 92
- Answer: 186 cm, 92
Common mistakes and misconceptions
- Using fₑ/fₒ for a telescope
- Forgetting the 1 + in the simple microscope
Practice questions
- (Basic) Find the magnifying power of a simple microscope of focal length 12.5 cm (image at 25 cm).
- (Basic) Find the magnifying power of a simple microscope of focal length 2.5 cm (image at 25 cm).
- (Standard) A telescope has fₒ = 104 cm and fₑ = 4 cm. Find its magnifying power in normal adjustment.
- (Standard) A telescope has fₒ = 99 cm and fₑ = 4 cm. Find its magnifying power in normal adjustment.
- (Challenge) A telescope (fₒ = 113 cm, fₑ = 4 cm) is in normal adjustment. Find its length and magnifying power.
- (Challenge) A telescope (fₒ = 133 cm, fₑ = 4 cm) is in normal adjustment. Find its length and magnifying power.
Answer key with full working
- Q1: m = 1 + 25/12.5 → = 3 → Answer: 3
- Q2: m = 1 + 25/2.5 → = 11 → Answer: 11
- Q3: m = fₒ/fₑ = 104/4 → = 26 → Answer: 26
- Q4: m = fₒ/fₑ = 99/4 → = 24.75 → Answer: 24.75
- Q5: L = 113 + 4 = 117 cm → m = 28.25 → Answer: 117 cm, 28.25
- Q6: L = 133 + 4 = 137 cm → m = 33.25 → Answer: 137 cm, 33.25
Syllabus note
This skill is listed under "Ray Optics" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is magnifying power of optical instruments in?
"Ray Optics", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.