# Reflection and Refraction of Light

> Physics · CIE IGCSE 0625 2026-2028
> Source: https://www.owlsprep.com/study/cie-0625-u3-reflection-and-refraction-of-light/

This guide covers core and extended content for reflection and refraction of light as specified in the CIE IGCSE Physics 0625 2026-2028 syllabus, including ray diagrams, reflection laws, and extended-level Snell's Law.

**Prerequisites:** [Basic properties of transverse waves (light)](https://www.owlsprep.com/study/cie-0625-u3-properties-of-waves/)

## Learning objectives

- State and apply the law of reflection for plane mirrors
- Draw and interpret ray diagrams for reflection by plane mirrors
- Describe refraction of light at boundaries between transparent media
- Apply core qualitative refraction direction rules for all tiers
- State the meaning of critical angle and describe internal and total internal reflection using everyday examples
- Extended: Use Snell's Law to calculate refractive index and angles, and calculate the critical angle using $\sin c = 1/n$

## 1. Law of Reflection (Core)

When light hits a smooth, shiny surface like a plane mirror, it reflects following a fixed, predictable rule called the law of reflection. We first define key terms for this process.

**Law of Reflection** — 1. The incident ray, reflected ray, and normal all lie in the same plane. 2. The angle of incidence $i$ is always equal to the angle of reflection $r$.

*Example:* If a light ray hits a plane mirror at 35° to the normal, it will reflect at 35° to the normal on the opposite side of the normal.

**Worked example:** A light ray hits a plane mirror at 40° to the mirror surface. State the angle of reflection, and describe how you would draw the corresponding ray diagram.

1. Step 1: Draw the plane mirror as a straight line with hatching on the non-reflective side, then draw a dashed normal line perpendicular to the mirror at the point of incidence.
2. Step 2: Calculate the angle of incidence: since the ray is 40° to the mirror, $i = 90° - 40° = 50°$.
3. Step 3: Apply the law of reflection: $r = i = 50°$. Draw the reflected ray at 50° to the normal on the opposite side of the incident ray.
4. Final answer: Angle of reflection = 50°

> **Exam tip:** Always label the normal as a dashed line, and measure angles from the normal, not the mirror surface, to avoid losing easy marks.

## 2. Plane Mirror Image Formation (Core)

Plane mirrors form consistent, predictable images that follow fixed rules you will need to apply for ray diagram and calculation questions.

**Plane Mirror Image Properties** — Images formed by plane mirrors are: virtual (cannot be projected on a screen), upright, same size as the object, same distance behind the mirror as the object is in front, and laterally inverted.

*Example:* If you stand 1m in front of a plane mirror, your image appears 1m behind the mirror, so the total distance between you and your image is 2m.

**Worked example:** A student stands 1.2 m in front of a plane mirror. They move 0.4 m closer to the mirror. Calculate the new distance between the student and their image.

1. Step 1: Calculate the new object distance from the mirror: $1.2m - 0.4m = 0.8m$.
2. Step 2: Apply the plane mirror rule: image distance behind mirror = object distance in front = 0.8m.
3. Step 3: Total distance between student and image = object distance + image distance = $0.8m + 0.8m = 1.6m$.
4. Final answer: 1.6 m

> **Exam tip:** Remember plane mirror images are laterally inverted: if you raise your left hand, your image raises its right hand, a common short-answer test point.

## 3. Refraction of Light (Core + Extended)

Refraction is the change in direction of light when it travels from one transparent medium to another of different optical density, caused by the change in speed of light between media.

**Core Refraction Rules** — 1. Light travelling from less dense to denser medium (e.g. air to glass) bends *towards* the normal, so $r < i$. 2. Light travelling from denser to less dense medium bends *away* from the normal, so $r > i$. 3. Light incident along the normal ($i=0°$) does not change direction.

**Worked example:** A light ray travels from glass into air, with an angle of incidence of 30° to the normal. State whether the ray bends towards or away from the normal, and justify your answer.

1. Step 1: Compare the optical density of the two media: glass is more optically dense than air.
2. Step 2: Apply the core refraction rule: light moving from denser to less dense medium bends away from the normal.
3. Final answer: Bends away from the normal, because glass is optically denser than air.

> **Extended Only Content**
>
> The following content is only required for Extended tier (Paper 2/4) learners. Core tier learners may skip this section.

**Snell's Law (Extended)** — The refractive index $n$ of a medium is the ratio of the sine of the angle of incidence in air to the sine of the angle of refraction in the medium. It is also equal to the ratio of the speed of light in vacuum $c$ to the speed of light in the medium $v$.

*Notation:* $n = \frac{\sin i}{\sin r} = \frac{c}{v}$

*Example:* Glass has a refractive index of ~1.5, meaning light travels 1.5 times faster in vacuum than in glass.

**Worked example:** A light ray travels from air into water, with an angle of incidence of 45°. The refractive index of water is 1.33. Calculate the angle of refraction, to 2 significant figures.

