Practical 20
Oceanography
CAPS block 13 · Term 2 · 2 hours
Light in Water — Bend It, Dim It, Steal Its Colours
Measure how far water lifts a coin, and test whether the textbook's reason for it is right. Turn a phone into a light meter and watch light fade layer by layer — then solve a puzzle hidden in the CAPS numbers. Make red things turn black with nothing but a blue screen, and read a diver's colour card from the surface to 30 m.
What CAPS asks for
Investigations column, block 13: "Refraction exploration." Resources: "Videos · Glass · Pencil." Depth column: "Learners should be able to apply these contexts to basic settings." Key concepts 11 and 12: "Light attenuation can be observed and measured with the use of a Secchi disk. Light is more accurately measured by a light meter."
The refraction exploration, with a glass and a pencil, is Investigation 1. Measuring light with a light meter — and the CAPS absorption figures — is Investigation 2. Colours at depth (key concepts 6–9) are Investigation 3, and Investigation 4 is the booklet's "Test your knowledge" questions. The Secchi disk itself was built in Practical 1 and used again in Practical 19.
What you need
- 🟢 kitchen A tall, straight-sided clear glass or jug, a pencil, two identical coins, a ruler, masking tape, and some cooking oil
- 🟢 kitchen An empty tin can (label off) or a cardboard tube, a bath towel, a smooth tiled or wooden floor, and a protractor
- 🟢 kitchen A phone with a free light-meter (lux) app, a torch, a roll of baking paper, a clear glass baking dish, and a little milk
- 🟢 kitchen A dark room, a second phone or tablet showing a plain blue screen, and some coloured things: a tomato, an orange, a banana, a green leaf, something blue, white paper and black cloth
- 🌊 dive Optional — a colour card (squares of red, orange, yellow, green, blue, violet, white and black, sealed in a zip-lock bag) taken on a snorkel or a qualified dive
- 🟢 kitchen Graph paper, a calculator and coloured pencils
No light-meter app, or no dive? That's fine.
Every investigation has a supplied results panel: apparent depths in water and oil, a rolling-tin run, light-meter readings through paper and milky water, a blue-light room, and a diver's colour-card log to 30 m. Most Android phones have the light sensor a lux app needs; on an iPhone, camera-based apps work less well — use the supplied readings.
Safety — eyes, phones and the sea
Never look at the sun, and never shine a torch into anyone's eyes. Keep phones away from the water dishes, or seal them in a zip-lock bag. Use cooking oil cold, and wipe up spills so nobody slips. Dive only within your qualification, with a buddy, and never go deeper for the colour card than your training allows — the snorkel version (0–3 m) still shows red fading.
1
Refraction Exploration — a Glass and a Pencil
CAPS: "Refraction exploration" · Resources: "Glass · Pencil"
Aim
To see refraction, to measure how much water makes things look shallower, and to test the reason CAPS gives for it.
Method
- The broken pencil. Stand a pencil at a slant in a glass half full of water. Look at it from the side, at the level of the water surface, and then from above. Sketch what you see.
- Apparent depth. Stick masking tape down the outside of the tall glass and mark it in centimetres from the bottom. Put a coin on the bottom and pour in water to 4 cm deep. Look straight down at the coin with one eye. Hold a pencil flat against the outside of the glass and slide it up or down until it seems to be at the same depth as the coin — when you move your head a little from side to side, the coin and the pencil should move together. Read the pencil's height on the tape. The apparent depth is the water depth minus that height.
- Repeat at 8, 12, 16 and 20 cm of water.
- Oil. In a second glass, pour 10 cm of cooking oil over a coin and measure its apparent depth the same way. Then pour a little water into a third glass, add some oil, and watch which layer floats.
- CAPS's rolling cylinder. Lay the towel flat on a smooth floor with one straight edge. Roll the tin across the floor so it meets the towel's edge at a slant — about 40° from a line drawn at right angles to the edge. Watch which end of the tin reaches the towel first and which way the tin turns. Measure the angle of its new path from the same right-angled line. Then roll it from the towel back onto the floor.
