The shape of the lesson
By the end of the lesson, learners can
- Trace the rays to the image: Move the object along the bench and watch the rays cross to build a real image, or spread to form a virtual one.
- Where the object sits sets the image: The same lens gives very different images depending on the object distance compared with the focal length.
- The three rays: parallel then through F, straight through the centre (undeviated), and through F then parallel.
- Focal length is the distance from the lens to the principal focus F, not to the image.
- Draw cleanly: single straight ruled lines with arrows. No curves, no doubled lines.
One picture every idea returns to
A converging lens refracts a parallel beam of light to a single point, the principal focus, a distance called the focal length from the lens. Where the object sits relative to the focal point decides whether the image is real or virtual, larger or smaller, upright or inverted.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Bending light to a point.. A convex lens is fatter in the middle, and that extra glass bends parallel light inward until it crosses at a single point. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Trace the rays to the image | Move the object along the bench and watch the rays cross to build a real image, or spread to form a virtual one. Project tab 6 of the lab (Lenses) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Where the object sits sets the image | The same lens gives very different images depending on the object distance compared with the focal length. Model the worked example Worked example on the board, one line at a time. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (7 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Where the object sits sets the image, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The magnifying glass image, formed when the object is closer than the focal length, is virtual, upright and enlarged, and cannot be caught on a screen. | Only a real image, formed when the object is beyond the focal length, is inverted and can be projected. |
The converging lens: worksheet
7 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms principal focus and focal length for a converging lens.
On a ray diagram, a student is asked to mark the focal length. They label the distance from the lens to the point where the image forms. Explain why this is wrong, and describe what they should have marked.
The diagram shows a converging lens with its principal foci F and the points 2F marked. An object stands beyond 2F.
State the three standard rays used to locate the image formed by a converging lens.
An object is placed between F and 2F in front of a converging lens. Describe the image formed, giving its nature, orientation, and size, and state one device that uses a lens in this way.
A converging lens is used as a magnifying glass to read small print.
(a) Where must the print be placed relative to the principal focus? [1] (b) Describe the image seen (nature, orientation, size). [2]A student draws a ray diagram in which the ray passing through the centre of the lens is shown bending slightly, and all of the rays are sketched freehand as slightly wavy lines. State the two drawing faults that would cost marks, and how to correct them.
Answer key and marking notes
- Principal focus: the point on the axis where rays travelling parallel to the principal axis converge after passing through the lens. ✓
- Focal length: the distance from the centre of the lens to the principal focus. ✓
- That distance is the image distance, which changes when the object moves. ✓
- The focal length is the fixed distance from the lens to the principal focus F, the point where parallel rays are focused. ✓
- Rays converge between F and 2F: a real, inverted, diminished image.
(a) Parallel ray bends through F, central ray goes straight through the lens centre. They cross between F and 2F. ✓✓
(b) The image is real, inverted, and diminished. ✓✓
Object beyond 2F always gives a real, inverted, diminished image between F and 2F. This is the camera case.
- A ray parallel to the axis, which then passes through the principal focus F on the far side. ✓
- A ray through the centre of the lens, which continues straight and undeviated. ✓
- A ray through the principal focus F in front of the lens, which then travels parallel to the axis. ✓
- Real and inverted. ✓
- Magnified (larger than the object), forming beyond 2F. ✓
- Used in a projector (or a photographic enlarger). ✓
(a) Closer to the lens than the principal focus (inside F). ✓
(b) The image is virtual, upright, and magnified. ✓✓
Inside F the rays diverge, so the image is virtual and on the same side as the object. This is the only converging-lens case giving an upright image.
- The central ray must pass straight through the centre, undeviated. It should not bend. ✓
- Rays must be single straight lines drawn with a ruler, not freehand wavy lines. ✓
7 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Parallel rays of light pass through a converging lens. Where do they meet?
- AAt the principal focus (focal point).
- BThey spread out and never meet.
- CAt the centre of the lens.
- DThey stay parallel and do not change direction.
Which describes a converging lens?
- AA convex lens that is thicker in the middle than at the edges.
- BA concave lens that is thinner in the middle than at the edges.
- CA flat sheet of glass.
- DA curved mirror.
An object is placed well beyond the focal length of a converging lens and a sharp image is formed on a screen. The image is:
- Areal and inverted
- Breal and upright
- Cvirtual and upright
- Dvirtual and inverted
A converging lens is used as a magnifying glass. Where must the object be placed, and what kind of image is seen?
- ACloser to the lens than the principal focus, giving a virtual, upright, enlarged image.
- BBeyond twice the focal length, giving a real, inverted image.
- CExactly at the principal focus, giving no image.
- DVery far away, giving a tiny real image.
An object is placed beyond twice the focal length (2F) of a converging lens. What is the nature of the image?
- Areal, inverted and diminished
- Breal, upright and enlarged
- Cvirtual, upright and enlarged
- Dvirtual, inverted and diminished
A diverging (concave) lens always forms an image that is:
- Avirtual, upright and diminished
- Breal, inverted and enlarged
- Creal, upright and the same size
- Dvirtual, inverted and enlarged
What does it mean to describe an image as virtual?
- AThe rays only appear to come from the image, so it cannot be formed on a screen.
- BThe image is always smaller than the object.
- CThe image is always upside down.
- DThe image is formed by reflection only.
Quiz key
A converging lens refracts parallel rays so that they meet at the principal focus. Rays that spread out (B) would describe a diverging lens.
A converging lens is convex, thicker in the middle, which bends rays inward to a focus. A concave lens (B) is thinner in the middle and diverges light.
An image formed on a screen is real, and a converging lens turns it upside down, so it is real and inverted. A virtual image (C, D) cannot be projected onto a screen.
When the object is inside the focal length, a converging lens forms a virtual, upright, enlarged image, which is how a magnifying glass works. Placing the object beyond 2F (B) gives a real, inverted, diminished image instead.
Beyond 2F, a converging lens forms a real, inverted, diminished image (as in a camera). A real image formed by a single converging lens is always inverted, ruling out the upright options.
A diverging lens always produces a virtual, upright, diminished image, whatever the object distance. It cannot form a real image because it spreads the rays apart.
A virtual image is one from which the rays only appear to diverge, so it cannot be projected onto a screen. Its size and orientation depend on the situation, so options B and C are not always true.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.