Science and Technology K–6 · Year 5

The bending pencil and the reappearing coin: refraction

Science understanding: Physical sciences

Practical, model not builtLow risk

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The idea

Light changes direction when it passes from air into water, so a straight pencil looks bent at the surface and a coin hidden by a cup's rim comes into view when water is poured in.

What you need

a clear glass of water and a pencil
an opaque cup, a coin and a jug of water
a rectangular clear plastic box of water on paper, and the slit torch from the reflection activity
a bowl of water 10 cm deep with a ruler standing in it
protractor, ruler, pencil

How to do it

Stand the pencil in the glass of water and view it from the side at eye level. Draw what you see. View from above and draw again.
Put the coin in the bottom of the opaque cup and step back until the rim just hides it. Hold still while a partner pours water in slowly. Record when the coin appears.
Shine the narrow beam across the paper into the side of the water box at 45 degrees to the normal. Mark the beam outside and inside the box, remove the box and measure both angles.
Repeat at 30 degrees and 60 degrees. Tabulate angle in air against angle in water.
Look at the ruler standing in a 10 cm deep bowl from above and estimate how deep the bottom looks.
Write the rule: light bends toward the normal going into water and away from it coming out.

What you should see

The pencil looks broken and shifted at the water line; the coin comes into view as the water rises because light from it bends at the surface toward the eye. Snell's law with n = 1.333 for water gives 32.0 degrees in the water for 45 degrees in air, 22.0 for 30 and 40.5 for 60. A 10 cm deep bowl looks about 7.5 cm deep (real depth divided by 1.333), which is why pools look shallower than they are.

What changes

What you change: angle of incidence in air
What you measure: angle of refraction in water
What you keep the same: same box and water
beam at the box centre
angles from the normal

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The pencil is bent by the water pressure.
Water magnifies everything.
The coin floats up when water is added.

Safety card

Low riskLearners carry it out

Hazards

water on electrical equipment
torch in eyes

Controls

torch away from the water
beam kept on the paper

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; a Stage 2 outcome, because NSW teaches the behaviour of light in Stage 2 ('investigate the behaviour of light', with reflection in a mirror and shadows as its examples), and neither the 2017 nor the 2024 Stage 3 content includes it; code read from the syllabus document on 22 September 2026ST2-8PW-ST
Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01
Australian Curriculum v9AC9S5U03AC9S5I01AC9S5I03

Sources

The pages the author read to write this activity.

All Concept Studio activities