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Discovering why light bends when it moves from one material to another, like from air to water.
Have you ever tried to grab a diving toy at the bottom of a pool and missed, even though you were looking right at it? It’s not your eyes playing tricks—it’s light doing a 'magic trick' called refraction!
Light usually travels in a straight line, but something strange happens when it moves from one material to another. This change is called refraction. Refraction is the bending of light as it passes from one medium (material) to another, such as from air into water or glass.
Why does it bend? It’s all about speed! Light travels incredibly fast, but it moves at different speeds depending on what it is traveling through. In empty space, light travels at about kilometers per second. However, when light hits a thicker material like water, it slows down. This change in speed causes the light path to tilt, just like a shopping cart tilts if one wheel hits a patch of grass while the other stays on the smooth sidewalk.
Quick Check
What is the main reason that light bends when it moves from air into water?
Answer
Light bends because it changes speed when it enters a new material.
You can see refraction in action with a simple glass of water: 1. Place a straight straw into a half-full glass of water. 2. Look at the straw from the side of the glass. 3. Notice that the straw looks like it has 'snapped' or shifted at the water's surface.
This happens because the light reflecting off the part of the straw underwater bends as it exits the water and enters the air before reaching your eyes. Your brain assumes light always travels in a straight line, so it 'sees' the straw in a different spot than it actually is!
Not all materials bend light the same way. The amount of bending depends on the optical density of the material. A denser material, like diamond, will slow light down much more than water will.
Imagine a ray of light hitting a glass block at an angle. As it enters the glass, it slows down and bends toward an imaginary line called the normal (a line perpendicular to the surface). When it leaves the glass and enters the air again, it speeds back up and bends away from the normal. This is why lenses in your glasses can focus light—they are carefully shaped to bend light exactly where your eyes need it to go.
Quick Check
If light moves from a thin material (like air) into a thick material (like glass), does it speed up or slow down?
Answer
It slows down.
This experiment shows how refraction can hide or reveal objects: 1. Place a coin at the bottom of an empty, opaque mug. 2. Move your head back until the rim of the mug just barely hides the coin from your view. 3. Stay still and have a friend slowly pour water into the mug. 4. The coin will 'magically' reappear!
As the water fills the mug, the light from the coin bends over the edge of the rim, allowing the light to reach your eyes even though the coin hasn't moved.
Refraction isn't just a science experiment; we use it every day! Lenses are pieces of glass or plastic curved to bend light in specific ways.
* Eyeglasses: Help people see clearly by bending light so it hits the back of the eye perfectly. * Magnifying Glasses: Bend light outward to make an object look much larger than it is. * Rainbows: These are created when sunlight enters a raindrop, slows down, bends, reflects off the back, and bends again as it leaves. This spreads the light into its beautiful colors!
The Archer Fish hunts by spitting water at bugs sitting on leaves above the water. 1. Because of refraction, the bug looks like it is higher in the air than it actually is. 2. If the fish aimed directly at where it saw the bug, it would miss! 3. The fish has learned to aim slightly below where the bug appears to be to account for the bending of light. This is a complex calculation that the fish does naturally to survive.
What happens to the speed of light as it moves from air into a glass of water?
Which of these is an example of refraction?
Light only bends when it hits a material at an angle.
Review Tomorrow
Tomorrow, try to explain to a friend or family member why a pool looks shallower than it actually is using the word 'refraction'.
Practice Activity
Find a clear glass and a coin. Try the 'Disappearing Coin' experiment at home to see if you can make the coin appear!