ICSE Class 10 Physics: Lens Refraction Rules & Ray Diagrams

Why does a lens bend light? (And why you’ll love it in your exam)

Imagine a straw in a glass of water. The straw looks broken at the surface, right? That “break” is light changing direction when it moves from air to water – we call that refraction. A lens does the same thing, only it’s shaped to bend light in a useful way, like turning a flashlight into a spotlight.

💡 In Simple Words: Refraction through a lens means light enters the glass, slows down, and changes direction. The lens’s curved surfaces make the light rays meet at a point (image) or spread out, depending on the lens shape.

What exactly is refraction through a lens?

Refraction is the bending of a wave when it passes from one medium (like air) into another (like glass). In a lens, two curved surfaces work together. When light hits the first surface, it bends toward the normal (an imaginary line perpendicular to the surface). Inside the glass it travels slower, then bends away from the normal when it exits back into air. The net effect can focus light to a point (converging lens) or make it diverge (diverging lens).

Key terms you need to know

• Principal axis: an imaginary straight line that passes through the centre of the lens and both its focal points. Think of it as the “road” that light follows.
• Focal point (F) / focal length (f): the spot where parallel rays either meet (converging) or appear to come from (diverging). The distance from the centre of the lens to this point is the focal length.
• Optical centre (O): the exact middle of the lens where a ray passes straight through without bending.
• Object distance (u): distance between the object and the lens.
• Image distance (v): distance between the lens and the formed image.

Rules for refraction in a lens

These are the “golden” shortcuts you can use to predict where an image will form without solving equations every time.

Quick mnemonic

Remember “Parallel‑to‑Focal, Centre‑Straight, F‑to‑Parallel.” It tells you what each of the three standard rays does.

How to draw a ray diagram for a convex lens

Drawing a clean diagram is half the battle in the ICSE exam. Follow these steps, and you’ll never get lost.

Let’s walk through an example.

Worked example: Object 30 cm from a convex lens of focal length 15 cm

1. Draw a horizontal line – this is the principal axis.
2. Place the lens at the centre and mark focal points 15 cm on each side.
3. Mark the object 30 cm to the left of the lens.
4. Ray 1: From the top of the object, draw a line parallel to the axis. After hitting the lens, bend it through the far‑right focal point.
5. Ray 2: From the same top point, draw a line through the optical centre – it stays straight.
6. The two refracted rays meet at a point 30 cm on the right side of the lens. That’s the image location.
7. Measure the image height: draw a line from the intersection down to the axis. You’ll see the image is inverted and the same size as the object (real, same size, inverted).

Notice how the object distance (u) equals the focal length (f) times 2, so the image distance (v) also equals 2f. That’s a handy symmetry rule for convex lenses.

Common pitfalls and how to avoid them

• Mixing up real and virtual images: Real images actually form on a screen; virtual images cannot be projected – they only appear when you look through the lens.
• Forgetting the sign convention: In the lens formula (1/f = 1/v + 1/u), distances measured against the direction of incoming light are negative. ICSE usually expects you to state signs clearly.
• Skipping the optical centre ray: It’s the easiest way to check your diagram. If that ray isn’t straight, you’ve probably drawn the lens at the wrong position.

Quick summary table

📝 Likely Exam Questions

1. State the three principal rays used to draw a ray diagram for a convex lens.
   Answer: (i) Ray parallel to the principal axis, which after refraction passes through the focal point; (ii) Ray through the optical centre, which continues undeviated; (iii) Ray through the focal point before reaching the lens, which emerges parallel to the principal axis.
2. A convex lens of focal length 10 cm forms a real image of an object placed 30 cm from it. Find the image distance and describe the nature of the image.
   Answer: Using 1/f = 1/v + 1/u → 1/10 = 1/v + 1/30 → 1/v = 1/10 – 1/30 = 2/30 = 1/15 → v = 15 cm on the opposite side. The image is real, inverted, and reduced.
3. Explain why a diverging lens always produces a virtual image.
   Answer: A diverging lens spreads incoming parallel rays outward. The refracted rays appear to originate from a point on the same side as the object, so they never actually meet on a screen. Hence the image formed is virtual, upright, and smaller.
4. Draw a ray diagram for a concave lens with an object placed 12 cm from the lens. The focal length is 6 cm. State the image distance.
   Answer: (Provide diagram following the three‑ray method.) Using the lens formula, 1/‑6 = 1/v + 1/12 → 1/v = –1/6 – 1/12 = –3/12 = –1/4 → v = –4 cm. The image forms 4 cm on the same side as the object, virtual, upright, and reduced.
5. What happens to the image when the object is placed at the focal point of a convex lens?
   Answer: The refracted rays become parallel after passing through the lens, so they never converge. The image is formed at infinity, meaning it cannot be captured on a screen; it appears highly enlarged and essentially at an infinite distance.