Small, Smaller, Smallest
The length ladder — from femtometre to light year
What this lesson covers
Why it matters
The same unit that measures your pencil measures a virus and a galaxy — you just bolt on a prefix and keep counting zeros.
The idea in plain words
The long and short of length. Tap the landmarks.
fm → Å → nm → mm → m → km → AU → ly
Express 2.5 light years in metres.
≈ 2.4 × 10¹⁶ m
- ångström: 1 Å — 10⁻¹⁰ m — the size of atoms; X-ray wavelengths
- astronomical unit: 1 AU — 1.496 × 10¹¹ m — Earth-to-Sun, the solar system's ruler
- light year: 1 ly — 9.46 × 10¹⁵ m — the distance LIGHT travels in a year (a length, not a time!)
- 2.5 × 9.46 × 10¹⁵
Predict first
Roughly how many times bigger is an atom (10⁻¹⁰ m) than its nucleus (10⁻¹⁵ m)?
Divide the powers: 10⁻¹⁰ ÷ 10⁻¹⁵ = 10⁵. If the atom were a stadium, the nucleus would be a pea at the centre.
- 100,000 times (10⁵) — correct
- 5 times
- 50 times
What you do
Slide from the nucleus to a light year — eleven objects, thirty orders of magnitude.
Check yourself
The light year is a unit of…
It's how FAR light gets in a year — the universe's favourite tape measure.
1 nanometre equals…
milli 10⁻³, micro 10⁻⁶, nano 10⁻⁹ — three steps of a thousand each.
Which instrument suits measuring a hair's THICKNESS?
A hair is ~0.1 mm; the rule's smallest count is 1 mm. You need the 0.01 mm machine — next lessons!
- distance — correct
- time
- speed
- 10⁻⁹ m — correct
- 10⁻⁶ m
- 10⁻³ m
- screw gauge — correct
- metre rule
- measuring tape