Everyday Science

What this topic covers

The more familiar something is, the less we ask why. A blue sky, hot summers, salt on winter roads, a warm patch behind the fridge: they seem too ordinary to need explaining. This topic picks seven such things and restates, in everyday language, the principles you may have learned at school and half forgotten. The point is not to memorise formulas but to show what changes once you understand the mechanism. Why you should stir food halfway through microwaving, why you check tide times before walking onto a mudflat: understanding leads straight to better decisions.

Three threads: light, heat and moving bodies

The seven guides fall into three groups. The first is light. Scattering, where light bounces off air molecules, and refraction, where it bends on entering water, produce the blue sky, red sunsets and rainbows. The second is heat. How heat enters and leaves a substance as it melts, freezes, boils or evaporates explains the freezing point of salt water and the workings of a fridge, and the way electromagnetic waves shake water molecules explains the microwave oven. The third is the movement of the Earth and Moon. A tilted Earth orbiting the Sun and a Moon orbiting the Earth give us seasons, lunar phases and tides.

• Light: scattering, refraction, dispersion
• Heat: changes of state and latent heat
• Bodies: orbit, rotation, tilt, gravity

What light does

Sunlight looks white, but it is a mixture of many wavelengths. The light our eyes can see runs from roughly 380 to 750 nanometres, with violet and blue at the short end and red at the long end. How strongly light scatters in air, and how much it bends on entering water or glass, depends on wavelength, and that is why colours separate. The blue sky, the red sunset and the rainbow look like different phenomena, but they all come from the same fact: light of different wavelengths behaves differently.

What heat does

Heat always flows from warmer to colder. A fridge or air conditioner seems to defy this, but it actually uses electrical work to pump heat in the opposite direction. The key is that a liquid absorbs heat from its surroundings when it evaporates. Salt water failing to freeze at zero is also a matter of heat: dissolved particles get in the way of water molecules locking into ice, so the water has to get colder before it freezes. A microwave oven works differently from conduction, shaking the water molecules inside food directly to generate heat.

What the Earth and Moon do

The Earth's axis is tilted about 23.4 degrees relative to its orbit. Because of this tilt, the angle of sunlight and the length of the day at any given place change through the year, and that is what creates seasons. The Moon returns to the same shape about every 29.5 days, and its changing shape is not caused by the Earth's shadow but by the changing angle from which we see its sunlit half. The difference in how strongly the Moon and Sun pull on different parts of the ocean produces, in most places, two high tides and two low tides a day.

Reading order

You can start anywhere, but reading the two light guides back to back makes the difference between scattering and refraction clear. We suggest the sky guide first, then the rainbow guide. For heat, read the freezing point guide before the fridge and air conditioner guide, so that changes of state and latent heat build naturally. The microwave guide stands on its own. The seasons and Moon guides both ask you to picture positions in space, so a torch and a ball will help you understand them far faster than words alone.

Common misconceptions

Plausible but wrong explanations are everywhere in everyday science. Here are the most common ones, each taken apart in the relevant guide. The best way to test an explanation is to ask what we should observe if it were true. If distance from the Sun caused the seasons, for example, both hemispheres would have summer at the same time, yet their seasons are opposite.

• Summer is hot because the Earth is closer to the Sun
• The sky is blue because it reflects the sea
• Microwaves cook food from the inside out
• Leaving the fridge open cools the room
• The Moon changes shape because the Earth's shadow covers it

The seven guides in this topic

Each guide shows the phenomenon first, explains the principle, then ends with practical judgement, common mistakes and safety notes. Only well-established figures are used, and values that vary by place or conditions point you to official sources.

• Why the sky is blue and sunsets are red
• How a microwave oven heats food
• Why we have seasons: the tilted axis
• Why salt water and sugar water freeze later
• Rainbows and the refraction of light
• Moon phases and the tides
• How fridges and air conditioners work: the heat pump