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Comparing how light reflects differently off smooth, shiny surfaces versus dull, rough surfaces.
Have you ever wondered why you can see your face clearly in a shiny silver spoon, but not in a piece of white paper, even though both are clean and bright?
When light hits a smooth and shiny surface, like a mirror or a polished metal tray, it behaves in a very organized way. Imagine a row of soldiers marching toward a wall; when they hit it, they all turn and march away in the exact same formation. This is called specular reflection. Because the light rays stay parallel (in a straight line) after they bounce, they carry a clear image to your eyes. This is why you can see yourself so clearly in a mirror. The surface must be incredibly flat—so flat that even the tiniest bumps are smaller than the wavelength of light!
1. Light from a lamp hits your face. 2. The light bounces off your face in many directions. 3. Some of that light hits the smooth glass of the mirror. 4. Because the mirror is flat, the light bounces back perfectly to your eyes, allowing you to see your reflection.
Quick Check
If light rays hit a surface and all bounce back in the same direction, what kind of reflection is occurring?
Answer
Specular reflection.
Most things around us, like your shirt, a brick wall, or a piece of paper, are rough. Even if a piece of paper feels smooth to your finger, under a microscope, it looks like a range of jagged mountains! When light hits these 'mountains,' the rays bounce off in a million different directions. This is called diffuse reflection. Because the light is scattered, it doesn't carry a clear image back to your eyes. This is actually very helpful! It allows us to see objects from any angle without being blinded by a reflection of the sun.
1. On a calm day, the water surface is flat ( waves). It acts like a mirror (specular reflection). 2. When the wind blows, it creates waves. These waves act like the 'mountains' on the paper. 3. The light hits the slanted sides of the waves and scatters. 4. Result: The clear image of the mountain disappears and becomes a blurry smudge of color.
Quick Check
Why can't you see your face in a piece of white paper?
Answer
Because the surface is microscopically rough, causing diffuse reflection which scatters the light.
Sometimes, specular reflection can be a problem. Have you ever been blinded by a bright flash of light off a car window or a tablet screen? This is called glare. Glare happens when a smooth surface reflects a concentrated beam of light (like sunlight) directly into your eyes. To fix this, many phone screens have an 'anti-glare' coating. This coating actually makes the surface slightly rough at a microscopic level to turn that harsh specular reflection into softer, diffuse reflection.
1. A dry road is rough and bumpy, creating diffuse reflection so you can see the pavement clearly. 2. When it rains, water fills in the tiny holes in the asphalt, creating a flat, liquid top layer. 3. This turns the rough road into a smooth surface. 4. At night, car headlights hit this smooth water and create a powerful specular reflection (glare) that makes it hard for drivers to see.
Which surface would most likely cause specular reflection?
What happens to light rays during diffuse reflection?
A surface can look smooth to our eyes but still be 'rough' to light rays.
Review Tomorrow
Tomorrow morning, look at your breakfast. Can you identify one object that uses specular reflection and one that uses diffuse reflection?
Practice Activity
Take a piece of aluminum foil. Look at your reflection while it is flat (smooth). Then, crumple it up and flatten it back out. Why did your reflection disappear?