The normal is a line drawn at right angles to the boundary, where the ray crosses it. Angles are measured from the normal, not from the surface. Your booklet's definition says light bends "towards the normal boundary between the two mediums" — the normal is not the boundary; it is the line perpendicular to it. On a ray diagram the rays carry arrows to show which way the light travels, even though Topic 5 says label lines never do.
Supplied results — apparent depths, oil, and a rolling tin
| Real depth (cm) | 4 | 8 | 12 | 16 | 20 |
|---|---|---|---|---|---|
| Apparent depth (cm) | 3.0 | 6.1 | 9.0 | 12.1 | 15.0 |
Data set 1.2 — cooking oil. Under 10.0 cm of sunflower oil the coin appeared 6.8 cm deep. In the third glass the oil floated on top of the water: oil is less dense than water (about 0.92 g/cm³ against 1.00 g/cm³). A pencil through both layers looked broken twice — at the air–oil surface and again at the oil–water boundary.
Data set 1.3 — the rolling tin. Rolled at 40° from the normal off the smooth floor, the tin's front end touched the towel first, slowed, and the tin swung round: on the towel it rolled at about 28° from the normal, noticeably slower. Rolled back from the towel at 28°, it came out onto the floor at about 40°.
Results
| Real depth (cm) | 4 | 8 | 12 | 16 | 20 | oil, 10 |
|---|---|---|---|---|---|---|
| Apparent depth (cm) | ||||||
| Real ÷ apparent |
Analysis
- Sketch the pencil in the glass as seen from the side, and explain why it looks broken at the water surface. 3
- Calculate real depth ÷ apparent depth for each water depth, and the mean. 3
- Plot apparent depth against real depth and draw the line of best fit. Calculate its gradient. 4
- Use your result: a spearfisher at the surface sees a fish that appears to be 1.5 m down. How deep is it really? 2
- Draw a ray diagram to show why the coin looks closer to the surface than it is. Show the normal, the real coin, the ray bending as it leaves the water, and where the eye thinks the coin is. 4
- Describe what happened to the tin at the edge of the towel, and use it to explain CAPS's rolling-cylinder model of refraction. 3
- What happened when the tin rolled from the towel back onto the floor? What does this predict about a ray of light leaving water into air? 2
- CAPS and the booklet say light bends "because of the different densities" of air and water. Calculate real ÷ apparent depth for the oil. Oil is less dense than water — so what does your result show about that explanation, and what is the real reason light bends? 4
- Underwater, a flat dive mask makes everything look about a third closer and a third bigger. A spearfisher sees a fish that looks 40 cm long. Estimate its real length, and give one way this effect could mislead a diver. 3
2
A Phone Light Meter, and the CAPS Numbers
CAPS key concepts 2–5 and 10–12 · "Light is more accurately measured by a light meter"
Aim
To measure attenuation with a light meter — through layers of paper and through milky water — and to use the result to make sense of the CAPS figures for light in the sea.
Method
- In a darkened room, lay the phone face-up with the light-meter app open. Fix the torch 20 cm above the phone's sensor (prop it on books) and record the reading in lux.
- Lay one sheet of baking paper over the sensor and record the reading. Add sheets one at a time, up to six, recording each time. Your booklet compares attenuation to looking through several pairs of sunglasses — this is that idea, measured.
- Stand the empty glass dish over the phone (on two books, so it does not touch). Record the reading. Add clear tap water 10 cm deep and record again.
- Empty the dish. Make milky water: 5 drops of milk in a litre of water. Record the reading with 2, 4, 6, 8 and 10 cm of milky water in the dish.
- In the dark, shine the torch sideways through a glass of clear water, then a glass of milky water. Look at each from the side.
