Ray Diagrams Revision Notes | GCSE AQA Higher Physics

1. Understanding Lenses Through Light

Why does a magnifying glass make things look bigger, while the peephole in your door makes everything look smaller? The answer lies in how different lens shapes bend light rays.

Lenses work by refracting (bending) light rays that pass through them. The amount and direction of bending depends entirely on the lens shape. This bending follows predictable patterns that we can map out using ray diagrams.

Ray diagrams are like roadmaps for light. They show us exactly where light rays go after hitting a lens, which tells us where images will form and what they'll look like. Once you master the rules, you can predict what any lens will do to an image.

2. The Architecture of Ray Diagrams

Every ray diagram follows the same basic structure, like a building blueprint. You need the lens (drawn as a vertical line with arrows showing its shape), the principal axis (horizontal line through the lens centre), and the focal points marked at equal distances on both sides.

The focal length is the distance from the lens centre to the principal focus. For convex lenses, this is where parallel rays actually meet. For concave lenses, it's where the rays appear to come from when you trace them backwards.

Think of the principal axis as the lens's 'spine' - it's the reference line that everything else relates to. Any ray travelling along this axis passes straight through without bending, because it hits the lens at exactly 90 degrees.

3. The Three Golden Rules for Ray Tracing

Every ray diagram uses three special rays that follow predictable rules. These aren't the only rays of light, but they're the easiest to draw accurately and they're sufficient to locate any image.

Rule 1: A ray parallel to the principal axis refracts through the principal focus (convex) or appears to come from the principal focus (concave). Rule 2: A ray through the centre of the lens continues straight without deviation. Rule 3: A ray through the principal focus emerges parallel to the principal axis (convex) or a ray aimed at the principal focus on the far side emerges parallel (concave).

These rules work because of how refraction occurs at curved surfaces. The lens shape determines the angle of the surface at each point, which determines how much each ray bends according to Snell's law.

**Trap**: Many students think rays 'bounce off' lenses like mirrors. Actually, rays pass through lenses and get refracted (bent) as they do so. The bending happens at both the front and back surfaces of the lens.

4. Constructing Convex Lens Ray Diagrams

Convex lenses are the 'converging' type - they bring light rays together. When you draw ray diagrams for convex lenses, you'll see that rays actually meet at real points where images form.

Start by drawing your lens as a vertical line with outward-pointing arrows (showing it's thicker in the middle). Mark the principal focus on both sides at equal distances. Place your object as an arrow to one side of the lens.

The image location depends on where you place the object. If the object is beyond 2F (twice the focal length), the image forms between F and 2F on the opposite side - real, inverted, and diminished. If the object is between F and 2F, the image forms beyond 2F - real, inverted, and magnified.

Worked example: Drawing a Convex Lens Ray Diagram

An object is placed 30cm from a convex lens with focal length 10cm. Construct the ray diagram to locate the image.

1. Draw lens, principal axis, mark F at ±10cm, object at -30cm — Object is at 3F, so beyond 2F from the lens
2. Parallel ray from object top → through F on far side — Parallel rays always refract through the principal focus
3. Ray through lens centre → continues straight — Central rays pass through undeviated
4. Ray through near F → emerges parallel to axis — This ray confirms where the other two intersect
5. Image forms where rays intersect at +15cm — Real, inverted, diminished image between F and 2F

Answer: Image located 15cm from lens on opposite side, real, inverted, and half the object height

5. Constructing Concave Lens Ray Diagrams

Concave lenses are the 'diverging' type - they spread light rays apart. This means they always produce virtual images that appear on the same side as the object.

Draw your concave lens as a vertical line with inward-pointing arrows (showing it's thinner in the middle). The focal points are still marked at equal distances, but now the rays diverge as if coming from the focus on the object side.

For concave lenses, the ray rules are modified: Ray 1 (parallel to axis) refracts as if coming from the near focus. Ray 2 (through centre) still goes straight. Ray 3 (aimed at far focus) emerges parallel to the axis.

The key difference is that concave lens rays never actually converge. To find the image, you must trace the refracted rays backwards (shown as dotted lines) to where they appear to meet. This creates a virtual image that's always upright, diminished, and on the same side as the object.

6. Comparing Convex and Concave Lens Behaviour

The fundamental difference between convex and concave lenses shows up clearly in ray diagrams. Convex lenses converge rays to real focal points, while concave lenses diverge rays from virtual focal points.

Both lens types follow the same basic ray rules, but with opposite effects. Both have rays that pass straight through the centre. Both interact with parallel rays and rays through focal points. The difference is whether rays converge or diverge.

Convex lenses can produce both real and virtual images depending on object position. Real images form when the object is beyond the focal point, virtual images when it's closer than the focal point. Concave lenses only ever produce virtual images, regardless of object position.

In terms of image characteristics: convex lenses can magnify or diminish, and can invert images. Concave lenses always diminish and never invert. This makes convex lenses useful for magnifying glasses and cameras, while concave lenses are used in glasses for short-sightedness.

7. Exam Technique for Ray Diagram Questions

Ray diagram questions are common in GCSE physics and they follow predictable patterns. Examiners typically ask you to construct the diagram, then describe the image characteristics or calculate magnification.

Always start by identifying the lens type from the shape or description. Set up your diagram with a ruler - accuracy matters for marks. Draw the lens, principal axis, and focal points first, then add the object as a clear arrow.

When drawing rays, use a ruler and make them clear. Label each ray (Ray 1, Ray 2, Ray 3) and show the direction with arrowheads. For virtual images, use dotted lines when tracing rays backwards and clearly mark where they appear to intersect.

Common mistakes include: forgetting to mark focal points, drawing rays that don't follow the rules, not using dotted lines for virtual rays, and failing to measure distances accurately. Always check your diagram follows the three ray rules before moving on.