O-Level Pure Physics: Light, Reflection & Refraction — Ray Diagrams Made Systematic

Why Light Questions Cost So Many Marks

In O-Level Pure Physics, light is tested frequently through ray diagrams, qualitative explanations and short calculations. Students often:

• Draw rays inaccurately or without rulers
• Forget to label angles and refractive indices
• Mix up angle of incidence and angle of refraction
• Confuse real vs virtual images
• Apply Snell’s Law with the wrong sine or wrong medium

This article gives you a systematic method to handle reflection and refraction questions, including plane mirrors, glass blocks, and converging lenses.

Core Concepts You Must Be Clear About

Key Definitions

• Ray: A line showing the direction of light travel.
• Normal: A line drawn perpendicular to the surface at the point where the ray hits.
• Angle of incidence, i: Angle between incident ray and normal.
• Angle of reflection, r: Angle between reflected ray and normal.
• Angle of refraction: Angle between refracted ray and normal in the second medium.
• Refractive index, n: Measure of how much light slows down in a medium compared to vacuum/air.

Always Remember These Laws

• Law of reflection: i = r (measured from the normal).
• Snell’s Law: n₁ sin i = n₂ sin r.
• Light bends towards the normal when entering a denser medium.
• Light bends away from the normal when entering a less dense medium.

Plane Mirror Questions: A Simple 4-Step Routine

Plane mirror questions are common and often combined with ray diagrams and image properties.

Properties of Image in a Plane Mirror

• Same size as object
• Same distance behind mirror as object is in front
• Laterally inverted (left-right reversed)
• Virtual (cannot be formed on a screen)

How to Draw a Plane Mirror Ray Diagram

1. Draw the mirror as a straight line. Mark the normal at the point where the ray hits.
2. Draw the incident ray towards the mirror.
3. Use i = r: Measure the angle of incidence from the normal, then draw the reflected ray making the same angle on the other side.
4. Extend the reflected ray backwards with a dotted line behind the mirror. The point where dotted rays meet is the virtual image.

Exam tip: Use a ruler and protractor. Many marks are for accuracy and clarity, not just the final answer.

Refraction at a Flat Boundary: Glass Block Questions

Rectangular glass block questions test your understanding of refraction, lateral displacement and emergent rays.

Standard Glass Block Setup

Typical question: A ray of light enters a glass block from air, refracts, then emerges into air again. You may be asked to:

• Draw the path of the ray
• Label angles of incidence and refraction
• Measure and calculate refractive index
• Determine lateral displacement

Systematic Method for Glass Block Diagrams

1. Draw the block as a rectangle. Mark the boundaries clearly.
2. At the first boundary (air to glass):
   • Draw the normal at the point of incidence.
   • Draw the incident ray in air.
   • Since glass is denser than air, draw the refracted ray bending towards the normal.
3. Inside the block: Draw the refracted ray as a straight line to the second boundary.
4. At the second boundary (glass to air):
   • Draw the normal at the exit point.
   • Glass to air: light goes into a less dense medium, so it bends away from the normal.
   • Extend the emergent ray into air.
5. Check parallelism: For a rectangular block, the incident ray and emergent ray should be parallel.

Using Snell’s Law Correctly

Given: Light passes from air (n ≈ 1.0) into glass (n ≈ 1.5).

Apply Snell’s Law at the first boundary:

n_air sin i = n_glass sin r

So,

sin r = (n_air / n_glass) sin i

Common mistake: Swapping n₁ and n₂ or using the wrong angle (using angle to surface instead of angle to normal).

Total Internal Reflection & Critical Angle

Total internal reflection (TIR) is tested in prisms, optical fibres and critical angle questions.

Conditions for TIR

• Light must travel from a denser medium to a less dense medium.
• Angle of incidence in the denser medium must be greater than the critical angle.

Critical Angle Formula

For light going from a medium (n) to air (n ≈ 1):

sin c = 1 / n

where c is the critical angle in the denser medium.

How to Handle TIR Diagrams

1. Identify media: Confirm light is going from denser to less dense.
2. Mark the normal at the boundary.
3. Compare i with c:
   • If i < c: light refracts out (bends away from normal).
   • If i = c: refracted ray travels along the boundary.
   • If i > c: total internal reflection occurs; draw reflected ray inside the denser medium with i = r.

Exam tip: Always label which medium is denser and show the direction of the ray with arrows.

Converging Lenses: Image Formation Without Memorising All Cases

Instead of memorising every object distance case, use a two-ray method for converging lenses.

Standard Rays for a Converging Lens

Draw the principal axis, lens, and focal points F on both sides.

1. Ray 1 (parallel then through F): From top of object, draw a ray parallel to principal axis to the lens, then refract it through the focal point on the other side.
2. Ray 2 (through centre): From top of object, draw a ray through the optical centre of the lens; it continues in a straight line (no deviation).

The point where the two refracted rays (or their backward extensions) meet is the image.

Quick Summary of Image Types

• Object beyond 2F: Image between F and 2F, real, inverted, smaller.
• Object at 2F: Image at 2F, real, inverted, same size.
• Object between F and 2F: Image beyond 2F, real, inverted, larger.
• Object at F: Rays emerge parallel, no image formed on screen (image at infinity).
• Object between lens and F: Image on same side as object, virtual, upright, larger.

Exam tip: Always use arrows to show object and image orientation, and label distances clearly (u, v, f) if required.

Common Exam Traps and How to Avoid Them

Trap 1: Measuring Angles from the Surface

Angles of incidence, reflection and refraction are always measured from the normal, not from the surface. Draw the normal first before drawing or measuring angles.

Trap 2: Forgetting Which Medium is Denser

Before drawing any refracted ray, write a small note beside each region, e.g. air (n=1.0), glass (n=1.5). Then apply:

• Denser → less dense: bends away from normal
• Less dense → denser: bends towards normal

Trap 3: Mislabelling Real vs Virtual Images

• Real image: Rays actually meet; can be formed on a screen.
• Virtual image: Rays only appear to come from a point (dotted extensions meet); cannot be formed on a screen.

In diagrams, use solid lines for real rays and dotted lines for backward extensions.

Trap 4: Inaccurate Diagrams

Examiners award marks for:

• Straight, neat rays (use a ruler)
• Clearly drawn normals
• Correct bending direction
• Proper labels (i, r, n, F, object, image)

Even if your numerical answer is wrong, a good diagram can still earn method marks.

How to Practise Light Questions Effectively

1. Group similar questions: Do a batch of only plane mirror questions, then only glass block questions, then only lens questions. This builds pattern recognition.
2. Mark your diagrams: After checking answers, compare your ray directions, bending and labels carefully, not just final values.
3. Re-draw wrong diagrams: For every mistake, redraw the correct diagram once from scratch. This helps you internalise the correct pattern.
4. Time yourself: Aim to complete standard ray-diagram questions in 3–4 minutes each under exam conditions.

Conclusion: Turn Light Diagrams into Easy Marks

Light questions in O-Level Pure Physics are highly predictable. Once you:

• Use a fixed routine for drawing rays and normals
• Apply i = r and Snell’s Law systematically
• Keep track of denser vs less dense media
• Practise lens and TIR diagrams until they become automatic

you can convert what many students see as a confusing topic into a reliable source of marks. Start by revising your definitions, then work through a set of past-year light questions using the step-by-step methods above.