Supplied results — paper, milky water, and the CAPS figures
| Sheets | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Light (lux) | 1 850 | 1 120 | 690 | 420 | 255 | 160 | 98 |
| Water depth (cm) | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| clear tap water (lux) | 1 850 | — | — | — | — | 1 640 |
| milky water (lux) | 1 850 | 1 310 | 925 | 660 | 465 | 330 |
The torch from the side. In clear water the beam was invisible from the side. In milky water the whole beam glowed, bluish-white from the side — and looking straight through the glass at the torch, its light looked yellow-orange.
| Depth (m) | 0 | 1 | 10 | 150 | 1 000 |
|---|---|---|---|---|---|
| Light remaining (%) | 100 | 40 | 20 | 1 | 0 |
Analysis
- For each sheet of paper, calculate the fraction of the light that got through (reading after ÷ reading before), and the mean fraction. 3
- Plot the light reading against the number of sheets, and describe the shape of the curve. 4
- Each sheet lets through the same fraction, not the same number of lux. Explain why the curve therefore never quite reaches zero, and predict the reading under 8 sheets. 3
- From Data set 2.2, find the depth of milky water that halves the light. 3
- Clear water lost about 11 % of the light in 10 cm; milky water lost about 82 %. What does the milk represent in the sea? Name the process that caused the extra loss, and explain why the beam could be seen from the side only in the milky water. 4
- Plot the CAPS figures (Data set 2.3). The 1 m point and the 150 m point are hard to show on one ordinary graph. Suggest two ways to plot the data so both can be read, and draw one of them. 4
- The 60 % puzzle. If the sea removed 60 % of the light in every metre, as it does in the first, what percentage would be left at 10 m? Compare your answer with the CAPS figure for 10 m. 3
- Resolve the puzzle: what does the first metre of seawater absorb that is no longer there below 10 m? Use the word wavelength. 3
- Between 10 m and 150 m the light falls from 20 % to 1 %. About how many halvings is that, and about how many metres does each halving take? Compare this with your milky water. 3
- CAPS says light is "more accurately measured by a light meter" than by a Secchi disk. Give three reasons. 3
3
The Colour Thief — Colours at Depth
CAPS key concepts 6–9 · booklet question 5 (Newton's spectrum taken to 30 m)
Aim
To show why red things look black at depth, and to read how the colours of a colour card change on a dive from the surface to 30 m.
Method
- The blue room. In a completely dark room, light your coloured objects with only a phone or tablet showing a plain blue screen, held 20 cm away. Write down what colour each object looks. Then switch on a white torch and look again.
- The colour card (divers and snorkellers). At the surface, look at your sealed colour card and photograph it without a flash. At each depth you reach — 3 m, 6 m, 10 m and deeper only within your training — look again, note each square's colour, and photograph it. At your deepest point, light the card with a dive torch held close.
Supplied results — the blue room, and a colour card to 30 m
| Object | Under blue light | Under the white torch |
|---|---|---|
| tomato | black | red |
| orange | very dark brown | orange |
| banana | dark grey | yellow |
| green leaf | dark blue-green | green |
| blue cup | bright blue | blue |
| white paper | blue | white |
| black cloth | black | black |
⚠️ About this dive log
This log is a composite of what divers commonly report on a clear-water descent at Sodwana Bay. It is not one person's record of one dive, and colour is partly in the eye of the diver — use it to practise reading the pattern, not as a measurement.
| Depth | Red | Orange | Yellow | Green | Blue | White |
|---|---|---|---|---|---|---|
| 0 m | red | orange | yellow | green | blue | white |
| 3 m | dull red | orange | yellow | green | blue | very pale blue |
| 6 m | brown | dull orange | yellow | green | blue | pale blue |
| 10 m | dark brown-black | brownish | pale yellow-green | green | blue | blue |
| 15 m | black | grey-brown | greenish-yellow | green | blue | blue |
| 20 m | black | grey | greenish-grey | blue-green | blue | blue-green |
| 30 m | black | grey | grey-green | dark blue-green | blue | blue-green |
At 30 m the diver held a torch 30 cm from the card: the red square was suddenly bright red and the orange square orange. Held 3 m away, the torch lit the card, but the red looked dull brown again.
