Reflection and Refraction of Light

Q: Do I need to learn Snell's Law for core tier?

No. Snell's Law () and critical-angle calculations () are only required for Extended tier. Core tier still needs the qualitative refraction direction rules, the meaning of the critical angle, and a qualitative description of internal and total internal reflection with everyday examples.

Physics· 3.2.1, 3.2.2· 18 min read

1. 1. Law of Reflection (Core)★★☆☆☆⏱ 4 min

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.

📘 Definition

Law of Reflection

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. 2. Plane Mirror Image Formation (Core)★★☆☆☆⏱ 4 min

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

📘 Definition

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.

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Step 1: Calculate the new object distance from the mirror: $1.2 m - 0.4 m = 0.8 m$ .
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 = $0.8 m + 0.8 m = 1.6 m$ .
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. 3. Refraction of Light (Core + Extended)★★★☆☆⏱ 5 min

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.

📘 Definition

Core Refraction Rules

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.

📘 Definition

Snell's Law (Extended)

$n = \frac{s i n i}{s i n r} = \frac{c}{v}$

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 $v$ .

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.

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Step 1: Write Snell's Law for air to water: $n_{w a t er} = \frac{s i n i}{s i n r}$
2
$1.33 = \frac{\sin 45°}{\sin r}$
3
Step 2: Rearrange to solve for $sin r$ : $sin r = \frac{s i n 45°}{1.33}$
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. 4. Critical Angle and Total Internal Reflection (Core + Extended)★★★☆☆⏱ 5 min

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.

📘 Definition

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°.

📘 Definition

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°.

📘 Definition

Calculating the Critical Angle (Extended)

$sin c = \frac{1}{n}$

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

📐 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.

5. Common Pitfalls

Wrong move:

Measuring angles of incidence/reflection/refraction from the boundary surface instead of the normal

Why:

All optical laws are defined relative to the normal, so this leads to incorrect angle values and lost marks

Correct move:

Always draw the dashed normal first, then measure all angles from the normal line

Wrong move:

Calculating the distance between plane mirror object and image as only the image distance behind the mirror

Why:

The total distance is the sum of the object distance in front and equal image distance behind the mirror

Correct move:

Add the object distance and image distance to get the total separation between object and image

Wrong move:

Applying TIR rules when light travels from less dense to denser medium

Why:

TIR only occurs when light moves from denser to less dense medium, so this leads to incorrect conclusions

Correct move:

First check the direction of travel of the light ray before applying any refraction or TIR rules

Wrong move:

Using raw angle values instead of their sines when applying Snell's Law

Why:

Snell's Law uses the ratio of sines of angles, not the angles themselves, leading to massively incorrect results

Correct move:

Always take the sine of the angles before substituting into the Snell's Law formula

Wrong move:

Stating that plane mirror images are real

Why:

Plane mirror images are virtual, formed by apparent crossing of light rays behind the mirror, and cannot be projected on a screen

Correct move:

Memorize the 5 core properties of plane mirror images: virtual, upright, same size, same distance behind, laterally inverted

6. Quick Reference 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 $sin c = 1/ n$

7. Frequently Asked

Do I need to learn Snell's Law for core tier?

No. Snell's Law ( $n = sin i / sin r$ ) and critical-angle calculations ( $sin c = 1/ n$ ) are only required for Extended tier. Core tier still needs the qualitative refraction direction rules, the meaning of the critical angle, and a qualitative description of internal and total internal reflection with everyday examples.

How do I tell if a light ray bends towards or away from the normal?

If light travels from a less dense to more dense medium (e.g. air to glass), it bends towards the normal. If it travels from more dense to less dense (e.g. glass to air), it bends away from the normal.

Going deeper

official_syllabusCIE IGCSE Physics 0625 2026-2028 SyllabusRefer to section 3.2 for full topic guidance

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.