Lesson overview
Lenses and images: How does a lens make a sharp picture on a screen?
Let in the light from a far window and slide a screen until the lens makes a sharp picture: that distance is the focal length. Then put a lamp with an arrow-shaped hole on the bench and find its image at 30, 20, 15 and 12 cm, and see what happens at the principal focus. Change to a diverging lens and trace its spreading rays back. Supplement: put the lamp inside the focal length and look through the lens, as through a magnifying glass.
Suggested sequence
- Lesson 1 · Lenses and real images. Converging and diverging lenses, the principal axis, the principal focus and the focal length; the focal length and real images found on an optical bench; ray diagrams for a real image, and describing images as enlarged, same size or diminished, upright or inverted, real or virtual. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Virtual images. A diverging lens spreading a parallel beam, and virtual images that no screen can catch; Supplement: the ray diagram for a virtual image, the magnifying glass, and lenses that correct long and short sight. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Lenses and real images
- Describe the action of thin converging and thin diverging lenses on a parallel beam of light; define and use the terms focal length, principal axis and principal focus (focal point) (3.2.3.1, 3.2.3.2)
- Draw and use ray diagrams for the formation of a real image by a converging lens; describe the characteristics of an image using the terms enlarged/same size/diminished, upright/inverted and real/virtual (3.2.3.3, 3.2.3.4)
Part 2: Virtual images
- Describe the action of a thin diverging lens on a parallel beam of light; know that a virtual image is formed when diverging rays are extrapolated backwards and does not form a visible projection on a screen (3.2.3.1, 3.2.3.5)
- Supplement: Draw and use ray diagrams for the formation of a virtual image by a converging lens; describe the use of a single lens as a magnifying glass (3.2.3.6, 3.2.3.7)
- Supplement: Describe the use of converging and diverging lenses to correct long-sightedness and short-sightedness (3.2.3.8)
Misconceptions to listen for
- “Distance 3 is the principal focus.” — The principal focus is a point, where the rays meet. The distance from the lens to it is the focal length.
- “Line 1 is a ray of light, because the rays travel along it.” — Line 1 is the principal axis, a construction line through the centre of the lens; a ray along it would go straight through.
- “Point 2 is the focal length.” — The focal length is a distance, so it is drawn as a double arrow. Point 2 is the principal focus.
- “The rays meet at one point beyond the lens, then spread apart again after that point.” — That is what a converging lens does: it is thicker in the middle. A diverging lens bends the rays away from each other, so they never meet beyond it.
- “The rays stay parallel to each other, but the lens squeezes them into a narrower beam.” — A narrower beam would need rays bent towards each other, and then they would not stay parallel. A diverging lens makes the beam wider.
- “The rays stay parallel to each other, because a thin lens hardly bends any of them.” — A thin lens still bends every ray except the one along the principal axis.
- “The focal length is 49.5 cm.” — That is the card’s mark on the rule. The focal length is the distance between the lens and the card: the difference of the two marks.
- “A lamp across the room would give the same distance.” — Only a very distant object gives its image at the principal focus; a nearer object’s image is further away.
- “A ray through the centre of the lens bends towards the principal focus.” — At the centre the two faces of a thin lens are parallel, so the ray goes straight on.
- “A ray parallel to the axis stays parallel to it.” — The lens bends it through the principal focus on the far side.
Scope
The action of thin converging and diverging lenses on a parallel beam of light (3.2.3.1); focal length, principal axis and principal focus (3.2.3.2); ray diagrams for the formation of a real image by a converging lens (3.2.3.3); describing an image as enlarged, same size or diminished, upright or inverted, real or virtual (3.2.3.4); a virtual image formed when diverging rays are extrapolated backwards, which cannot be projected onto a screen (3.2.3.5); Supplement: ray diagrams for a virtual image formed by a converging lens (3.2.3.6), a single lens as a magnifying glass (3.2.3.7), and converging and diverging lenses correcting long-sightedness and short-sightedness (3.2.3.8). The lens equation and lens power are not in the syllabus; ray diagrams for images formed by a diverging lens are not drawn here; the structure of the eye is in Biology; dispersion of light is in Light and the electromagnetic spectrum.