Analysis
- Explain why the tomato looked black under the blue screen, while the white paper looked blue. 4
- What does the blue screen represent in the sea, and which CAPS statement does the blue room demonstrate? 2
- From the dive log, give the depth by which each of red, orange and yellow had lost its own colour. 3
- CAPS says the long-wave (red) end of the spectrum "is lost within the first 10 m", and the booklet says no red, orange or yellow light reaches 10 m. Does the dive log fully agree? Suggest two reasons for any difference — and say which statement to write in an exam. 3
- Explain why the white square looked blue by 10 m. 2
- Explain why the red square was bright red with the torch 30 cm away, but dull brown with the torch 3 m away. 3
- Answer the booklet's question 5: a diver takes a laminated sheet of Newton's spectrum down to 30 m. Explain what she sees changing on it as she descends. 4
- Many fish and crabs that live below 20 m are red. Explain why red is good camouflage there. 2
- Explain why the open ocean looks deep blue, but the water off the Swartkops river mouth after rain looks brown-green. 3
4
The Booklet's Questions
"Test your knowledge" questions 1–4, 6–8 · CAPS: "apply these contexts to basic settings"
Aim
To answer the booklet's own questions — the ones most like an exam — and to check two of its statements against the physics.
Your diagram
Analysis
- Draw the electromagnetic spectrum as a labelled band from the shortest to the longest wavelength. Mark visible light (400–750 nm) and show its seven colours in order. 5
- (a) Explain what light energy is and how an electromagnetic wave travels. (b) The booklet says light from the sun reaches Earth "years later". The sun is 150 million km away and light travels at 300 000 km/s. Calculate the real travel time and correct the statement. 4
- The booklet's question 4: two divers compare notes. The visibility at Sodwana Bay is 10 m and at Simon's Town is 1 m. Explain the possible reasons for the difference. 4
- Explain the terms light penetration and light attenuation, and how they differ. 4
- Describe how light changes when it passes from water into air, and give two challenges this creates for observing what happens in the water. 4
- Name the process described by each: (a) light bounces off a smooth surface at the angle it arrived; (b) light is taken up by a material and turned into heat; (c) light is sent off in many directions by suspended particles; (d) light changes direction as it changes speed between two media; (e) the total loss of light intensity with distance through a medium. 5
Memo — worked answers and mark allocation (does not print)
Total: 113 marks — Investigation 1: 28 · Investigation 2: 33 · Investigation 3: 26 · Investigation 4: 26.
Investigation 1 — refraction exploration
- Q1 (3): Sketch: the part of the pencil in the water appears shifted and bent upwards at the surface (1). Light from the underwater part changes speed and bends as it leaves the water into the air (1), so the eye traces it back along a straight line to a place higher than the pencil really is — the two parts no longer line up (1).
- Q2 (3): 4 ÷ 3.0 = 1.33; 8 ÷ 6.1 = 1.31; 12 ÷ 9.0 = 1.33; 16 ÷ 12.1 = 1.32; 20 ÷ 15.0 = 1.33 (2). Mean ≈ 1.33 (1).
- Q3 (4): Axes labelled with units (1); points accurate (1); a straight line through the origin (1); gradient ≈ 0.75 — the coin appears at three-quarters of its real depth (1).
- Q4 (2): Real = apparent × 1.33 (1) = 1.5 × 1.33 ≈ 2.0 m (1).
- Q5 (4): The normal drawn at right angles to the surface where the ray leaves (1); a ray from the coin bending away from the normal as it passes from water into air (1); the ray traced straight back (dashed) to an image above the real coin (1); arrows on the rays, from the coin to the eye, and both coin and image labelled (1).
- Q6 (3): The end of the tin that reached the towel first slowed down while the other end kept rolling fast on the floor (1), so the tin swung round towards the normal, from 40° to 28° (1). Light is the same: the side of a beam that enters the water first slows first, so the beam turns towards the normal (1).
- Q7 (2): It turned away from the normal, from 28° back to 40°, and sped up (1). Light leaving water into air speeds up and bends away from the normal (1).
