Talha's Physics Academy
Geometrical Optics - Optical Instruments
Describe construction, working and uses of Telescope.
Telescope
"An astronomical telescope is an optical instrument used to view distant heavenly bodies such as the Moon, stars, planets, and other celestial objects."
Figure: Ray diagram showing image formation by an astronomical telescope using objective and eyepiece lenses.
Construction
An astronomical telescope consists of two converging lenses mounted coaxially:
- Objective Lens: A convex lens of large focal length ($f_o$) and large aperture to collect maximum light from distant objects. It is often made of two convex lenses in contact to reduce chromatic and spherical aberrations.
- Eyepiece Lens: A convex lens of shorter focal length compared to the objective. It is also typically a combination of two lenses.
The objective lens is mounted on a wide outer metallic tube, while the eyepiece is mounted on a smaller inner tube. The distance between the eyepiece and the objective can be smoothly adjusted by sliding the tubes.
Working
Parallel rays of light coming from a distant object fall on the objective lens at an angle. After refraction, the objective converges these rays at its principal focus to form a real, inverted intermediate image ($AB$). This image acts as a virtual object for the eyepiece. The position of the eyepiece is adjusted so that this image lies within its focal length ($f_e$). Consequently, the eyepiece forms a magnified, virtual, and inverted final image with respect to the original object, typically formed at infinity.
Mathematically, the magnifying power ($M$) of an astronomical telescope is given by:
$M = \frac{f_o}{f_e}$
Where $f_o$ = focal length of the objective lens and $f_e$ = focal length of the eyepiece lens.
Uses
- Have extended our vision deep into the universe, revealing craters and mountains on the Moon.
- Showed that Earth is not the center of the universe, revolutionizing astronomy.
- Modern telescopes reveal weather patterns and geography on planets, as well as new asteroids and dwarf planets within our solar system.
- Provide profound evidence of billions of galaxies, each containing billions of stars.
- Used in discovering exoplanets orbiting distant stars and exploring potential habitable worlds.
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