Simple microscope (magnifying glass)
A simple microscope is a single convex lens used with the object inside its focal length. It forms a virtual, upright, magnified image. The magnification is about 1 + D ÷ f, where D = 25 cm is the least distance of distinct vision (the closest the normal eye can focus comfortably).
Compound microscope
A compound microscope uses two convex lenses of short focal length. The objective forms a real, inverted, magnified image of the object. The eyepiece acts as a magnifying glass on that image and gives a final virtual image that is greatly magnified. The magnifying power is about the product of the two magnifications.
Astronomical telescope
An astronomical telescope looks at very distant objects. The objective has a long focal length and forms a small real image of the distant object. The eyepiece has a short focal length and magnifies that image. In normal adjustment the final image is at infinity and the lenses are separated by f₀ + fₑ.
Choose the focal lengths of the objective and the eyepiece.
Projection lantern and the lens camera
| Instrument | Lens | How it works |
|---|---|---|
| Projection lantern (projector) | Convex lens | A bright, inverted slide placed just beyond F; a large, real image is formed on a screen. Put the slide in upside down so the picture is upright |
| Lens camera | Convex lens | Object beyond 2F; a small, real, inverted image forms on the film or sensor. Focus by moving the lens; the diaphragm controls the light and the shutter the time |
| Human eye | Convex lens (flexible) | A real, inverted image forms on the retina; the brain turns it the right way up |
The human eye
The cornea and lens focus light on the retina, which has light-sensitive rods (dim light) and cones (colour). The iris controls the size of the pupil. The ciliary muscles change the shape of the lens: it becomes thicker to focus on near objects and thinner for distant ones (accommodation).
| Defect | What happens | Correction |
|---|---|---|
| Short sight (myopia) | Distant objects blurred: image forms in front of the retina | Concave (diverging) lens |
| Long sight (hypermetropia) | Near objects blurred: image forms behind the retina | Convex (converging) lens |
| Astigmatism | Uneven cornea gives a blurred image in one direction | Cylindrical lens |
| Old-age long sight (presbyopia) | The lens becomes stiff | Convex reading glasses |
Key terms
- Objective
- The lens nearest the object in a microscope or telescope.
- Eyepiece
- The lens nearest the eye.
- Accommodation
- The eye’s ability to change focus by changing the shape of the lens.
- Short sight
- A defect in which distant objects look blurred.
Summary
- A magnifying glass forms a virtual, upright, magnified image.
- Compound microscope: objective makes a real image, eyepiece magnifies it. Telescope magnification = fₒ ÷ fₑ.
- Projector, camera and eye all form real images with convex lenses.
- Short sight is corrected with concave lenses and long sight with convex lenses.