How a magnifying glass makes things bigger

A convex lens bends parallel light inward to a focus. How it forms an image depends on where the object is placed relative to its focal point.

When the object is closer to the lens than the focal point, the refracted rays spread apart instead of meeting. Your eye traces them back to where they seem to come from, and sees a larger, upright image on the same side as the object. This is a virtual image, because the light does not actually pass through it. That is exactly how a magnifying glass works when you hold it near small print.

Move the lens too far from the page and the image flips and shrinks, because the object is now beyond the focal point. Keeping it close is the trick to a clear, enlarged view.

In SPM you should use ray diagrams to explain the enlarged virtual image formed by a convex lens used as a magnifying glass.

Light and Optics

The physics behind it

A magnifying glass is a convex (converging) lens, and how much bigger it makes something is measured by the linear magnification, m. Magnification is the ratio of the image height to the object height, m = hᵢ/hₒ, and it has no unit because it is a ratio of two lengths.

It can equally be written as the ratio of image distance to object distance, m = v/u.

Suppose the print you are reading is hₒ = 4 mm tall and the image you see is hᵢ = 12 mm tall; then m = 12 mm ÷ 4 mm = 3, so the letters look three times as tall. To get an upright, enlarged image the object must lie inside the focal length, that is the object distance u must be less than the focal length f, measured from the lens.

A lens with a shorter focal length bends the light more strongly and gives a larger magnification for the same position, which is why a strong magnifier is a small, fat lens rather than a large, flat one.

In everyday life

You do not need a proper lens to see this. Place a single drop of water on a sheet of clear plastic laid over a newspaper, and the curved drop acts as a tiny convex lens, making the letters beneath it swell.

The rounded surface of the water bends the light inward just as glass does.

A jeweller inspecting a ring uses the same physics in a loupe, a small high-power lens held close to the eye and close to the object, so that a hallmark only a fraction of a millimetre across becomes readable. Older relatives reading the fine expiry date on a medicine box rely on it too, holding the glass near the box so the object stays inside the focal length.

In every case the rule is the same: keep the lens close to what you are looking at. Slide it too far away and the enlarged upright view is lost, because the object then moves beyond the focal point and the lens starts forming a small inverted image instead.

How this comes up in SPM

In Paper 2 this belongs to the Light and Optics chapter, where command words such as describe, explain, state and compare are common. You might be asked to describe how a convex lens forms the image seen in a magnifying glass, to state the nature of that image (virtual, upright and enlarged), or to compare the images formed when the object is inside and outside the focal length.

Drawing an accurate ray diagram is often required, using the standard rays through the optical centre and parallel to the principal axis.

The idea connects to its neighbours in the same chapter: the general study of image formation by convex and concave lenses, the meaning of focal length and optical centre, and linear magnification as m = hᵢ/hₒ. It also leads into optical instruments built from lenses, where a magnifying glass is the simplest case.

Learn to link the object position to the type of image produced, so you can reason through any lens arrangement rather than memorising single results.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)