Sec 1 Light & Refraction Notes

Comprehensive revision guide · Secondary 1 Physics
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📝 Instructions: Read each section carefully. Use the space provided to write your answers. Refer to the diagrams to help you understand how light behaves when it passes between different media.

Section 1 — Introduction to Refraction

What is Refraction?

Refraction is the bending of light rays as they pass from one transparent medium to another of different optical density. This occurs because light travels at different speeds in different materials.

Medium A (air — less dense) Medium B (glass/water — denser) Normal Incident ray Refracted ray i r Light bends TOWARD the normal when entering a denser medium

💡 Key Point: Why Does Light Bend?

  • Light travels faster in air (less optically dense) and slower in glass/water (more optically dense)
  • When light enters a denser medium at an angle, one side of the ray slows down first → the ray bends
  • If light enters perpendicularly (along the normal), it does NOT bend — only slows down or speeds up

Conditions for Refraction

For refraction to occur, the following conditions must be met:

  1. The two media must have different optical densities
  2. The incident ray must not be perpendicular to the boundary between the two media (i.e., it must not travel along the normal)
  3. Both media must be transparent or translucent
⚠️ Important: If light travels along the normal (perpendicular to the boundary), it does not bend — even though its speed changes. Refraction only occurs when light enters at an angle.

Section 2 — Laws of Refraction & Direction of Bending

Direction of Bending: The Two Cases

Air Glass Bends TOWARD normal
Glass Air Bends AWAY from normal

📌 Summary Rule

  • Air → Glass/Water (less dense to denser): Light bends toward the normal. Speed decreases.
  • Glass/Water → Air (denser to less dense): Light bends away from the normal. Speed increases.

The Two Laws of Refraction

  1. The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
  2. The ratio of sin(i) to sin(r) is constant for a given pair of media. This is known as Snell's Law: n₁sin(i) = n₂sin(r)
Medium A (n₁ — less dense) Medium B (n₂ — denser) Normal Incident ray Refracted ray Reflected ray i r n₁ sin(i) = n₂ sin(r)

Speed of Light in Different Media

The speed of light varies depending on the optical density of the medium:

Medium Optical Density Speed of Light
Air (vacuum) Least dense Fastest (~3 × 10⁸ m/s)
Water Moderately dense Slower (~2.25 × 10⁸ m/s)
Glass More dense Slowest (~2 × 10⁸ m/s)
🔑 Remember: The more optically dense the medium, the slower light travels through it. This change in speed is what causes refraction (bending).

Section 3 — Apparent Depth & Real Depth

Why Does a Pool Look Shallow?

When you look at an object underwater from above, the light rays coming from the object bend as they pass from water into air. Your brain assumes light travels in straight lines, so it perceives the object to be at a different position than where it actually is.

Water (denser) Air (less dense) Real position (real depth) Apparent position (apparent depth) 👁️ 👁️ Real depth Apparent depth The object appears closer to the surface than it really is

📌 Key Takeaway

  • Real depth = actual distance of the object below the surface
  • Apparent depth = how deep the object appears to be (always less than real depth)
  • The relationship: Apparent depth < Real depth
  • This is why a swimming pool looks shallower than it actually is!

Why Does a Straw Look Bent in Water?

When you place a straw in a glass of water, the part underwater appears to be at a different position than the part above water. This is because light from the submerged part bends as it exits the water into air, making the straw appear "broken" or bent at the water surface.

🔍 Try this: Place a pencil in a glass of water and look from the side. You will see the pencil appears to be broken at the water surface! This is refraction in action.

Section 4 — Dispersion of White Light

What is Dispersion?

Dispersion is the splitting of white light into its component colours (the spectrum) when it passes through a prism or other transparent medium. This happens because different colours of light travel at slightly different speeds in the same material, so they refract (bend) by different amounts.

White light Prism Red Orange Yellow Green Blue Indigo Violet ROYGBIV — Red bends least, Violet bends most

🌈 The Spectrum of Colours

  • White light is a mixture of all seven colours: Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV)
  • Red bends the least (travels fastest in glass) → appears at the top
  • Violet bends the most (travels slowest in glass) → appears at the bottom
  • This is why we see a rainbow after rain — water droplets act like tiny prisms!
🌈 Rainbow Formation: A rainbow forms when sunlight passes through water droplets in the atmosphere. Each droplet acts as a prism, dispersing white light into its component colours. The observer sees the rainbow when facing away from the Sun with rain in front of them.