- Q8 (4): 10.0 ÷ 6.8 = 1.47 (1) — oil bends light more than water does (1), yet oil is less dense, so mass density cannot be the reason (1). The real reason is the speed of light in the material: light travels more slowly in oil than in water, and more slowly in water than in air. Physicists call this "optical density", which is not the same as density in g/cm³ (1). In an exam, write that light slows down as it enters water.
- Q9 (3): About 40 × ¾ = 30 cm (accept 30 cm, or 40 ÷ 1.33) (1). Any two: a fish can look legal-size when it is too small; objects look closer than they are, so a diver reaches short or misjudges a gap; a diver can misjudge distances to rocks, a boat or a buddy (2).
Investigation 2 — a phone light meter
- Q10 (3): 1 120 ÷ 1 850 = 0.61; 690 ÷ 1 120 = 0.62; 420 ÷ 690 = 0.61; 255 ÷ 420 = 0.61; 160 ÷ 255 = 0.63; 98 ÷ 160 = 0.61 (2). Mean ≈ 0.61 — each sheet lets through about 61 % (1).
- Q11 (4): Axes labelled with units (1); points accurate (1); a smooth curve (1); described as falling steeply at first, then more and more gently — it curves towards zero without reaching it (1).
- Q12 (3): Each sheet removes the same fraction (about 39 %) of whatever light reaches it, so there is always some fraction left — the light gets smaller and smaller but never mathematically reaches zero (1). In practice it falls below what the meter can detect (1). Under 8 sheets: 98 × 0.61 × 0.61 ≈ 36 lux (accept 35–38) (1).
- Q13 (3): Half of 1 850 is 925 (1), reached at 4 cm (1); and it halves again over the next 4 cm (925 → 465 at 8 cm), so each 4 cm halves the light (1).
- Q14 (4): The milk stands for suspended particles — silt, plankton and organic matter (1). The extra loss is scattering: the particles send the light off in all directions, so less goes straight through (1). In clear water almost no light is scattered sideways, so the beam is invisible from the side (1); in milky water the particles scatter some of the beam towards your eye, so the whole beam glows (1).
- Q15 (4): Any two ways: a logarithmic scale for the light; a broken depth axis; two graphs, one for 0–10 m and one for 10–1 000 m; plotting depth on a log scale (2). One of them drawn correctly, depth increasing downwards or across, axes labelled (2).
- Q16 (3): 40 % remains after 1 m, so 0.410 of the light would remain after 10 m (1) = about 0.01 % (1). CAPS says 20 % remains — two thousand times more — so the sea cannot be removing the same fraction in every metre (1).
- Q17 (3): The first metre absorbs the long wavelengths — the infrared (about half of the sun's energy) and most of the red — which water absorbs very strongly (1). What is left below that is mostly blue-green light, which water absorbs only weakly (1). So the loss per metre is huge at first and much smaller afterwards; the 60 % and 80 % figures are about the total of all wavelengths (1).
- Q18 (3): 20 → 10 → 5 → 2.5 → 1.25 %: just over 4 halvings (1) in 140 m, so about 33 m per halving (accept 30–35 m) (1). Milky water halved the light every 4 cm — hundreds of times faster, because of all the particles (1).
- Q19 (3): Any three: a light meter gives a number, a Secchi disk depends on the observer's eyesight and judgement; a meter can measure the light at any depth, not just where a disk vanishes; it is not affected by glare, waves or the angle of the sun on the surface; it can be repeated exactly by anyone; some meters measure separate wavelengths (colours); it works in any water, a disk only to where it disappears.
Investigation 3 — the colour thief
- Q20 (4): An object's colour is the light it reflects (1). A tomato reflects red light and absorbs the rest (1); the blue screen gave it no red light to reflect, so it reflected almost nothing and looked black (1). White paper reflects every colour it receives, but only blue arrived, so it reflected blue (1).
- Q21 (2): The light left at depth, once the water has absorbed the red end (1). CAPS key concept 9: objects deeper than 10 m receive and reflect only blue light, so red objects appear black (1).
- Q22 (3): Red — by about 10–15 m (brown by 6 m, black by 15 m) (1). Orange — by about 10 m, brownish then grey (1). Yellow — by about 20 m, greenish-grey (1). Accept answers that follow the log.
