The sky isn't actually blue, a straw in a glass of water isn't actually bent, and white light secretly contains every color at once — light rarely behaves the way it first appears to.
Sunlight looks colorless, the sky above looks blue, and a straw dropped into a glass of water looks bent at an angle it clearly isn't. None of these are illusions exactly — they're light obeying precise physical rules that simply don't match casual intuition. Once those rules are visible, a rainbow, a mirror, and a sunset all turn out to be explained by the very same handful of principles.
Light is a form of electromagnetic energy that travels as a wave, and it's the only part of the much larger electromagnetic spectrum (which also includes radio waves, microwaves, and X-rays) that human eyes can actually detect — known as visible light. In empty space, light travels at its maximum possible speed, roughly 299,792 kilometres per second, faster than anything else in the universe. Under normal conditions, light travels in straight lines, which is why shadows have sharp edges and why you can't see around a corner without a mirror.
Reflection happens when light hits a surface and bounces back, rather than passing through or being absorbed. It follows one precise rule, called the Law of Reflection: the angle at which light arrives (the angle of incidence) always equals the angle at which it leaves (the angle of reflection), measured from an imaginary line perpendicular to the surface.
The Law of Reflection: the angle the light comes in at always equals the angle it bounces off at, measured from the perpendicular "normal" line.
A flat, smooth mirror reflects light in an organised way, producing a clear image — this is called regular reflection. A rough surface, like paper or an unpolished wall, reflects light in many different directions at once because the surface itself is uneven at a microscopic level — this is called diffuse reflection, and it's why you can see a piece of paper from almost any angle, but a mirror's image only from specific ones.
Refraction is the bending of light as it passes from one transparent material into another — for example, from air into water, or from air into glass. This happens because light travels at different speeds in different materials, and that change in speed causes the light to bend at the boundary between them.
A straw placed in a glass of water appears to "bend" sharply at the water's surface. The straw itself hasn't moved at all — light from the underwater portion of the straw refracts (bends) as it exits the water and enters the air, changing direction before it reaches your eye, which makes the straw only appear broken.
The same principle, refraction, is what makes eyeglasses and camera lenses work: precisely shaped curved glass bends light in a controlled way, to focus an image sharply.
Sunlight looks white or colorless, but it actually contains every color of visible light mixed together. As sunlight enters Earth's atmosphere, it collides with tiny gas molecules (mostly nitrogen and oxygen), and this scatters the light in all directions — a phenomenon called Rayleigh scattering, named after the physicist Lord Rayleigh, who described it in 1871.
Crucially, shorter wavelengths of light (toward the blue and violet end of the spectrum) scatter far more strongly than longer wavelengths (toward the red end). Blue light gets bounced around the sky from every direction, which is why the sky looks blue no matter where you look, on a clear day.
Blue light scatters strongly off air molecules in every direction (why the daytime sky looks blue), while red light mostly travels straight through, only becoming dominant when sunlight passes through much more atmosphere at sunset.
At sunset, sunlight has to pass through far more atmosphere to reach your eyes, since it's travelling in at a low angle. By that point, almost all the blue light has already been scattered away in other directions, leaving mostly the longer red and orange wavelengths to reach you directly — which is exactly why sunsets glow red and orange.
White light is really a mixture of every visible wavelength combined. Passing it through a glass prism separates it back out into its individual colors — the same effect that creates a rainbow.
| Color | Approximate Wavelength |
|---|---|
| Violet | ~380–450 nm |
| Blue | ~450–495 nm |
| Green | ~495–570 nm |
| Yellow | ~570–590 nm |
| Orange | ~590–620 nm |
| Red | ~620–700 nm |
This entire range — roughly 380 to 700 nanometres — is called the visible spectrum, and it's only a tiny slice of the full electromagnetic spectrum. Just beyond violet lies ultraviolet light; just beyond red lies infrared — both invisible to human eyes, but very real and used constantly in technology, from UV sterilisation to infrared remote controls.
An object's color depends on which wavelengths of light it reflects and which it absorbs. A red apple looks red because its surface absorbs most wavelengths of light but reflects red wavelengths back to your eyes. A white object reflects nearly all wavelengths; a black object absorbs nearly all of them, which is also why black surfaces heat up faster in sunlight — they're absorbing far more light energy rather than reflecting it away.
A rainbow forms when sunlight enters a raindrop, refracts (bends) as it slows down entering the water, reflects off the inside back surface of the drop, and refracts again on the way back out — and because each wavelength of light bends by a very slightly different amount, the drop separates white sunlight into its full range of colors. Millions of raindrops doing this simultaneously, each one bouncing a particular color toward your eyes at a particular angle, is what produces the complete arc of a rainbow.
Light seems like the simplest thing in the world to understand, right up until you ask why the sky is blue, why a straw looks bent, or where a rainbow's colors actually come from. Reflection, refraction, and scattering are only three ideas, but between them they explain nearly everything light does — from a mirror's clear image to a sunset's deep red glow.
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