Colour Absorption & Reflection

The colour we see an object to be depends on which wavelengths of light it reflects and which it absorbs:

Object Colour Reflects Absorbs
White object All colours None
Black object None All colours
Red object Red only All other colours
Green object Green only All other colours
🎨 Key Point: A red apple looks red because it reflects red light and absorbs all other colours. Under blue light, the red apple would appear black because there is no red light to reflect!

Section 5 — Free Response Questions (Tutorial)

📝 Instructions: Write your answers in the spaces provided. Show all working where applicable.
Question 1

(a) Define refraction of light.

(b) State two conditions necessary for refraction to occur.

(c) A light ray travels from air into a glass block. Describe what happens to the speed and direction of the light ray.

📝 Show/Hide Answer

(a) Refraction is the bending of light rays as they pass from one transparent medium to another of different optical density.

(b) Any two:

  • The two media must have different optical densities
  • The incident ray must not be perpendicular to the boundary (must not travel along the normal)
  • Both media must be transparent or translucent

(c) The speed of light decreases as it enters the glass (glass is more optically dense than air). The ray bends towards the normal.

Question 2

A student looks down into a swimming pool and notices that the bottom of the pool appears to be shallower than it actually is. Explain this observation using the concept of refraction.

📝 Show/Hide Answer

Light rays from the bottom of the pool travel from water (denser medium) to air (less dense medium). As they pass through the boundary, the light bends away from the normal. The observer's brain assumes light travels in straight lines, so it perceives the bottom to be at a shallower position than its actual location. This is called apparent depth — the apparent depth is always less than the real depth.

Question 3

(a) What is dispersion of white light?

(b) Name the seven colours of the spectrum in order.

(c) Which colour of light bends the most when passing through a prism? Which bends the least?

📝 Show/Hide Answer

(a) Dispersion is the splitting of white light into its component colours (the spectrum) when it passes through a prism or transparent medium.

(b) Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).

(c) Violet bends the most. Red bends the least.

Question 4

A red shirt is viewed under blue light. What colour does the shirt appear? Explain your answer.

📝 Show/Hide Answer

The shirt appears black. A red shirt only reflects red light and absorbs all other colours. Under blue light, there is no red light available to be reflected, so the shirt absorbs all the blue light and appears black.

Question 5

(a) State the two laws of refraction.

(b) Draw a labelled diagram showing a light ray passing from air into glass, indicating the incident ray, refracted ray, normal, angle of incidence (i), and angle of refraction (r).

📝 Show/Hide Answer

(a)

  1. The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
  2. The ratio sin(i)/sin(r) is constant for a given pair of media (Snell's Law: n₁sin(i) = n₂sin(r)).

(b) See the diagram in Section 2 — the incident ray comes from air, bends towards the normal when entering glass. The angle of incidence (i) is between the incident ray and the normal. The angle of refraction (r) is between the refracted ray and the normal. Note that i > r because light bends toward the normal.

Section 6 — Summary & Key Takeaways

📌 Key Concepts Covered

  • Refraction: Bending of light when passing between media of different optical densities due to change in speed
  • Bending rules: Less dense → denser = bends toward normal; Denser → less dense = bends away from normal
  • Laws of refraction: (1) All rays and normal lie in same plane; (2) Snell's Law: n₁sin(i) = n₂sin(r)
  • Speed of light: Fastest in air/vacuum, slower in water, slowest in glass
  • Apparent depth: Objects underwater appear closer to the surface than they really are (apparent depth < real depth)
  • Dispersion: White light splits into ROYGBIV through a prism — violet bends most, red bends least
  • Colour reflection/absorption: Objects reflect their own colour and absorb others; black absorbs all, white reflects all
📌 Remember for Next Week's Lesson: Bring this booklet with you! We will build on these concepts to explore lenses and optical instruments.
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