GCSE Physics revision

Lenses (physics only)

Electromagnetic waves

AQA 4.6.2.5

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Construct ray diagrams to illustrate the similarities and differences between convex and concave lenses.

MOST 🎯🎯

Apply the specified scientific knowledge of lenses to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of lenses using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Two Types of Lenses

A convex (converging) lens causes light rays to converge (come together) to a focal point, whilst a concave (diverging) lens causes light rays to diverge (spread apart).
Both types of lens work due to refraction — the bending of light as it passes from a less dense medium (air) into a more dense medium (glass or perspex).
A helpful memory trick: a concave lens caves inwards, much like the entrance to a cave.

1. Two Types of LensesAQA

Notes

A convex (converging) lens causes light rays to converge (come together) to a focal point, whilst a concave (diverging) lens causes light rays to diverge (spread apart).
Both types of lens work due to refraction — the bending of light as it passes from a less dense medium (air) into a more dense medium (glass or perspex).
A helpful memory trick: a concave lens caves inwards, much like the entrance to a cave.

Real-World Applications of Lenses

Convex lenses are used in cameras, telescopes, microscopes, magnifying glasses, and spectacles to correct long-sightedness.
Concave lenses are used in spectacles to correct short-sightedness and in certain telescope designs.
Understanding lenses is essential for explaining how we see the world, from viewing distant galaxies to observing microscopic bacteria.

2. Real-World Applications of LensesAQA

Notes

Convex lenses are used in cameras, telescopes, microscopes, magnifying glasses, and spectacles to correct long-sightedness.
Concave lenses are used in spectacles to correct short-sightedness and in certain telescope designs.
Understanding lenses is essential for explaining how we see the world, from viewing distant galaxies to observing microscopic bacteria.

Magnification

Magnification is calculated using the formula: magnification = frac{text{image height}}{object height} Magnification is a ratio and therefore has no units, provided both heights are measured in the same unit (e.g. both in centimetres).
A magnification greater than 1 means the image is magnified (larger), whilst less than 1 means it is diminished (smaller).

3. MagnificationAQA

Notes

Magnification is calculated using the formula: magnification = frac{text{image height}}{object height} Magnification is a ratio and therefore has no units, provided both heights are measured in the same unit (e.g. both in centimetres).
A magnification greater than 1 means the image is magnified (larger), whilst less than 1 means it is diminished (smaller).

Key Terms in Ray Diagrams

The focal point (F) is the point at which parallel rays of light converge after passing through a convex lens.
The focal length (f) is the distance between the centre of the lens and the focal point.
The principal axis is the horizontal straight line passing through the centre of the lens.
The point 2F is located at twice the focal length from the centre of the lens.

4. Key Terms in Ray DiagramsAQA

Notes

The focal point (F) is the point at which parallel rays of light converge after passing through a convex lens.
The focal length (f) is the distance between the centre of the lens and the focal point.
The principal axis is the horizontal straight line passing through the centre of the lens.
The point 2F is located at twice the focal length from the centre of the lens.

Drawing Ray Diagrams for a Convex Lens

Ray 1 travels parallel to the principal axis and then refracts through the focal point (F) on the far side of the lens.
Ray 2 passes straight through the centre of the lens without bending, as it strikes the lens at a right angle.
The image is formed where the two rays intersect, and arrows should be drawn on rays to show direction.
Always use a ruler and pencil when drawing ray diagrams, as accuracy is essential in exams.

5. Drawing Ray Diagrams for a Convex LensAQA

Notes

Ray 1 travels parallel to the principal axis and then refracts through the focal point (F) on the far side of the lens.
Ray 2 passes straight through the centre of the lens without bending, as it strikes the lens at a right angle.
The image is formed where the two rays intersect, and arrows should be drawn on rays to show direction.
Always use a ruler and pencil when drawing ray diagrams, as accuracy is essential in exams.

Describing Images: Three Key Criteria

Criterion 1 — orientation: is the image upright (same way up as the object) or inverted (upside down)?
Criterion 2 — size: is the image magnified (larger than the object) or diminished (smaller than the object)?
Criterion 3 — type: is the image real (rays actually cross over and can be projected onto a screen) or virtual (rays do not truly cross over and cannot be projected)?

6. Describing Images: Three Key CriteriaAQA

Notes

Criterion 1 — orientation: is the image upright (same way up as the object) or inverted (upside down)?
Criterion 2 — size: is the image magnified (larger than the object) or diminished (smaller than the object)?
Criterion 3 — type: is the image real (rays actually cross over and can be projected onto a screen) or virtual (rays do not truly cross over and cannot be projected)?

Convex Lens — Object Beyond 2F (Real Image)

When the object is placed far from the lens (beyond 2F), the two rays cross over on the opposite side of the lens, forming a real image.
This image is inverted, diminished, and real — similar to how a camera or the human eye forms an image.
A real image can be projected onto a screen, just like in a cinema projector.

7. Convex Lens — Object Beyond 2F (Real Image)AQA

Notes

When the object is placed far from the lens (beyond 2F), the two rays cross over on the opposite side of the lens, forming a real image.
This image is inverted, diminished, and real — similar to how a camera or the human eye forms an image.
A real image can be projected onto a screen, just like in a cinema projector.

Convex Lens — Object Inside F (Virtual Image)

When the object is placed close to the lens (inside the focal point), the refracted rays diverge and do not cross over on the far side.
The rays must be traced back (shown as dotted lines) to find where they appear to meet behind the lens — this is where the virtual image forms.
This virtual image is upright, magnified, and virtual — exactly how a magnifying glass works, as the brain perceives the object as larger.

8. Convex Lens — Object Inside F (Virtual Image)AQA

Notes

When the object is placed close to the lens (inside the focal point), the refracted rays diverge and do not cross over on the far side.
The rays must be traced back (shown as dotted lines) to find where they appear to meet behind the lens — this is where the virtual image forms.
This virtual image is upright, magnified, and virtual — exactly how a magnifying glass works, as the brain perceives the object as larger.

Concave Lens — Ray Diagram

For a concave lens, Ray 1 travels parallel to the principal axis but diverges away from the axis after the lens, directed in line with the focal point on the same side as the object.
Ray 2 still passes straight through the centre of the lens without bending.
Because the rays diverge, they must be traced back with dotted lines to find the virtual image on the same side as the object.
The image produced by a concave lens is always upright, diminished, and virtual — this is how a concave lens in a telescope reduces a large object to a smaller image.

9. Concave Lens — Ray DiagramAQA

Notes

For a concave lens, Ray 1 travels parallel to the principal axis but diverges away from the axis after the lens, directed in line with the focal point on the same side as the object.
Ray 2 still passes straight through the centre of the lens without bending.
Because the rays diverge, they must be traced back with dotted lines to find the virtual image on the same side as the object.
The image produced by a concave lens is always upright, diminished, and virtual — this is how a concave lens in a telescope reduces a large object to a smaller image.

Lens Symbols in Diagrams

In exam diagrams, a convex (converging) lens is represented by a line with outward-pointing arrows at each end (like a double-headed arrow bowing outwards).
A concave (diverging) lens is represented by a line with inward-pointing arrows at each end (bowing inwards).
You may be asked to draw or identify these symbols, so it is important to learn them alongside the full lens shape.

10. Lens Symbols in DiagramsAQA

Notes

In exam diagrams, a convex (converging) lens is represented by a line with outward-pointing arrows at each end (like a double-headed arrow bowing outwards).
A concave (diverging) lens is represented by a line with inward-pointing arrows at each end (bowing inwards).
You may be asked to draw or identify these symbols, so it is important to learn them alongside the full lens shape.