Telescope and Microscope – Working Principle, Types, and Magnification

Telescope and Microscope – Both telescopes and microscopes are optical instruments that use lenses (or mirrors) to magnify objects. While a telescope helps us observe distant celestial bodies, a microscope allows us to study tiny, close objects invisible to the naked eye.

This article explains their principle, structure, magnifying power, and differences in a clear and student-friendly way.

Introduction to Optical Instruments

Optical instruments are devices that process light to form clear and magnified images of objects. They use lenses, mirrors, or both to manipulate light rays.

The two most important magnifying instruments are:

• Telescope: For viewing faraway objects (like stars and planets).
• Microscope: For viewing minute objects (like cells and bacteria).

Despite having different purposes, both work on the principle of refraction of light through lenses.

Part A: Telescope

What Is a Telescope?

A telescope is an optical instrument used to observe distant objects by collecting and magnifying light. It provides angular magnification, making faraway celestial or terrestrial objects appear closer and clearer.

Basic Components:

Objective lens: A large convex lens with a long focal length that collects light from distant objects and forms a real image.

Eyepiece lens: A smaller convex lens with a short focal length that magnifies the image formed by the objective.

Working Principle of Telescope

The telescope works on the refraction of light and angular magnification.

• Light from a distant object enters through the objective lens, forming a real, inverted image at its focal plane.
• The eyepiece lens acts like a magnifier to enlarge this image for the observer’s eye.
• The final image appears magnified, inverted, and formed at infinity for comfortable viewing.

Magnifying Power of a Telescope

The magnifying power (m) of a telescope is defined as:

$$m=\frac{\beta }{\alpha }=\frac{f_0}{f_e}$$

where,

• $\beta$ = angle subtended by the image at the eye
• $\alpha$ = angle subtended by the object at the naked eye
• $f_{\mathrm{0 }}$= focal length of the objective lens
• $f_e$ = focal length of the eyepiece lens

Example:
If $f_0=100\text{ cm}$ and $f_e=1\text{ cm}$,
then $m=100\mathrm{/}1=100$.
This means the telescope magnifies 100 times.

Length of Telescope Tube:

$$L=f_0+f_e$$

Types of Telescopes

A. Refracting Telescope

• Uses two convex lenses (objective and eyepiece).
• Objective has a large focal length; eyepiece has a small focal length.
• The image formed is real, inverted, and magnified.
• Used for both terrestrial and astronomical observations.

B. Reflecting Telescope

• Uses a concave mirror instead of a lens as the objective.
• Avoids chromatic aberration (color distortion).
• Provides clearer, sharper images.
• Commonly used in modern astronomy.

Key Features of an Astronomical Telescope

1. Light-Gathering Power:
Determines how much light the telescope can collect. Larger objectives capture more light, making faint stars visible.

2. Resolving Power:
The ability to distinguish between two closely spaced objects.
A larger diameter of the objective lens increases resolution.

3. Example:
The largest lens objective telescope is at Yerkes Observatory (Wisconsin, USA), with a diameter of 40 inches (~1.02 m).

4. Limitation:

• Large lenses are heavy and hard to support.
• Expensive to make without distortions.
• Hence, most modern telescopes use mirrors instead of lenses.

Terrestrial vs Astronomical Telescopes

Feature	Terrestrial Telescope	Astronomical Telescope
Purpose	Used to view distant objects on Earth	Used to view celestial objects
Image Formed	Erect image	Inverted image
Lenses Used	Has additional inverting lens	No extra lens used
Example	Spyglass, binoculars	Hubble Space Telescope

Part B: Microscope

What Is a Microscope?

A microscope is an optical instrument used to view small, close objects that cannot be seen clearly with the naked eye. It magnifies the fine details of objects such as microorganisms, cells, and crystals.

Main Parts:

• Objective lens: Near the object; forms a real, inverted, magnified image.
• Eyepiece lens: Magnifies the image produced by the objective to form a final virtual, enlarged image.

Working Principle of Microscope

The microscope works on the principle of refraction of light through convex lenses.

• The objective lens first creates a real, inverted, magnified image of the object.
• The eyepiece lens then acts like a magnifier to produce a final enlarged and virtual image.

The combination of both lenses provides high total magnification.

Magnification in a Microscope

For a compound microscope, magnifying power is the product of magnifications of both lenses:

$$M=M_{\text{objective}}\times M_{\text{eyepiece}}$$

Since the objective forms a magnified real image and the eyepiece enlarges it again, overall magnification is very high.

In general:

$$M\propto \frac{1}{f_0\times f_e}$$

Smaller the focal lengths, greater the magnification.

Practical Note:
In real microscopes, both lenses have small focal lengths (less than 1 cm) to achieve large magnification.

Types of Microscopes

A. Simple Microscope (Magnifying Glass)

• Consists of a single convex lens.
• Used for magnifications less than 9×.
• Produces virtual, erect, and enlarged images.
• Example: Reading lenses, jeweller’s lenses.

B. Compound Microscope

• Uses two convex lenses (objective and eyepiece).
• Objective has very short focal length.
• Produces enlarged, inverted, and virtual images.
• Used in laboratories for biological and material study.

Ray Diagram for a Compound Microscope

1. Object placed slightly beyond the focal point of the objective lens.
2. Objective forms a real, inverted, magnified image.
3. This image acts as an object for the eyepiece lens.
4. Eyepiece forms a virtual, enlarged image seen by the eye.

Hence, the microscope provides very high magnification and fine detail.

Convex and Concave Lenses

Convex Lens (Converging Lens):

• Converges light rays inward to a focus.
• Used in microscopes and telescopes.
• Produces real and inverted images (in most cases).

Concave Lens (Diverging Lens):

• Diverges light rays outward.
• Produces virtual and erect images.
• Used in spectacles and optical corrections.

Differences Between Telescope and Microscope

Feature	Telescope	Microscope
Purpose	Viewing distant objects	Viewing very small nearby objects
Distance of Object	Infinite or very far	Very close
Lens Type	Objective with long focal length	Objective with short focal length
Magnification	Moderate	Very high
Image Formed	Inverted and real	Enlarged, inverted, and virtual
Used In	Astronomy, navigation	Biology, materials science

Applications

Telescopes:

• Used in astronomy to observe planets, stars, and galaxies.
• In navigation and defense for distant viewing.
• In satellite communication and space research (e.g., Hubble Space Telescope).

Microscopes:

• Used in medical and biological studies to observe cells, bacteria, and tissues.
• In forensic science for trace analysis.
• In material research for studying crystal structures.

Summary

Instrument	Type of Lens Used	Purpose	Nature of Image	Magnification Principle
Telescope	Convex (or mirror)	View distant objects	Real, inverted	$m=f_0\mathrm{/}{f}_e$
Microscope	Two convex lenses	View small close objects	Virtual, inverted	$M=M_o\times M_e$

FAQ

Q1. What is the difference between refracting and reflecting telescopes?

A refracting telescope uses lenses, while a reflecting telescope uses mirrors to collect and focus light.

Q2. What is the principle of a microscope?

It works on refraction of light through convex lenses to form magnified virtual images.

Q3. Why is the image in a telescope inverted?

Because the objective lens forms a real image that gets inverted when magnified by the eyepiece.

Q4. What determines the magnifying power of a microscope?

The focal lengths of the objective and eyepiece lenses — shorter focal lengths provide higher magnification.

Q5. Why do astronomers prefer reflecting telescopes?

Mirrors eliminate chromatic aberration, provide clearer images, and are easier to build in large sizes.