This topic explains how light travels in straight lines as rays, and what happens when a ray meets a mirror, a block of glass or a lens. You draw the path of the ray, calculate an angle where a refractive index is given, decide when light is trapped inside a material, and build an image with ray construction.
The exact equations and the depth expected depend on your syllabus year and on whether you study the Core or the Extended route. Check the current Cambridge IGCSE Physics 0625 page for your exam year. The Physics learning guide shows where this module sits in the whole subject.
What do I need to know first?
You need the wave ideas in wave behaviour, especially that a wave can reflect, refract and change speed when it enters a new material. You also need basic angle work and a sine key on your calculator set to degrees.
A sharp pencil, a protractor and a ruler matter here as much as the physics. Most lost marks in this topic come from drawing, not from ideas.
One orienting worked example
A ray of light in air hits a flat glass block with an angle of incidence of 50°, and the angle of refraction inside the glass is 31°. (Invented example data.) Find the refractive index of the glass.
Step 1, name the angles: both angles are measured from the normal, the line at 90° to the surface. i = 50° and r = 31°.
Step 2, choose the relationship: refractive index n = sin i ÷ sin r.
Step 3, calculate: sin 50° = 0.766 and sin 31° = 0.515, so n = 0.766 ÷ 0.515 = 1.49.
Step 4, check: light bends toward the normal when it enters glass, so r must be smaller than i, which it is. A value a little above 1 is reasonable for glass. The answer is n ≈ 1.5.
Every lesson in this module is a variation on those moves: measure from the normal, pick the rule that fits the situation, calculate or construct carefully, then check that the result makes physical sense.
In what order should I study the lessons?
- Construct a reflected ray: the law of reflection and the normal, which every later diagram depends on.
- Use a refraction relationship where applicable: the bending of light and the sine relationship, using the same normal habit.
- Explain total internal reflection conditions: the special case of refraction where all the light stays inside, with the critical angle.
- Draw a simple lens image: the three standard rays and where they meet.
- Distinguish real and virtual image construction: when the rays really meet and when they only appear to, including dashed-line conventions.
Then test yourself with the light and imaging practice set.
Which traps catch most students?
- Measuring an angle from the mirror or glass surface instead of from the normal.
- Forgetting that light bends toward the normal in a denser material and away from it in a less dense one.
- Dividing the angles instead of the sines when finding a refractive index.
- Claiming total internal reflection for light that is going from air into glass.
- Drawing a real image with dashed lines, or a virtual image with solid lines, and forgetting arrows on the rays.
How should I use the practice set?
Draw every ray diagram on paper with a ruler before you open the worked answer. Write the equation, the angles in degrees and the unit on each calculation line. Use the error-routing section at the end of the set to return to the right lesson.
Afterwards, log any repeated slip in the mistake log and retest queue so it comes back later as a fresh question.
Students who understand the rules yet still lose marks on the drawing or the angle can gain from a teacher watching the pencil work as it happens. That is what our online one-to-one Physics tuition is designed to offer.