🌟 CIE IGCSE Physics – 3.2 LIGHT

Light is a form of energy that allows us to see objects. In IGCSE Physics, you study how light behaves when it reflects, refracts, passes through lenses, and disperses into colours.

🔹 3.2.1 Reflection of Light

📘 Key Terms and Definitions

Normal
An imaginary line drawn perpendicular (90°) to the surface at the point where the light ray hits.

Angle of incidence (i)
The angle between the incident ray (incoming ray) and the normal.

Angle of reflection (r)
The angle between the reflected ray and the normal.

⚠️ Important:

• Angles are always measured from the normal, not the mirror surface.

📐 Law of Reflection

The angle of incidence is equal to the angle of reflection
i = r

This law applies to all reflecting surfaces, including plane mirrors.

🪞 Plane Mirrors and Image Formation

A plane mirror is a flat mirror.

🖼️ Characteristics of the Image Formed

An image formed by a plane mirror is:

1. Same size as the object
2. Same distance behind the mirror as the object is in front
3. Virtual (cannot be formed on a screen)
4. Upright (not inverted)
5. Laterally inverted (left and right are swapped)

📌 Definition: Virtual image
An image formed where light rays appear to meet, but do not actually meet.

✏️ Ray Diagram for a Plane Mirror (Description)

To draw a ray diagram:

1. Draw the mirror as a straight vertical line
2. Draw the object in front of the mirror
3. Draw two rays from the object to the mirror
4. Reflect the rays using i = r
5. Extend the reflected rays backwards with dotted lines
6. The point where the dotted lines meet is the virtual image

📊 Simple Measurements and Calculations

You may be asked to:

• Measure angles using a protractor
• Calculate angles using i = r
• Compare object distance and image distance

📌 Example Question:
If the angle of incidence is 35°, what is the angle of reflection?

✅ Answer: 35°

📝 Exam Tips – Reflection

• Always draw the normal first
• Label i and r clearly
• Use a ruler and protractor for accuracy
• Say “virtual”, not “imaginary”

🔹 3.2.2 Refraction of Light

📘 Key Terms

Refraction
The bending of light when it passes from one medium to another because its speed changes.

Angle of refraction (r)
The angle between the refracted ray and the normal.

Medium
A substance through which light travels (e.g. air, glass, water).

🔬 Experiment to Show Refraction

Using a rectangular glass block:

1. Place the glass block on paper and trace around it
2. Shine a narrow ray of light into the block
3. Mark the incident and emergent rays
4. Remove the block and join the lines
5. Draw normals at the boundaries

Observation:

• Light bends towards the normal when entering glass
• Light bends away from the normal when leaving glass

🌊 Passage of Light Between Two Media

• Light slows down when entering a denser medium
• Light speeds up when entering a less dense medium

From	To	Direction of bending
Air	Glass	Towards the normal
Glass	Air	Away from the normal

🔺 Critical Angle (c)

Definition:
The angle of incidence in the denser medium for which the angle of refraction is 90°.

At this angle:

• Refracted ray travels along the boundary

🔁 Internal Reflection and Total Internal Reflection

Internal reflection
Occurs when light reflects inside a medium instead of refracting out.

Total internal reflection (TIR) occurs when:

1. Light travels from denser to less dense medium
2. Angle of incidence greater than the critical angle

📌 Everyday Examples:

• Optical fibres
• Periscopes
• Diamond sparkle
• Endoscope in medicine

📐 Refractive Index (n)

Definition:
The refractive index is the ratio of the speed of light in air (or vacuum) to the speed in the medium.

(where c is the critical angle)

✏️ Example Calculation

If:

• Angle of incidence = 40°
• Angle of refraction = 25°

📡 Optical Fibres

How they work:

• Use total internal reflection
• Light signals bounce repeatedly inside the fibre

Uses:

• Internet and telephone communication
• Medical instruments (endoscopes)

Advantages:

• Very fast
• Low signal loss
• Not affected by electrical interference

📝 Exam Tips – Refraction

• Critical angle is only in the denser medium
• Always check direction of travel
• Use sin, not cos or tan
• State both conditions for TIR

🔹 3.2.3 Thin Lenses

🔍 Types of Lenses

Lens	Shape	Effect on parallel rays
Converging (convex)	Thicker in middle	Rays meet
Diverging (concave)	Thinner in middle	Rays spread

📘 Key Definitions

Principal axis
A straight line through the centre of the lens.

Principal focus (focal point)
The point where parallel rays converge (or appear to diverge from).

Focal length (f)
Distance from the centre of the lens to the principal focus.

✏️ Ray Diagrams – Converging Lens (Real Image)

Rules for drawing rays:

1. Ray parallel to axis → passes through focal point
2. Ray through centre → undeviated
3. Ray through focal point → emerges parallel

Real image characteristics:

• Inverted
• Can be projected on a screen

🖼️ Image Characteristics

Describe images using:

• Size: enlarged / same size / diminished
• Orientation: upright / inverted
• Nature: real / virtual

👓 Virtual Image (Converging Lens)

• Object placed inside focal length
• Rays diverge
• Image formed by extrapolating rays backwards
• Image is upright, enlarged, virtual

📌 Cannot be formed on a screen

🔍 Magnifying Glass

A single converging lens used to:

• Produce a large virtual image
• Object placed inside focal length

👀 Vision Defects and Correction

Short-sightedness (myopia):

• Distant objects blurred
• Corrected using a diverging lens

Long-sightedness (hyperopia):

• Near objects blurred
• Corrected using a converging lens

📝 Exam Tips – Lenses

• Label focal points clearly
• Use three rays if asked
• State real or virtual explicitly
• Don’t mix up concave and convex

🔹 3.2.4 Dispersion of Light

🌈 Dispersion

Definition:
The separation of white light into its component colours when refracted.

Occurs because:

• Different colours have different wavelengths
• They refract by different amounts

🔺 Prism and White Light

• Violet bends the most
• Red bends the least

🎨 Visible Spectrum

Order (frequency increasing):

Red → Orange → Yellow → Green → Blue → Indigo → Violet

Mnemonic: ROYGBIV

Colour	Wavelength
Red	Longest
Violet	Shortest

🔴 Monochromatic Light

Definition:
Light of a single frequency (single colour).

Examples:

• Laser light
• Sodium lamp (yellow)