Ever wondered why a magnifying glass makes things look bigger?
💡 In Simple Words: When light passes from air into a curved piece of glass, it bends. That bending (refraction) lets lenses focus light to form images. By following a few easy rules, you can draw the picture of where the image will appear.
What is Refraction?
Refraction is the change in direction of a light ray when it moves from one medium (like air) to another (like glass). Think of it like a car turning when it hits a patch of mud – the speed changes, so the direction shifts.
Lens Rules You Must Remember
All lenses share a few basic ideas. Below are the rules for the two most common lenses you’ll see in ICSE questions.
- Convex (converging) lens: It’s thicker in the middle. Parallel rays (rays that never meet) bend and meet at the focal point (F) on the other side.
- Concave (diverging) lens: It’s thinner in the middle. Parallel rays spread out as if they came from a focal point (F) on the same side as the object.
- Principal axis: an invisible line that runs through the centre of the lens and both focal points.
- Optical centre: the exact middle of the lens where a ray passes straight through without bending.
- Focal length (f): the distance between the optical centre and the focal point. Positive for convex, negative for concave.
How to Draw a Ray Diagram for a Convex Lens
Grab a ruler and follow these five steps. A quick flowchart is included to keep you on track.
Here’s what each ray does:
- Parallel ray: Hits the lens parallel to the axis, then passes through the far focal point.
- Focal ray: Starts from the object’s top, heads toward the near focal point, then leaves the lens parallel to the axis.
- Central ray: Passes straight through the optical centre without bending.
The point where any two of these rays cross gives you the image location. If they meet on the opposite side of the lens, the image is real (can be projected on a screen). If they appear to meet on the same side as the object, the image is virtual (you can’t project it).
Worked Example: Find Image Position
Object height = 2 cm, placed 15 cm in front of a convex lens of focal length 10 cm. Where does the image form?
Use the lens formula (1/f = 1/v + 1/u). Remember:
- u = object distance (negative because the object is on the incoming side). So u = –15 cm.
- f = +10 cm for a convex lens.
Plug in:
1/10 = 1/v + 1/(–15) → 1/v = 1/10 + 1/15 = (3+2)/30 = 5/30 = 1/6 → v = +6 cm.
Positive v means the image is on the opposite side, 6 cm from the lens. Since v
Convex vs Concave Lens – Quick Comparison
| Property | Convex Lens | Concave Lens |
|---|---|---|
| Shape | Thicker in the middle | Thinner in the middle |
| Focal length sign | Positive (+) | Negative (–) |
| Parallel ray behaviour | Converges at far focal point | Diverges as if from near focal point |
| Image type (object beyond 2f) | Real, inverted, smaller | Virtual, upright, smaller |
| Common use | Magnifying glass, camera | Eyeglasses for near‑sightedness |
📝 Likely Exam Questions
- State the three principal rays used to draw a convex lens diagram and describe what each ray does.
Answer: Parallel ray → passes through far focal point; Focal ray → passes through near focal point then emerges parallel; Central ray → goes straight through the optical centre without deviation. - An object is placed 30 cm from a concave lens of focal length –15 cm. Find the image distance and nature of the image.
Answer: Using 1/f = 1/v + 1/u, 1/(–15) = 1/v + 1/(–30) → 1/v = –1/15 + 1/30 = –1/30 → v = –30 cm. Image is virtual, upright, same size, located 30 cm on the same side as the object. - Explain why a convex lens can form a real image on a screen while a concave lens cannot.
Answer: A convex lens converges parallel rays to a point, so the rays actually meet and can be captured on a screen (real image). A concave lens only diverges rays; they only appear to meet when traced backward, so no real image forms. - Draw a ray diagram for a convex lens with an object placed at 2f. State the image position, size, and nature.
Answer: Image forms at 2f on the opposite side, same size as object, inverted, real. - Define focal length and explain its sign convention for convex and concave lenses.
Answer: Focal length is the distance between the optical centre and the focal point. It is taken as positive for convex (converging) lenses and negative for concave (diverging) lenses.