Optical instruments such as the telescope and microscope have changed the way we explore the universe and study the microscopic world. Both rely on the principles of refraction and lens magnification, but they serve opposite purposes: telescopes bring distant celestial objects closer to our eyes, while microscopes reveal the hidden details of tiny specimens. Understanding their working principles, lens arrangements, and applications helps students build a strong foundation in physics and astronomy.
Telescope: An Instrument to See the Distant Universe
A telescope is designed to provide angular magnification of distant objects. It typically consists of two lenses:
Objective lens: A convex lens with a large focal length and large aperture, responsible for collecting light.Light from a distant source enters the objective, forming a real image at its focal plane. The eyepiece then magnifies this image, producing the final view. In an astronomical telescope, the final image is inverted, while in a terrestrial telescope, additional inverting lenses are used to make the image erect.
Magnifying Power of a Telescope
The magnifying power is given by:
where:
- = focal length of the objective
- = focal length of the eyepiece
For example, if the objective has a focal length of 100 cm and the eyepiece 1 cm, the magnification is .
Types of Telescopes
- Refracting Telescope – Uses convex lenses for both objective and eyepiece.
- Reflecting Telescope – Uses a concave mirror as the objective to avoid chromatic aberration.
Key Features of Astronomical Telescopes
- Light Gathering Power: Larger aperture objectives collect more light, making faint stars visible.
- Resolving Power: Ability to distinguish two close objects. Higher for large diameter lenses/mirrors.
The largest refracting telescope objective has a diameter of 40 inches at the Yerkes Observatory in the USA. However, such large lenses are difficult to manufacture, heavy, and prone to distortions. This is why modern astronomical telescopes mostly use mirrors instead of lenses.
Microscope: An Instrument to See the Invisible
Unlike telescopes, microscopes are designed to magnify very small, nearby objects. They too use a combination of convex lenses but with much smaller focal lengths.
Simple Microscope
- Consists of a single convex lens of small focal length.
- Provides limited magnification (less than 9x).
- Produces an enlarged, virtual, and erect image of the object.
Compound Microscope
Consists of two convex lenses:
- Objective (close to the object) – produces a real, inverted, magnified image.
- Eyepiece – further magnifies this image, acting like a simple magnifier.
Magnification in a Microscope
Magnification increases when the focal length of the objective decreases, allowing the lens to be closer to the object and capture finer details.
Converging and Diverging Lenses
- Convex Lens (Converging Lens): Bends incoming parallel rays inward to a focus point. Used in both telescopes and microscopes.
- Concave Lens (Diverging Lens): Spreads out light rays; used in corrective optics.
Differences Between Telescope and Microscope
| Feature | Telescope | Microscope |
|---|---|---|
| Purpose | To magnify distant objects (e.g., stars, planets) | To magnify small nearby objects (e.g., cells, bacteria) |
| Objective Lens | Large focal length | Very small focal length |
| Eyepiece Lens | Small focal length | Small focal length |
| Image Nature | Inverted (astronomical), erect (terrestrial) | Enlarged, inverted, and virtual |
| Applications | Astronomy, space exploration, surveillance | Biology, medicine, research, forensic science |
Applications of Telescopes
- Observing stars, galaxies, and planets.
- Navigation and military purposes.
- Space exploration through observatories and satellites.
Applications of Microscopes
- Studying microorganisms, tissues, and cells in biology.
- Forensic analysis and material science.
- Medical diagnostics and research in pharmaceuticals.
FAQs About Telescope and Microscope
Q1. What is the main difference between a telescope and a microscope?
A telescope magnifies distant celestial objects, while a microscope magnifies nearby microscopic objects.
Q2. Why do telescopes need large objective lenses?
Large objectives gather more light, making faint stars and galaxies visible.
Q3. Why are modern telescopes mostly reflecting instead of refracting?
Reflecting telescopes use mirrors, which avoid chromatic aberration and are easier to manufacture in large sizes.
Q4. What type of lenses are used in microscopes?
Microscopes use convex lenses with small focal lengths for both the objective and eyepiece.
Q5. Which instrument has higher magnification: a telescope or a microscope?
A compound microscope can achieve much higher magnification compared to a telescope, but only for nearby objects.
