Old sunlight
LIGHT is the fastest thing there is, travelling about 300,000 kilometres every second. Even so, sunlight takes about eight minutes to reach us, so the Sun you see is the Sun as it was eight minutes ago. The first person to show that light takes time to travel was the Danish astronomer Ole Rømer in 1676. He noticed that the moons of Jupiter slipped behind the planet a little late when Earth was far from Jupiter, and a little early when it was close.
Light travels in straight lines, so a SHADOW has sharp edges. When a RAY of light bounces off a MIRROR, that is REFLECTION. When it passes from air into water or glass it slows down and changes direction, which is REFRACTION. It makes a straw in a glass look bent, and how a LENS in glasses or a camera focuses light.
Why the sky is blue
White sunlight is a mixture of every COLOUR, as a PRISM shows by splitting it into a SPECTRUM. When sunlight passes through the air, tiny gas molecules scatter the short blue waves far more than the long red ones, sending blue light across the whole sky. At sunset the light reaches us through much more air, so most of the blue is scattered away before it arrives, leaving the reds and oranges. The RAINBOW has its own story on our colours page.
Screens mix colour differently from paint. Your eyes have three kinds of colour-sensing cells, most sensitive to red, green and blue light. A screen fools them: when red and green dots glow side by side, both sets of cells are triggered at once, and your brain sees yellow, even though no yellow light is there. Our painting page covers how mixing paints works the other way.
Sound needs something to travel through
SOUND is a VIBRATION passed on from particle to particle as a WAVE. In air it travels at about 343 metres a second, around a kilometre every three seconds, and more than four times faster through water. In space, where there is no air, there is no sound at all. In 1947 the American pilot Chuck Yeager became the first person to fly faster than sound, and anything that does so drags a shock wave behind it, heard on the ground as a sonic boom.
An ECHO is sound bouncing back from a hard surface. Bats find their food by listening for echoes, as our nocturnal animals page explains.
High, low, loud and soft
PITCH depends on FREQUENCY, the number of vibrations each second. Young people can hear from about twenty vibrations a second up to about twenty thousand, but the top end fades with age, so some very high-pitched ringtones can be heard by teenagers but not by their parents. Breathing helium does not actually change the pitch of your vocal cords; the sound travels faster through the light gas, which changes the tone of your voice and makes it sound squeaky.
VOLUME depends on the AMPLITUDE, how big the vibrations are, and it is measured in DECIBEL units. The scale jumps in multiples: every ten decibels is ten times more intense, so a loud concert at about 110 decibels is around a hundred million times more intense than a whisper. The loudest sound in recorded history was the eruption of Krakatoa in 1883, heard nearly 5,000 kilometres away.