Reflection and Refraction

Core Idea

When a wave hits a boundary between two materials, two things can happen: reflection (the wave bounces back) and refraction (the wave passes through but changes direction). The law of reflection says the angle of incidence equals the angle of reflection. Refraction occurs because waves change speed when entering a new medium — the change in speed causes the wave to bend. Both phenomena apply to light, sound, and water waves.

How It's Best Learned

Shine a flashlight at a mirror at various angles and trace the incoming and reflected beams. Place a pencil in a glass of water and observe how it appears bent at the surface. Use a laser pointer and a semicircular glass block to measure reflection and refraction angles directly.

Common Misconceptions

• Light always travels in perfectly straight lines. (Light bends when it passes from one medium to another, like from air to water. It travels in straight lines only within a single uniform medium.)
• Refraction changes the color of light. (Refraction changes the direction. It can separate colors because different wavelengths bend by different amounts, but it does not change the light's color.)
• Reflection only happens with mirrors. (Every surface reflects some light. Mirrors are special because they reflect light in an organized way rather than scattering it.)
• Waves always bend toward the denser material. (Waves bend toward the normal when entering a slower medium and away from the normal when entering a faster medium. Density is often correlated with speed but is not the direct cause.)

Explainer

When a basketball bounces off a wall, it comes back at a predictable angle. Light behaves the same way. When a light ray hits a smooth surface like a mirror, it bounces off in a specific direction governed by the law of reflection: the angle of incidence (incoming angle measured from the perpendicular, called the normal) equals the angle of reflection (outgoing angle measured from the same normal). This is why mirrors form clear images — they reflect all incoming rays in an orderly, predictable way.

But not every surface is a mirror. Rough surfaces also reflect light, but they scatter it in many random directions. This is called diffuse reflection, and it is why you can see most objects: they reflect light toward your eyes from every part of their surface, not just at one specific angle. The law of reflection still applies to each individual ray hitting a tiny patch of the surface — but because the surface is uneven, the normals point in different directions.

Refraction is what happens when a wave passes from one medium into another and changes speed. Light travels at about 300,000 km/s in air but slows to about 225,000 km/s in water. When a light beam enters water at an angle, one side of the beam hits the water and slows down before the other side does. This causes the beam to pivot and change direction — bending toward the normal as it enters the slower medium.

This bending explains many familiar phenomena. A straw in a glass of water appears broken or shifted at the surface because light from the submerged portion refracts as it exits the water, reaching your eyes from a different direction than expected. A rainbow forms because sunlight entering a raindrop refracts, reflects off the back of the drop, and refracts again upon exiting — and different colors (wavelengths) refract by slightly different amounts, separating white light into its component colors.

Reflection and refraction are not just about light. Sound waves reflect off walls (creating echoes) and refract when passing through layers of air at different temperatures (which is why sounds can seem to carry farther over a lake on a calm evening — the temperature gradient bends sound waves downward). Water waves reflect off harbor walls and refract when moving from deep to shallow water. These wave behaviors are universal, and mastering them for light opens the door to understanding lenses, fiber optics, cameras, and the entire field of optics.