- Q23 (3): Not fully — red was gone by about 10 m, but some yellow was still seen at 10–15 m (1). Any one reason: this was very clear water, where light reaches further; colours fade gradually rather than switching off at one depth, and a little dim yellow light still arrives; the human eye adjusts to dim light and still picks out colour; CAPS gives a simplified rule (1). In an exam, write the CAPS statement: the red end is lost within the first 10 m (1).
- Q24 (2): White reflects whatever colours arrive (1); by 10 m most of the red, orange and yellow have been absorbed, so only blue (and green) light reaches the card to be reflected (1).
- Q25 (3): The torch makes white light containing red (1). With the torch 30 cm away the light travels only a short way through water, so little red is absorbed and the card reflects red back (1). From 3 m away the light travels further through water to the card and back to the eye, and most of the red is absorbed on the way (1).
- Q26 (4): At the surface all the colours show (1). As she descends, red fades first — dull, then brown, then black by about 10 m (1); then orange and yellow fade to brown and grey (1); by 30 m only green, blue and violet still look like themselves, and the white parts look blue-green (1).
- Q27 (2): No red light reaches that depth, so a red animal reflects almost nothing (1) and looks black — it disappears against the dark water to predators and prey (1).
- Q28 (3): In the open ocean, with few particles, the water absorbs the long wavelengths and scatters blue light back up to our eyes, so it looks blue (1). After rain the river brings silt and organic matter, and plankton may bloom (1); these particles scatter and reflect their own colours — brown from silt, green from plankton — before the light can go deep (1).
Investigation 4 — the booklet's questions
- Q29 (5): Gamma rays → X-rays → ultraviolet → visible → infrared → microwaves → radio waves (TV, FM, AM and long radio) in that order (2); visible light marked as a narrow band, 400–750 nm (1); the seven colours violet, indigo, blue, green, yellow, orange, red, from short to long wavelength (1); the direction of increasing wavelength shown (1).
- Q30 (4): (a) Light is energy radiated by the sun (1) as an electromagnetic wave — changing electric and magnetic fields that keep generating each other and so travel, even through the vacuum of space (1). (b) 150 000 000 ÷ 300 000 = 500 s ≈ 8.3 minutes (1); sunlight reaches Earth about eight minutes after leaving the sun, not years later (1).
- Q31 (4): Any four: Sodwana is warm, clear, low-nutrient tropical water brought by the Agulhas Current, so there are few plankton (1); it has little suspended sediment, so little light is scattered (1); at Simon's Town nutrient-rich water can feed dense plankton blooms (1); swell and storms stir up sediment, and runoff and kelp debris add particles (1); more suspended particles mean more scattering and absorption, so light is attenuated within a short distance (1); visibility on any day depends on the weather, so these are typical, not fixed, values.
- Q32 (4): Penetration — how deep light can travel into a medium (1). Attenuation — the loss of light intensity as distance through a medium increases (1), the combined effect of absorption and scattering (1). Penetration is a depth reached; attenuation is the loss along the way — the more the attenuation, the less the penetration (1).
- Q33 (4): Light leaving water speeds up and bends away from the normal (1), so objects in the water appear shallower and in a different place from where they are (1). Any two challenges: you cannot judge where a fish or the bottom really is from above; surface reflection and glare hide what is below; ripples bend the light unevenly, so the view wobbles; measurements or photos taken through the surface are distorted (2).
- Q34 (5): (a) reflection · (b) absorption · (c) scattering · (d) refraction · (e) attenuation.
Marking note. Q8 and Q16–Q17 are the reasoning this sheet is built around. CAPS's "different densities" fails the oil test, and its "60 % in the first metre, 80 % in the first ten" cannot both describe the same light — until the learner realises the first metre takes the long wavelengths. A learner who resolves both has applied the content to a setting, which is what the Depth column asks for. Q23 is not a trap: the CAPS statement is the exam answer, and the mark is for seeing why a real dive is gentler than the rule.