ICSE Class 10 Physics: Lens Refraction Rules & Ray Diagrams

Why refraction through a lens matters

Ever wondered how glasses help you see clearly or why a camera can zoom in? It all comes down to light bending, or refraction, as it passes through a lens.

💡 In Simple Words: When light hits a lens, it changes direction. This bending lets the lens bring light together or spread it apart, forming an image that can be bigger, smaller, upright, or inverted.

Key terms you should know

• Refraction: the change in direction of a wave (like light) when it passes from one material to another.
• Lens: a piece of transparent material (usually glass or plastic) that bends light. Lenses come in two main shapes: convex (bulging outward) and concave (curved inward).
• Principal axis: an imaginary straight line that passes through the centre of the lens and is perpendicular to its surfaces.
• Focal point (F): the point where parallel rays of light either meet (convex) or appear to diverge from (concave) after passing through the lens.
• Focal length (f): the distance between the centre of the lens and its focal point. Positive for convex, negative for concave.
• Optical centre (O): the exact middle of the lens where a ray passes straight through without bending.

Rules for refraction in a lens

Think of a lens like a water pipe that redirects water flow. Light behaves similarly, but we follow a few handy rules to predict where it will go.

Convex (converging) lens rules

• A ray parallel to the principal axis will pass through the focal point on the other side.
• A ray passing through the focal point before reaching the lens will emerge parallel to the principal axis.
• A ray through the optical centre goes straight, unchanged in direction.

Concave (diverging) lens rules

• A ray parallel to the principal axis appears to diverge from the focal point on the same side as the object.
• A ray aimed toward the focal point on the opposite side leaves the lens parallel to the principal axis.
• Again, a ray through the optical centre continues straight.

How to draw a ray diagram – step by step

Drawing a ray diagram is like sketching a roadmap for light. Follow these steps and you’ll always end up at the right image point.

Worked example: image formed by a convex lens

Suppose an object 3 cm tall stands 12 cm in front of a convex lens with a focal length of 6 cm. Where does the image appear and how big is it?

1. Use the lens formula: 1/f = 1/v + 1/u. Here, f = +6 cm (positive for convex), u = -12 cm (object distance is negative by sign convention).
2. Plug in: 1/6 = 1/v + 1/(-12) → 1/6 = 1/v – 1/12 → 1/v = 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4.
3. Thus, v = +4 cm. The positive sign tells us the image is formed on the opposite side of the lens (real image).
4. Now find magnification (m) using m = v/u = 4/(-12) = -1/3.
5. The image height = m × object height = -1/3 × 3 cm = -1 cm. The minus sign means the image is inverted, and its size is one‑third of the object.

So, the lens produces a real, inverted image 4 cm behind the lens, 1 cm tall.

Quick comparison table

📝 Likely exam questions

1. State the three ray rules for a convex lens.
   Answer: (i) Parallel ray → passes through focal point; (ii) Focal ray → emerges parallel; (iii) Ray through centre → passes undeviated.
2. Draw a ray diagram for a concave lens with an object placed 10 cm from the lens. Focal length = –5 cm.
   Answer: Sketch principal axis, mark focal point 5 cm on object side, draw object, then draw parallel ray diverging as if from focal point, ray through centre straight, and ray aimed toward focal point emerging parallel. The image is virtual, upright, and reduced.
3. Calculate the image distance for a convex lens of focal length 8 cm when an object is placed 24 cm in front.
   Answer: Using 1/f = 1/v + 1/u → 1/8 = 1/v + 1/(-24) → 1/v = 1/8 + 1/24 = 3/24 + 1/24 = 4/24 = 1/6 → v = +6 cm.
4. Explain why a ray passing through the optical centre of any lens does not change direction.
   Answer: The optical centre is the point where the two surfaces of the lens intersect; the material thickness is symmetric there, so the incident angle equals the emergent angle, causing no refraction.