Ever wondered why you can see yourself in a mirror but a straw looks bent in a glass of water? That’s the magic of reflection and refraction at work.
💡 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 simple rules called the laws of reflection and refraction.
What is Reflection?
Reflection is when a light ray hits a surface and then goes back into the same medium, like a ball bouncing off a wall. Imagine a playground slide: if you roll a ball down, it goes forward; if the slide were a perfectly smooth wall, the ball would bounce straight back.
Law of Reflection
The law of reflection says two things:
- Incident ray (the incoming light) and reflected ray (the outgoing light) lie in the same plane.
- The angle of incidence (i) equals the angle of reflection (r). In other words, the ray bounces off at the same angle it arrived.
Think of a pool cue hitting a billiard ball. The direction you aim (incident angle) is the same as the direction the ball rolls after hitting the cushion (reflected angle).
What is Refraction?
Refraction is the bending of light when it passes from one transparent material to another—like moving from air into water. Picture a marching band walking from a smooth floor onto a muddy patch; the front slows down first, causing the line to tilt.
Law of Refraction (Snell’s Law)
Snell’s Law tells us how much the light bends. It says:
n₁ sin i = n₂ sin r
Here, n₁ and n₂ are the refractive indices of the first and second medium (a number that tells how much a medium slows light). i is the angle of incidence, and r is the angle of refraction.
Imagine a car entering a sandpit at an angle. The wheels on the side that hits sand first slow down, turning the car toward the sand. The sand’s “refractive index” is higher than the road, so the car (light) bends toward the normal (an imaginary line perpendicular to the surface).
Worked Example: Finding the Angle of Refraction
Problem: A ray of light travels from air (n≈1.00) into water (n≈1.33) with an incidence angle of 30°. Find the refraction angle.
Solution:
- Write Snell’s Law:
n₁ sin i = n₂ sin r. - Plug values:
1.00 × sin 30° = 1.33 × sin r. - Since sin 30° = 0.5, we have
0.5 = 1.33 × sin r. - Solve for sin r:
sin r = 0.5 / 1.33 ≈ 0.376. - Find r:
r = arcsin(0.376) ≈ 22°.
So the ray bends towards the normal and emerges in water at about 22°.
Key Differences Between Reflection and Refraction
| Aspect | Reflection | Refraction |
|---|---|---|
| What happens to light? | Light bounces back into the same medium. | Light passes into a new medium and changes direction. |
| Law governing it | Angle of incidence = angle of reflection. | n₁ sin i = n₂ sin r (Snell’s Law). |
| Typical example | Mirror, calm lake surface. | Straw in a glass of water, prism. |
| Effect on speed | No change; speed stays the same. | Speed changes according to the medium’s refractive index. |
Quick Summary (Bullet Points)
- Reflection: ray bounces, i = r, occurs on smooth surfaces.
- Refraction: ray bends, governed by Snell’s Law, depends on refractive indices.
- Refractive index tells how much a medium slows light; higher index → more bending.
- Both phenomena obey the principle of least time (light takes the quickest path).
- Use the law of reflection for mirrors; use Snell’s Law for lenses, prisms, and water.
📝 Likely Exam Questions
- State the law of reflection.
Answer: The incident ray, reflected ray and the normal all lie in the same plane, and the angle of incidence equals the angle of reflection. - What is the refractive index of a medium?
Answer: It is the ratio of the speed of light in vacuum to the speed of light in that medium; it indicates how much the medium slows down light. - Derive the angle of refraction when a ray enters glass (n=1.5) from air at 45°.
Answer: Using Snell’s Law, 1×sin45° = 1.5×sin r → sin r = 0.707/1.5 ≈ 0.471 → r ≈ 28°. - Explain why a straw appears broken in a glass of water.
Answer: Light from the part of the straw under water refracts as it moves from water (higher n) to air (lower n), bending away from the normal and making the submerged portion appear displaced. - List two practical applications of each law.
Answer: Reflection – plane mirrors, periscopes. Refraction – lenses in cameras, fiber‑optic communication.