8th Grade Reflection , Refraction and Dispersion of light
How Light Changes Direction
Light travels fast almost 300,000 kilometers per second in a vacuum. But what happens when it hits a surface or moves from one material to another, like from air into water? It can either bounce back or bend. These two effects are known as reflection and refraction.
🔁 What Is Reflection?
Reflection happens when light bounces off a surface. You see your face in a mirror because light rays hit the smooth surface and bounce back in the same pattern. This makes your reflection appear almost identical to you.
📏 The Laws of Reflection:
The angle of incidence equals the angle of reflection.
That means the angle the light hits the surface is the same as the angle it bounces off.
The incident ray, reflected ray, and the normal line all lie in the same plane.
The “normal” is an imaginary line perpendicular to the surface at the point where light hits.
✨ Fun Fact: Smooth surfaces like mirrors create regular reflection, while rough surfaces like paper scatter light in many directions called diffuse reflection.
🔀 What Is Refraction?
Refraction happens when light bends as it passes from one material to another. This bending occurs because light changes speed depending on the medium it travels through.
👓 Example: Why a Straw Looks Bent in Water
When you place a straw in a glass of water, it looks like it’s broken or bent. That’s because light slows down when it moves from air (less dense) into water (more dense), causing it to change direction.
According to The Physics Classroom:
Light travels fastest in a vacuum, slower in air, and even slower in water or glass.
The greater the change in speed, the more the light bends.
Our brain assumes light travels in a straight line, so it misinterprets the bent light path causing optical illusions.
🧠 Why Does It Matter?
Understanding how light behaves helps us in daily life:
Reflection lets us see ourselves in mirrors, design better lighting, and build laser tools.
Refraction allows for the creation of eyeglasses, microscopes, cameras, and even fiber-optic internet cables.
📘 What is Refractive Index?
The refractive index is a number that tells us how much light slows down when it passes from one material into another.
🔬 Definition:
Refractive index (n) is the ratio of the speed of light in a vacuum (c) to the speed of light in a material (v). It’s written as:
n = c / v
This means if light travels slower in a material than it does in a vacuum, the refractive index will be greater than 1.
📏 Does It Have Units?
Nope refractive index has no units. That’s because both speeds (in a vacuum and in the material) are measured in meters per second (m/s), so the units cancel out when you divide them.
🧪 What Does nD Mean?
Sometimes you’ll see a little "D" after the n, like this: nD.
This just means the refractive index was measured using a specific color of light: the sodium D-line, which has a wavelength of 589.3 nanometers (a yellow light).
Scientists use this specific wavelength because it's stable and consistent, making it a good reference point.
💡 Why Is Refractive Index Important?
Refractive index tells us how much light bends when it enters a new material. This is important in:
Optics – for making lenses and glasses
Chemistry – to study substances using refractometers
Physics – to understand light behavior in different media
For example:
Air has an n ≈ 1.0003
Water has an n ≈ 1.33
Diamond has an n ≈ 2.42 (light slows down a lot!)
🧪 Try It Yourself using the interactive diagram below:
Using this interactive simulation, explore the properties of reflection and refraction from different angles. You can observe how light behaves as both a wave and a ray. We often illustrate it as a ray for learning purposes, but it’s important to remember that light is both a wave and a ray.
