Microscopes and telescopes

Simple microscope

A simple microscope is a convex lens of short focal length. It is also called a magnifying glass or reading lens. The object is placed between F and the optic centre. The lens forms a virtual, erect and magnified image on the same side as the object. When the image is at the near point (D = 25 cm), the magnification m = 1 + D/f. Watch repairers and jewellers use it, and so do people reading small print.

Compound microscope: how it forms the image

A **compound microscope** has two convex lenses at the ends of a tube. The **objective** (near the object) has a **short focal length** and a small aperture (width). The **eyepiece** (near the eye) has a longer focal length and a larger aperture.

1. 1
   The object is placed just beyond the focus of the objective.
2. 2
   The objective forms a real, inverted and magnified image inside the tube.
3. 3
   This image lies within the focal length of the eyepiece. It acts as the object for the eyepiece.
4. 4★
   The eyepiece works as a simple microscope and gives a virtual, enlarged final image. Your textbook calls it erect: it is upright compared with the first image. Compared with the object itself, it is still inverted.
5. 5
   It magnifies in two stages, so it magnifies much more than a simple microscope.

★ key step

Telescopes: types, uses, advantages and disadvantages

• A telescope helps us see distant objects clearly. The first one was made by Lippershey, a Dutch spectacle maker, in 1608. Galileo built his own and saw the moons of Jupiter.
• By optical parts: refracting telescopes use lenses (Galilean, Keplerian). Reflecting telescopes use curved mirrors (Newtonian, Cassegrain).
• Astronomical telescope: used to see stars, planets and galaxies. Its objective has a long focal length and a large aperture to collect more light. The final image is inverted, which does not matter for stars.
• Terrestrial telescope: used to see distant objects on the Earth, such as ships and birds. An extra erecting lens makes the final image erect, so its tube is longer.
• Advantages: detailed views of planets, stars and galaxies; a camera can be attached to photograph them; useful even in low light.
• Disadvantages: needs frequent maintenance; not easy to carry.

How to draw the compound microscope

• Draw a principal axis and two convex lenses: a small objective on the left and a larger eyepiece on the right. Mark Fₒ and Fₑ.
• Draw a small upright object AB just beyond Fₒ. Draw two rays from its top. They meet to form a large, inverted, real image A′B′ between the lenses, inside the focal length of the eyepiece.
• From A′B′, draw rays through the eyepiece. Trace them back with dotted lines to a large final image A″B″, the same way up as A′B′. Label it virtual, enlarged.
• Label objective, eyepiece, AB, A′B′ and A″B″, and put arrows on every ray.

Magnification of a simple microscope