Question 1

Why is the sky blue?

Accepted Answer

Sunlight contains all visible wavelengths. As it passes through the atmosphere it collides with molecules of nitrogen and oxygen, which scatter it in all directions. The critical detail is that scattering is strongly wavelength dependent: shorter wavelengths scatter far more than longer ones, by a factor related to the fourth power of the frequency.

Blue and violet light therefore bounce around the atmosphere far more than red and orange, and reach your eye from every direction in the sky rather than only from the direction of the sun. The sky glows blue because you are seeing scattered short-wavelength light coming from everywhere.

A reasonable question is why the sky is not violet, since violet scatters even more strongly. Two reasons combine. The sun emits less violet than blue, and human eyes are considerably less sensitive to violet, with our cone response peaking well away from that end. The result our visual system constructs is blue.

The same mechanism explains sunsets, which is the satisfying part. When the sun is low, its light travels through much more atmosphere to reach you. Over that longer path the blue is scattered away almost entirely, leaving the longer wavelengths to continue. What remains is red and orange, which is why the sun itself appears red at the horizon while the overhead sky is still blue.

It also explains why the sky is black in space and on the moon: with no atmosphere there is nothing to scatter light, so you see only the direct source against darkness.

Clouds appear white rather than blue because their water droplets are much larger than air molecules and scatter all wavelengths roughly equally.

Question 2

Why does the sea look blue when water is colourless?

Accepted Answer

Two effects combine, and one of them surprises people: water is not actually colourless. Water genuinely absorbs red light. Water molecules absorb light at the red end of the visible spectrum through

Question 3

What causes rainbows, and why are they always curved?

Accepted Answer

Refraction, reflection and dispersion inside raindrops — and the curve is a consequence of geometry rather than of anything about the sky. What happens in a single drop: Light enters the drop and

Question 4

What causes the northern lights?

Accepted Answer

Charged particles from the Sun colliding with gases in Earth's upper atmosphere, guided to the polar regions by the planet's magnetic field. The sequence: The Sun emits the solar wind — a continuous

Question 5

Why does hot metal glow red and then white?

Accepted Answer

Because everything emits light according to its temperature, and as temperature rises the emission shifts toward shorter wavelengths and broadens across the spectrum. Any object above absolute zero

Question 6

What actually causes the seasons?

Accepted Answer

The seasons are caused by the tilt of Earth's rotational axis, about twenty-three and a half degrees relative to its orbital plane. They are not caused by distance from the sun, which is the most

Question 7

Why does the moon look bigger near the horizon?

Accepted Answer

It does not actually get bigger. The moon's angular size when it is low is essentially identical to when it is high, and if anything it is marginally smaller near the horizon, because you are

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