SingaporePhysicsSyllabus dot point

How does a converging lens form an image, and what kind of image does it produce?

Describe how a thin converging lens forms images and use ray diagrams and the focal length

A focused answer to the O-Level Physics outcome on converging lenses. The principal focus and focal length, drawing ray diagrams, the nature of real and virtual images, and how images change as the object moves.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe how a thin converging lens forms images, to know the principal focus and focal length, to draw ray diagrams locating the image, and to state the nature of the image (real or virtual, upright or inverted, magnified or diminished) for different object positions. The big idea is that a converging lens bends parallel light to a single point, and where the object sits decides the image.

The answer

The converging lens and its focal point

A converging (convex) lens is thicker in the middle and bends parallel rays of light inward so they meet at a single point, the principal focus (focal point). The focal length $f$ is the distance from the centre of the lens to the principal focus. A more strongly curved lens has a shorter focal length and bends light more.

Drawing ray diagrams

To locate the image of the top of an object, draw two of these standard rays and find where they cross:

A ray parallel to the principal axis, which passes through the principal focus after the lens.
A ray through the centre of the lens, which goes straight on undeviated.
A ray through the principal focus on the object side, which leaves parallel to the axis.

Real and virtual images

A real image is formed where rays actually meet; it can be caught on a screen and is inverted.
A virtual image is where rays only appear to come from; it cannot be caught on a screen and is upright.

How the image depends on object distance

Object position	Image nature
Beyond $2F$	Real, inverted, diminished
At $2F$	Real, inverted, same size
Between $F$ and $2F$	Real, inverted, magnified
Inside $F$	Virtual, upright, magnified

When the object is inside the focal length, the lens acts as a magnifying glass, giving an enlarged upright virtual image.

Worked example

An object is placed $15\ \text{cm}$ from a converging lens of focal length $10\ \text{cm}$ . Determine the nature of the image and explain which device uses this.

Step 1: Compare the object distance with f and 2f

The focal length is $f = 10\ \text{cm}$ , so $2f = 20\ \text{cm}$ . The object at $15\ \text{cm}$ is between $f$ and $2f$ .

Step 2: Recall the image for this region

For an object between $F$ and $2F$ , the two construction rays cross on the far side beyond $2F$ , giving a real, inverted, and magnified image.

Step 3: Name a device

This arrangement, a real magnified inverted image, is used in a projector (and is the basis of how a film or slide is thrown enlarged onto a screen).

So the image is real, inverted, and magnified, exactly the behaviour a projector relies on.

Examples in context

Example 1. The human eye and a camera. Both use a converging lens to throw a real, inverted, diminished image of a distant scene onto a light-sensitive surface, the retina in the eye and the sensor in a camera. The brain (or camera software) interprets the inverted image the right way up.

Example 2. A magnifying glass. Hold a converging lens close to small print, inside its focal length, and you see an enlarged, upright image. This works because, with the object inside $F$ , the rays diverge after the lens and only appear to come from a larger virtual image behind the object, which the eye sees magnified.

Try this

Q1. Define the principal focus of a converging lens. [2 marks]

Cue. The point on the principal axis where rays travelling parallel to the axis are brought to a focus after passing through the lens.

Q2. State the nature of the image when an object is placed beyond $2F$ from a converging lens. [2 marks]

Cue. Real, inverted, and diminished.

Q3. Explain why a converging lens used as a magnifying glass gives an upright image. [2 marks]

Cue. With the object inside the focal length, the refracted rays diverge and only appear to come from an enlarged virtual image behind the object, which is upright.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksAn object is placed

30\ \text{cm}

from a converging lens of focal length

10\ \text{cm}

. (a) Describe how you would locate the image using a ray diagram. (b) State the nature of the image formed (real or virtual, upright or inverted, magnified or diminished).

Show worked answer →

(a) Draw two rays from the top of the object: one parallel to the principal axis that passes through the principal focus after the lens, and one through the centre of the lens that goes straight on. Where these rays cross is the top of the image.

(b) With the object beyond twice the focal length ( $30\ \text{cm} > 20\ \text{cm}$ ), the image is real, inverted, and diminished (smaller than the object), formed on the far side of the lens.

Markers reward the two correct construction rays meeting to locate the image, and the image described as real, inverted, and diminished for an object beyond $2F$ .

Original4 marks(a) Define the focal length of a converging lens. (b) An object is placed between a converging lens and its principal focus. State the nature of the image and name one device that uses this arrangement.