Have you ever wondered how a microscope allows you to see tiny bacteria or how a telescope helps you observe distant stars? These instruments use lenses to magnify objects, making them appear larger than they actually are. The ability of an optical instrument to enlarge an image is called its magnifying power.
In this post, we will break down the concept of magnifying power, its formula, and how it applies to different optical instruments like magnifying glasses, microscopes, and telescopes. We will also include mnemonics, real-life examples, and practice questions to help you master the topic!
What is Magnifying Power?
Magnifying power (also called angular magnification) is the ability of an optical instrument to make an object appear larger. It is defined as:
Magnifying Power (M) = Ratio of the size of the image to the size of the object
Mathematically, it is written as:
- M=Image size / Object size
In simpler terms, if an object appears 5 times larger when viewed through an instrument, its magnifying power is 5.
Magnifying Power of Different Optical Instruments
1. Simple Magnifying Glass (Convex Lens)
A magnifying glass is a convex lens that enlarges objects when held close to them. Its magnifying power formula is:
M=1+D/f
Where:
- M = Magnifying power
- D = Near point distance (usually 25 cm for a normal human eye)
- f = Focal length of the lens
Example: If a magnifying glass has a focal length of 5 cm, then: M=1+255=1+5=6M = 1 + \frac{25}{5} = 1 + 5 = 6 So, the object appears 6 times larger.
2. Compound Microscope
A compound microscope has two convex lenses:
- Objective lens (near the object)
- Eyepiece lens (near the eye)
The total magnifying power is given by:
M=Mo×Me
- Mo = Magnification by the objective lens
- Me = Magnification by the eyepiece lens
If the objective lens magnifies 10 times and the eyepiece lens magnifies 5 times, then: M=10×5=50, This means the object appears 50 times larger.
3. Astronomical Telescope
A telescope helps us see distant objects like stars and planets. The magnifying power is given by:
M=fo / fe
Where:
- fo = Focal length of the objective lens
- fe = Focal length of the eyepiece lens
Example: If a telescope has an objective lens of 100 cm focal length and an eyepiece lens of 5 cm, then: M=100/ 5=20, This means the telescope makes the object appear 20 times larger.
Mnemonics to Remember the Formulas
Here’s a fun way to remember these formulas:
“Many Daring Frogs Jump (M = 1 + D/f)”
Use this phrase to recall the formula for magnifying glass.
“OET = Objective Eyepiece Telescope (M = Mo × Me or M = fo/fe)”
This helps you remember the compound microscope and telescope formulas.
Real-Life Examples
- Magnifying Glass: Used by detectives to examine clues.
- Microscope: Used by scientists to study bacteria.
- Telescope: Used by astronomers to explore space.
Exam-Oriented Tips
- Remember that higher magnifying power means a more enlarged image.
- Convex lenses are used in magnifying glasses, microscopes, and telescopes.
- Use the formulas correctly—check whether you are solving for a magnifying glass, microscope, or telescope.
- Practice numerical problems to strengthen your understanding.
Quick Quiz
Q1: What is the formula for magnifying power of a magnifying glass?
A. M=D/f
B. M=1+D/f
C. M=fo / fe
D. M=Mo×Me
Q2: A microscope has an objective lens magnification of 20x and an eyepiece magnification of 10x. What is its total magnification?
Q3: A telescope has an objective lens focal length of 120 cm and an eyepiece focal length of 10 cm. What is its magnifying power?
Conclusion
Understanding magnifying power is essential for competitive exams. Optical instruments like magnifying glasses, microscopes, and telescopes use lenses to enlarge images. Remembering formulas with mnemonics, practicing numerical problems, and relating them to real life will make learning easy and fun!
