Ever wonder why you can see yourself in a bathroom mirror but a straw looks bent in a glass of water?
💡 In Simple Words: When light hits a surface, it either bounces back (reflection) or bends as it passes through (refraction). The way it bounces or bends follows two easy rules that you’ll find on every exam.
What is Reflection?
Reflection is simply light bouncing off a surface, like a ball hitting a wall. The surface can be smooth (a mirror) or rough (a wall).
The Law of Reflection
The law says: the angle of incidence equals the angle of reflection. Let’s break that down.
- Angle of incidence – the angle between the incoming ray (the light coming towards the surface) and an imaginary line called the normal (a line drawn perpendicular to the surface).
- Normal – think of it as a ruler standing straight up at the point where the light hits.
- Angle of reflection – the angle between the reflected ray (the light that bounces off) and the normal.
Because the two angles are equal, you can predict exactly where the reflected ray will go. It’s like playing pool: the angle at which the cue ball hits the cushion decides where it rolls back.
What is Refraction?
Refraction is light changing direction when it passes from one medium (like air) into another (like water). Imagine a marching band moving from a smooth floor onto a thick carpet; they slow down and turn a bit.
The Law of Refraction (Snell’s Law)
Snell’s Law tells us how much the ray bends. It states: n₁ sin θ₁ = n₂ sin θ₂. Here’s what each symbol means:
- n₁, n₂ – refractive indices of the first and second medium. A refractive index measures how much a material slows down light, like how syrup slows down a spoon compared to water.
- θ₁ – angle of incidence (same as before).
- θ₂ – angle of refraction, the angle the ray makes with the normal after entering the new medium.
If light goes from a less dense medium (air, n≈1) to a denser one (water, n≈1.33), it bends toward the normal. If it goes the other way, it bends away.
Why the Bending Happens
Light travels slower in denser media. The part of the wave that hits the new medium first slows down first, while the other part is still fast, causing the whole ray to change direction. It’s like a line of runners entering mud – the front slows, the back keeps running, and the line tilts.
Key Differences at a Glance
| Aspect | Reflection | Refraction |
|---|---|---|
| What happens? | Light bounces back | Light bends while passing through |
| Law governing it | Angle of incidence = angle of reflection | n₁ sin θ₁ = n₂ sin θ₂ (Snell’s law) |
| Normal line | Used to measure both angles | Used to measure both angles |
| Effect of medium | No change in speed | Speed changes, causing bend |
| Everyday example | Seeing yourself in a mirror | Straw appearing broken in a glass of water |
Worked Example: Finding the Refraction Angle
Suppose a ray of light travels from air (n₁=1.00) into glass (n₂=1.50) with an angle of incidence of 30°.
Using Snell’s law: 1.00 × sin 30° = 1.50 × sin θ₂.
sin 30° = 0.5, so 0.5 = 1.50 × sin θ₂ → sin θ₂ = 0.5 / 1.50 = 0.333.
θ₂ = sin⁻¹(0.333) ≈ 19.5°.
So the ray bends toward the normal and emerges inside the glass at about 19.5°.
Quick Revision Checklist
- Remember the normal – it’s the invisible line at right angles to the surface.
- Reflection: angle of incidence = angle of reflection.
- Refraction: n₁ sin θ₁ = n₂ sin θ₂. Higher refractive index → more bending toward the normal.
- Use the table to compare the two phenomena.
📝 Likely Exam Questions
- State the law of reflection and give a real‑life example.
Answer: The angle of incidence equals the angle of reflection. Example – a mirror shows your image because light reflects obeying this law. - Define refractive index and explain why light bends when it enters water from air.
Answer: Refractive index is a number that tells how much a medium slows light compared to vacuum. Light slows in water, so the part that enters first bends toward the normal, causing the whole ray to change direction. - A ray strikes a glass slab (n=1.5) from air at 45°. Calculate the angle of refraction.
Answer: Using Snell’s law, sin θ₂ = (1.00/1.5) × sin 45° = 0.666 × 0.707 = 0.471. θ₂ = sin⁻¹(0.471) ≈ 28°. - What happens to the reflected and refracted rays when the incident ray is perpendicular to the surface?
Answer: The reflected ray goes back along the same line (angle of incidence = 0°). The refracted ray continues straight into the new medium (no bending) because the angle with the normal is zero. - Explain why a swimming pool appears shallower than it really is.
Answer: Light from the bottom refracts at the water‑air interface, bending away from the normal. Our eyes trace the rays back in a straight line, making the bottom appear higher (shallower) than it actually is.