1. Step 1: Write Snell's Law for air to water: $n_{water} = \frac{\sin i}{\sin r}$
2. $$1.33 = \frac{\sin 45°}{\sin r}$$
3. Step 2: Rearrange to solve for $\sin r$: $\sin r = \frac{\sin 45°}{1.33}$
4. $$\sin r = \frac{0.707}{1.33} = 0.532$$
5. Step 3: Calculate the inverse sine: $r = \sin^{-1}(0.532) = 32.1°$
6. Final answer: 32°

> **Exam tip:** Always verify your Snell's Law results match the direction rule: for air to glass, refracted angle should be smaller than incident angle, to catch calculation errors.

## 4. Critical Angle and Total Internal Reflection (Core + Extended)

As the angle of incidence inside a denser medium is increased, the refracted ray bends further away from the normal. At one particular angle of incidence, called the critical angle, the refracted ray travels exactly along the boundary (angle of refraction = 90°). The meaning of the critical angle and a qualitative description of total internal reflection are required for all learners.

**Critical Angle** — The critical angle $c$ is the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is exactly 90°.

**Total Internal Reflection (TIR)** — When light travels from a denser to a less dense medium and the angle of incidence is greater than the critical angle, none of the light passes through the boundary: all of it is reflected back into the denser medium, obeying the law of reflection. Everyday examples include reflecting prisms in periscopes and binoculars, road and bicycle reflectors, and light travelling along optical fibres.

*Example:* In a 45° glass prism periscope, light strikes the sloping face at an angle of incidence of 45°, which is greater than the critical angle of glass (~42°), so it is totally internally reflected and turns through 90°.

> **Extended Only Content**
>
> The following calculation is only required for Extended tier (Paper 2/4) learners. Core tier learners may skip this block.

**Calculating the Critical Angle (Extended)** — For Extended tier, the critical angle $c$ can be calculated from the refractive index $n$ of the denser medium using $\sin c = \frac{1}{n}$.

*Notation:* $\sin c = \frac{1}{n}$

**Worked example:** **[Extended]** The refractive index of a type of glass is 1.52. Calculate its critical angle at the glass-air boundary, to 2 significant figures.

1. Step 1: Write the critical angle formula: $\sin c = \frac{1}{n}$
2. $$\sin c = \frac{1}{1.52} = 0.658$$
3. Step 2: Calculate inverse sine: $c = \sin^{-1}(0.658) = 41.1°$
4. Final answer: 41°

> **Exam tip:** Total internal reflection only occurs when light travels from a denser to a less dense medium, never the reverse. This is a common exam trick question.

## Common pitfalls

- **Wrong:** Measuring angles of incidence/reflection/refraction from the boundary surface instead of the normal
  - Why it fails: All optical laws are defined relative to the normal, so this leads to incorrect angle values and lost marks
  - Correct: Always draw the dashed normal first, then measure all angles from the normal line
- **Wrong:** Calculating the distance between plane mirror object and image as only the image distance behind the mirror
  - Why it fails: The total distance is the sum of the object distance in front and equal image distance behind the mirror
  - Correct: Add the object distance and image distance to get the total separation between object and image
- **Wrong:** Applying TIR rules when light travels from less dense to denser medium
  - Why it fails: TIR only occurs when light moves from denser to less dense medium, so this leads to incorrect conclusions
  - Correct: First check the direction of travel of the light ray before applying any refraction or TIR rules
- **Wrong:** Using raw angle values instead of their sines when applying Snell's Law
  - Why it fails: Snell's Law uses the ratio of sines of angles, not the angles themselves, leading to massively incorrect results
  - Correct: Always take the sine of the angles before substituting into the Snell's Law formula
- **Wrong:** Stating that plane mirror images are real
  - Why it fails: Plane mirror images are virtual, formed by apparent crossing of light rays behind the mirror, and cannot be projected on a screen
  - Correct: Memorize the 5 core properties of plane mirror images: virtual, upright, same size, same distance behind, laterally inverted

## Cheatsheet

| Concept | Core Rule | Extended Rule |
| --- | --- | --- |
| Law of Reflection | $i = r$, all rays/normal in same plane | Same as core |
| Plane Mirror Image | Virtual, upright, same size, same distance behind, laterally inverted | Same as core |
| Refraction Direction | Less → denser: bend towards normal; Denser → less: bend away | Same as core + $n = \sin i / \sin r = c/v$ |
| Total Internal Reflection | Meaning of critical angle; TIR occurs if $i > c$, only denser → less dense (qualitative) | Calculate the critical angle with $\sin c = 1/n$ |

## What's next

Now that you have mastered reflection and refraction of light, you can move on to studying their real-world applications, including lenses and optical devices, which are also part of the CIE IGCSE Physics 0625 waves unit. Practice drawing ray diagrams regularly, as these are high-mark questions in both core and extended papers, and make sure you can apply Snell's Law and critical angle calculations quickly for extended tier exams. You should also review the electromagnetic spectrum to understand how light fits into the wider family of transverse waves.

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