Magnification and actual size: Class 9 practice

What you should know first

• Units

The idea behind magnification and actual size

Total magnification = eyepiece × objective. Actual size = image size ÷ magnification. Convert units: 1 mm = 1000 µm.

Step-by-step method

1. Multiply eyepiece and objective powers.
2. Convert the image size to µm.
3. Divide by magnification.

Worked example 1 (Basic)

A microscope has a 15× eyepiece and a 100× objective. What is the total magnification?

1. 15 × 100
2. Answer: 1500×

Worked example 2 (Standard)

At 50×, a cell's image is 12 mm long. Find its actual length in µm.

1. 12 mm = 12000 µm
2. 12000 ÷ 50 = 240 µm
3. Answer: 240 µm

Worked example 3 (Challenge)

A 19 µm cell is viewed at 400×. How long is the image in mm?

1. Image = 19 × 400 µm = 7600 µm
2. = 7.6 mm
3. Answer: 7.6 mm

Common mistakes and misconceptions

• Adding the lens powers
• Forgetting mm → µm

Practice questions

1. (Basic) A microscope has a 15× eyepiece and a 4× objective. What is the total magnification?
2. (Basic) A microscope has a 10× eyepiece and a 10× objective. What is the total magnification?
3. (Standard) At 1500×, a cell's image is 9 mm long. Find its actual length in µm.
4. (Standard) At 20×, a cell's image is 12 mm long. Find its actual length in µm.
5. (Challenge) A 31 µm cell is viewed at 400×. How long is the image in mm?
6. (Challenge) A 28 µm cell is viewed at 400×. How long is the image in mm?

Answer key with full working

• Q1: 15 × 4 → Answer: 60×
• Q2: 10 × 10 → Answer: 100×
• Q3: 9 mm = 9000 µm → 9000 ÷ 1500 = 6 µm → Answer: 6 µm
• Q4: 12 mm = 12000 µm → 12000 ÷ 20 = 600 µm → Answer: 600 µm
• Q5: Image = 31 × 400 µm = 12400 µm → = 12.4 mm → Answer: 12.4 mm
• Q6: Image = 28 × 400 µm = 11200 µm → = 11.2 mm → Answer: 11.2 mm

Syllabus note

This skill is listed under "The Fundamental Unit of Life" for Class 9 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

• All Class 9 practice

Frequently asked questions

Which chapter is magnification and actual size in?

"The Fundamental Unit of Life", Class 9 Biology (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.