IGCSE Physics: Light Practice Questions
Light reflects so that the angle of incidence equals the angle of reflection, and refracts when it changes speed on entering a new medium. Total internal reflection occurs when light strikes a boundary from the denser side at an angle greater than the critical angle.
Sub-topic 3.2 of Cambridge IGCSE Physics 0625 carries both a calculation and a set of ray diagram skills. Snell's law and the critical angle equation are the two calculations that recur. The questions below cover them plus the ray diagram conventions examiners expect.
What you need to know for Light
- Law of reflectionThe angle of incidence equals the angle of reflection. Both angles are measured from the normal, which is the line drawn perpendicular to the surface at the point where the ray hits.
- Image in a plane mirrorVirtual, upright, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front.
- RefractionLight changes speed when it enters a new medium. Entering a denser medium it slows and bends towards the normal. Leaving into a less dense medium it speeds up and bends away from the normal.
- Refractive indexn = sin i divided by sin r, where i is the angle in air and r is the angle in the medium. It can also be written n = speed of light in a vacuum divided by speed in the medium.
- Critical angleThe angle of incidence in the denser medium for which the angle of refraction is 90 degrees. sin c = 1 divided by n.
- Total internal reflectionWhen light travels from a denser to a less dense medium at an angle of incidence greater than the critical angle, all of it is reflected back. This is how optical fibres work.
IGCSE Physics Light questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A ray of light travels from air into a glass block. The angle of incidence in air is 42 degrees and the angle of refraction in the glass is 26 degrees. Calculate the refractive index of the glass.
Show the worked answer
- Use n = sin i divided by sin r, where i is the angle in air.
- sin 42 = 0.6691 and sin 26 = 0.4384.
- n = 0.6691 divided by 0.4384.
- n = 1.53, which rounds to 1.5. Refractive index has no units because it is a ratio.
Calculate the critical angle for a material of refractive index 1.5, and explain what happens to a ray striking the boundary from inside the material at 50 degrees.
Show the worked answer
- Use sin c = 1 divided by n = 1 divided by 1.5 = 0.6667.
- c = inverse sine of 0.6667 = 41.8 degrees, which is 42 degrees to 2 significant figures.
- The ray strikes the boundary at 50 degrees, which is greater than the critical angle of 42 degrees.
- No light is refracted out. All of it is reflected back into the material, obeying the law of reflection. This is total internal reflection.
Explain why a ray of light bends towards the normal when it passes from air into glass.
Show the worked answer
- Light travels more slowly in glass than in air, because glass is optically denser.
- When the ray meets the boundary at an angle, one side of each wavefront enters the glass and slows down before the other side does.
- The wavefront therefore pivots, changing the direction of travel.
- Because the light slows on entering, it bends towards the normal. The frequency is unchanged, so the wavelength decreases in the glass.
Explain how an optical fibre carries a light signal along a curved path, and give one advantage of using optical fibres for communication.
Show the worked answer
- The fibre has a core of higher refractive index surrounded by cladding of lower refractive index.
- Light entering the fibre strikes the core cladding boundary at an angle greater than the critical angle.
- It is therefore totally internally reflected rather than escaping, and this repeats many thousands of times along the fibre, so the light follows the fibre even around bends.
- An advantage is that optical fibres carry far more information per second than copper cables, with less signal loss over long distances and no interference from external electrical fields.
Common mistakes in this topic
- Measuring angles from the surface rather than from the normal.
- Inverting the refractive index fraction.
- Forgetting arrows on rays in a ray diagram, or drawing them without a ruler.
- Saying the frequency of light changes during refraction. Frequency is set by the source.
- Describing an image in a plane mirror as real. It is virtual.
Exam tips
- Draw the normal first as a dashed line, then measure every angle from it.
- Refractive index is always greater than 1 for light entering a denser medium. Use that to check your answer.
- For critical angle, remember sin c = 1 divided by n, and that total internal reflection needs the angle to exceed c.
- Describe images with four words: real or virtual, upright or inverted, magnified or diminished, and the position.
- In ray diagrams use a sharp pencil and a ruler, and always mark the direction of travel with an arrowhead.
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Light FAQs
How do I calculate refractive index?
Divide the sine of the angle in air by the sine of the angle in the medium, using n = sin i divided by sin r. The angles are measured from the normal. Refractive index has no units and is always greater than 1 for light passing into a denser medium.
What is the critical angle?
The critical angle is the angle of incidence inside the denser medium at which the refracted ray would travel along the boundary at 90 degrees to the normal. It is calculated using sin c = 1 divided by n. Beyond that angle, no light escapes and total internal reflection occurs.
Why does light bend when it enters glass?
Light travels more slowly in glass than in air. When a ray meets the boundary at an angle, one side of each wavefront slows before the other, which pivots the wavefront and changes the direction of travel. Slowing down means the ray bends towards the normal.
How do optical fibres work?
A fibre has a core of higher refractive index surrounded by cladding of lower refractive index. Light striking that boundary at an angle greater than the critical angle is totally internally reflected. Repeating this thousands of times guides the light along the fibre, even around bends.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.