ICSE Class 10 Physics – Refraction Through a Lens Ray Diagrams

Why refraction through a lens matters

Ever wondered how glasses help you see far objects clearly, or why a camera can zoom in? All that magic comes from light bending, or refraction, as it passes through a lens.

💡 In Simple Words: When light hits a lens, it changes direction because the lens material slows it down a bit. By drawing a few straight lines, we can predict where the light will meet and form an image.

What is refraction?

Refraction is the bending of a wave – in our case light – when it moves from one material to another. Imagine water flowing in a garden hose that suddenly widens; the water spreads out and changes direction. Light behaves the same way when it goes from air into glass.

How lenses bend light

A convex lens is thicker in the middle than at the edges, like a gentle hill. It makes parallel rays of light converge (come together) at a point called the focal point. A concave lens is thinner in the middle, like a shallow dip, and it makes parallel rays diverge (spread apart) as if they came from a focal point behind the lens.

Key parts of any lens diagram:

Principal axis: an invisible line that runs straight through the centre of the lens.
Focal points (F and F′): the spots where rays meet (real focus) or appear to meet (virtual focus).
Optical centre (O): the exact middle of the lens where a ray passes straight through without bending.

Drawing a ray diagram for a convex lens

Let’s walk through the steps you’ll use in the exam. Follow the flowchart at the end for a quick visual guide.

Draw a horizontal line – this is the principal axis.
Mark the centre of the lens (O) on the axis and sketch the convex shape.
Mark the focal points (F on the right side, F′ on the left) at a distance equal to the focal length (f) from O.
Choose an object point above the axis (usually an arrow) and draw three rays:

Parallel ray: starts from the top of the object, travels parallel to the axis, hits the lens, then passes through the far focal point.
Focal ray: starts from the top of the object, heads toward the near focal point, hits the lens, then emerges parallel to the axis.
Central ray: goes straight through the optical centre without bending.

Where the refracted rays intersect on the other side of the lens is where the image forms. Use a dotted line to join the intersection points if they don’t meet perfectly.

Label the image with its height, orientation (upright or inverted), and nature (real or virtual).

Example: An arrow 3 cm tall stands 12 cm in front of a convex lens with f = 6 cm. Draw the three rays as described. The rays meet 6 cm behind the lens, giving an inverted image of the same size. That tells you the magnification is –1 (negative means upside‑down).

Drawing a ray diagram for a concave lens

Concave lenses are a bit trickier because the image is always virtual (you can’t catch it on a screen). The steps are similar, but the rays behave differently.

Draw the principal axis and the concave shape, marking O.
Mark the focal points (F on the right, F′ on the left) – remember they lie on the same side as the incoming light for a concave lens.
Place the object arrow on the left side.
Draw three rays:

Parallel ray: travels parallel to the axis, hits the lens, then diverges as if it came from the near focal point.
Focal ray: aims toward the far focal point, hits the lens, and emerges parallel to the axis.
Central ray: passes straight through O without bending.

Extend the diverging rays backward (use dotted lines) – they appear to meet at the virtual image location on the same side as the object.

Label the image; it will be upright, smaller, and virtual.

Example: An arrow 4 cm tall stands 15 cm from a concave lens with f = –5 cm. After drawing, the back‑extended rays meet 3.33 cm behind the object, giving an upright image of about 1.33 cm height.

Quick reference table

Feature	Convex Lens	Concave Lens
Shape	Thicker at centre	Thinner at centre
Image type	Real or virtual depending on object distance	Always virtual
Image orientation	Inverted (real) or upright (virtual)	Upright
Magnification (\|m\|)	>1, =1, or <1	<1
Ray behaviour	Parallel → focal, focal → parallel, centre → straight	Parallel → appears from focal, focal → parallel, centre → straight

Step‑by‑step flowchart for drawing a convex lens diagram

graph TD A[Identify lens type] --> B[Draw principal axis] B --> C[Mark focal points] C --> D[Draw at least two rays] D --> E[Find image where rays intersect] E --> F[Label image]

📝 Likely Exam Questions

Explain why a ray passing through the optical centre of a lens does not change direction.
Model answer: The optical centre is the point where the thickness of the lens is uniform, so the light experiences the same speed on both sides and continues in a straight line.
Draw a ray diagram for a convex lens of focal length 5 cm when an object is placed 10 cm in front of it. State the nature of the image.
Model answer: (Student draws diagram). Image forms at 10 cm on the other side, inverted, same size – therefore real, inverted, magnification –1.
How does the image formed by a concave lens differ from that formed by a convex lens when the object is placed at infinity?
Model answer: For a concave lens, parallel rays diverge and appear to come from the focal point, giving a virtual, upright, reduced image at the focal point. A convex lens would focus parallel rays at its focal point, producing a real point image.
State three rules for drawing ray diagrams of lenses.
Model answer: (i) Draw the principal axis. (ii) Mark the focal points and optical centre. (iii) Use at least two of the three standard rays (parallel, focal, central) for each object point.

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