An optical instrument uses lenses to give the eye a larger or clearer image. To explain any of them, say what each lens does and where the image forms at each stage.
This lesson is part of SPM Physics light and optics. The image rules come from drawing ray diagrams for lenses.
What answer pattern works for every instrument?
Write one sentence for each stage.
- The object: where it is placed, and what type of lens it meets first.
- The first image: its type and position.
- The eyepiece: how it treats that first image.
- The final image: real or virtual, upright or inverted, and magnified.
Worked example 1: the magnifying glass
A magnifying glass is a single convex lens. The object is placed inside the focal length. The lens forms a virtual, upright, magnified image behind the object, which the eye sees through the lens.
Worked example 2: a simple astronomical telescope
An original description for a telescope with an objective of focal length 80 cm and an eyepiece of focal length 4 cm.
The object is very far away, so its rays are almost parallel. The objective forms a real, inverted, diminished image at its focal point, 80 cm from it.
The eyepiece is positioned so that this image falls at its own focal point, 4 cm away. The eyepiece works as a magnifying glass and gives a final virtual, magnified image that is inverted relative to the object. The lenses are 80 + 4 = 84 cm apart.
When the final image is at infinity, the magnification is f(objective) ÷ f(eyepiece) = 80 ÷ 4 = 20. Use this if your syllabus asks for it.
Worked example 3: a compound microscope
The object is placed just outside the focal point of the objective, which has a short focal length. The objective forms a real, inverted, magnified first image. The eyepiece, acting as a magnifying glass, magnifies this first image again into a virtual final image.
The mistake that costs marks
The common slip is to swap the roles: saying the eyepiece forms the first image of the object. In every two-lens instrument, the objective is the lens nearer the object and it forms the first image.
| Item | Wrong | Right |
|---|---|---|
| First image | Formed by the eyepiece | Formed by the objective |
| Eyepiece job | Collects light from the object | Magnifies the first image like a magnifying glass |
Another slip is listing the parts without saying what kind of image each one forms. Name each image as real or virtual.
Check yourself
Explain how a compound microscope forms the final image of a small specimen. Name the type of image at each stage.
Answer
The specimen is placed just outside the focal point of the objective lens. The objective forms a real, inverted, magnified first image.
The eyepiece is used as a magnifying glass, with the first image inside its focal length. It forms a virtual, magnified final image that the eye sees.
What to study next
Practise all the optics skills with the light and optics practice set. If instrument explanations still feel loose, revisit calculating image position and magnification.
The units and significant-figure checker helps with answer presentation. If you want a teacher to review your written explanations, see online one-to-one Physics tuition.