Ever wonder why you can see yourself in a calm lake but not in a foggy puddle?
💡 In Simple Words: Light bounces off smooth surfaces like a ball off a wall (reflection) and bends when it passes from one material to another (refraction). These two behaviours follow simple, predictable rules that help us design mirrors, lenses, and even your phone camera.
What is Reflection?
Reflection happens when a light ray hits a surface and bounces back into the same medium. Imagine a game of billiards: the cue ball (light) hits the cushion (the surface) and changes direction, but stays on the table (the same medium).
Law of Reflection
The law of reflection says two things:
- The angle between the incoming ray and an imaginary line called the normal (a line drawn perpendicular to the surface) is equal to the angle between the reflected ray and the normal.
- The incoming ray, the reflected ray, and the normal all lie in the same plane (flat surface).
In plain language: the angle you “throw” the light at a smooth surface is the angle it “comes back” with.
What is Refraction?
Refraction is the bending of light as it passes from one transparent medium to another – like a straw looking “bent” in a glass of water.
Law of Refraction (Snell’s Law)
The rule governing this bending is called Snell’s Law. It connects the angle of the incoming ray, the angle of the refracted ray, and the two media’s refractive indices (a number that tells how much a material slows down light).
Snell’s Law formula: n₁ sin θ₁ = n₂ sin θ₂
- n₁ and n₂ are the refractive indices of the first and second medium.
- θ₁ is the angle of incidence (the angle the incoming ray makes with the normal).
- θ₂ is the angle of refraction (the angle the bent ray makes with the normal).
Put simply: the product of a medium’s refractive index and the sine of the angle stays the same as light moves from one material to another.
Worked Example – Finding the Angle of Refraction
Suppose a ray of light travels in air (refractive index ≈ 1.00) and hits a glass block (refractive index ≈ 1.50) at an angle of 30° to the normal. What’s the angle inside the glass?
- Write down what you know: n₁ = 1.00, n₂ = 1.50, θ₁ = 30°.
- Plug into 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.
- Find the angle whose sine is 0.333. Using a calculator, θ₂ ≈ 19.5°.
So the light bends towards the normal, making a smaller angle inside the denser glass.
Key Differences Between Reflection and Refraction
| Feature | Reflection | Refraction |
|---|---|---|
| What happens to the ray? | Ray bounces back into the same medium. | Ray passes into a new medium and changes direction. |
| Typical example | Mirror, calm water surface. | Straw in water, lens, prism. |
| Governing law | Angle of incidence = angle of reflection. | n₁ sin θ₁ = n₂ sin θ₂ (Snell’s Law). |
| Depends on surface type? | Yes – needs a smooth (specular) surface. | Yes – depends on refractive indices of the two media. |
| Direction of the ray relative to normal | Symmetrical on both sides of the normal. | Usually bends towards the normal if entering a denser medium, away if entering a rarer medium. |
Quick Summary – Remember the Two Laws
- Law of Reflection: Angle of incidence = angle of reflection; both measured from the normal.
- Snell’s Law (Refraction): n₁ sin θ₁ = n₂ sin θ₂; the product stays constant across the boundary.
- Reflection keeps the ray in the same medium; refraction sends it into a new medium and bends it.
📝 Likely Exam Questions
- State the law of reflection.
Answer: The angle of incidence equals the angle of reflection, and both the incident and reflected rays lie in the same plane as the normal. - Define refractive index.
Answer: Refractive index is a dimension‑less number that indicates how much a medium slows down light compared to its speed in vacuum. - A ray of light passes from water (n = 1.33) to air. If the angle of incidence is 45°, find the angle of refraction.
Answer: Using Snell’s Law, 1.33 sin 45° = 1.00 sin θ₂ → sin θ₂ = 1.33 × 0.707 ≈ 0.94 → θ₂ ≈ 70° (bends away from the normal). - Explain why a swimming pool looks shallower than it actually is.
Answer: Light from the bottom is refracted as it moves from water to air, bending away from the normal, so the eye perceives the point of origin higher, making the depth appear less. - Draw a ray diagram showing a ray incident on a plane mirror at 30° to the normal and label the angles.
Answer: (Student draws a straight line meeting a vertical line, marks the normal, shows incident ray at 30°, reflected ray also at 30° on the other side.)