Instruments that use lenses
An optical instrument is a device that uses lenses to help us see things we could not see clearly with the naked eye. Two important examples studied in school are the compound microscope, for very small nearby objects, and the astronomical telescope, for very distant objects such as planets and stars.
Key idea
Both the microscope and the astronomical telescope use two convex lenses: an objective lens (near the object) and an eyepiece (near the eye). The eyepiece acts like a magnifying glass on the image formed by the objective.
The compound microscope
A microscope magnifies tiny objects such as cells. Its objective lens has a short focal length and the object is placed just beyond the objective's focal point. The objective forms a real, magnified image inside the tube, and the eyepiece then magnifies this image further, working just like a magnifying glass. The final image is virtual, inverted and greatly magnified, which is why cells and other tiny structures can be studied in a school laboratory.
The astronomical telescope
A telescope makes distant objects appear larger and closer. Its objective lens has a long focal length, while the eyepiece has a short focal length. In normal adjustment the distance between the two lenses equals the sum of their focal lengths, and the magnification is the objective focal length divided by the eyepiece focal length.
Example
A telescope with an objective focal length of 100 cm and an eyepiece focal length of 5 cm has a magnification of 100 ÷ 5 = 20 times. In normal adjustment its length is 100 + 5 = 105 cm.
Remember
For high magnification a telescope needs a long objective focal length and a short eyepiece focal length, while a microscope needs both lenses to have short focal lengths.