Refraction, Color & Transparent Materials — Print · Grade 6 Science

Refraction, Color & Transparent Materials

Learning goals

I can explain why light changes direction when it enters water or glass.
I can explain why a submerged object appears shallower than it is.
I can explain why a red object looks black under green light.

Readings for this unit

A Reading Lamp Is More Than a Bright Object

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Light travels at about 300,000 km/s in a vacuum and very nearly that in air. In water it slows to roughly 225,000 km/s, and in ordinary glass to about 200,000 km/s. In diamond it is slower still, around 124,000 km/s, which is why diamonds bend light so dramatically and sparkle as they do.

When light meets the boundary between two materials at an angle, the change of speed makes it change direction, which is called refraction. Entering a slower material it bends towards the normal, the perpendicular line at the surface; leaving into a faster one it bends away.

The amount of bending depends slightly on colour, because the colours that make up white light travel at marginally different speeds in glass. Violet slows most and bends most, red slows least and bends least, and white light entering a prism therefore spreads into a spectrum.

Build your understanding

Where this fits

The last lesson relied on light travelling in straight lines. This one deals with the exception: at a boundary between two materials the path bends, and everything from a swimming pool looking shallow to the existence of rainbows follows. It closes the chapter and the course's treatment of light.

Use three flexible 40-45-minute sessions: refraction at a boundary; apparent depth and the everyday illusions it causes; dispersion, colour and why objects look coloured, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Why a change of speed bends the path

Picture a line of people walking arm in arm across a field and stepping onto soft sand at an angle. The ones who reach the sand first are slowed while the others are still on firm ground, so the whole line pivots and ends up travelling in a new direction. Light does the same at a boundary. A beam entering water at an angle meets the surface on one side first, that part slows to about 225,000 km/s while the rest is still at 300,000 km/s, and the beam swings towards the normal. Entering glass at about 200,000 km/s the effect is stronger still.

Two details decide what you will observe. First, the bending happens only at the boundary: once inside the water the beam travels perfectly straight again at the new speed, so a ray diagram shows a kink at the surface and nothing more. Second, a beam arriving exactly along the normal, straight down onto the surface, does not bend at all, because every part of it slows at the same instant and there is nothing to pivot about. That is why you can see the bottom of a pool undistorted when looking straight down and why everything looks displaced when you look in at an angle. The strength of the bending is summarised by a number called the refractive index, which is simply how many times slower light travels in the material than in a vacuum. Water is about 1.33, ordinary glass about 1.5 and diamond about 2.42, and the larger the number the more sharply light is bent. Those three figures predict everything in this lesson: water bends gently, glass more, and diamond so strongly that light entering a cut stone bounces around inside before escaping.

The dashed line is what makes the bend visible as a change rather than just a drawing. The same effect explains the broken-looking straw and, because colours bend by different amounts, a rainbow.

Pause and explain

Why does a beam entering water straight down not bend, while one entering at an angle does?

Check your reasoning after answering

Because bending comes from one part of the beam slowing before another. Arriving along the normal, every part of the beam crosses the boundary at the same moment and slows together, so the direction is unchanged. At an angle, one edge reaches the water first and slows to about 225,000 km/s while the rest is still at 300,000, and the beam pivots.

Part 2 · Not where it appears to be

Put a straw in a glass of water and it looks broken at the surface. Nothing is bent except the light: rays leaving the submerged part refract as they pass out into the air, and your brain, which assumes light always travels in straight lines, traces them straight back and places that part of the straw somewhere it is not. The same reasoning explains why a swimming pool looks shallower than it is, typically about three quarters of its true depth, so a pool 2.0 m deep appears roughly 1.5 m deep from the side.

This has consequences beyond party tricks. People misjudge the depth of water they are about to step into, which is a genuine hazard at the edge of a pool or a river. Anyone spearing fish has to aim below where the fish appears to be, a correction that traditional fishing communities worked out long before anyone could explain it. And a coin placed at the bottom of an opaque mug, just out of sight over the rim, becomes visible when water is poured in, because the light from it now bends at the surface and reaches your eye over an edge it could not previously clear. Lenses are the same effect used deliberately. A lens is a piece of glass with curved surfaces shaped so that rays arriving across its whole width are bent by different amounts and brought together at one point, and that is the entire working principle of spectacles, a magnifying glass, a camera and the lens in your own eye. Every one of them depends on light slowing from 300,000 km/s to about 200,000 km/s on entering the glass.

Pause and explain

Why must someone spearing a fish aim below where the fish appears to be?

Check your reasoning after answering

Because light from the fish bends as it leaves the water, and the brain traces it back in a straight line. That places the image higher and nearer the surface than the fish actually is. Aiming at the apparent position sends the spear above the real one, so the aim has to be corrected downward.

Part 3 · Splitting white light apart

White light is not a single thing but a mixture of all the visible colours, and a prism separates them because they do not all slow by the same amount in glass. Violet is slowed most and therefore bends most, red is slowed least and bends least, and the others fall in between, so a narrow beam of white light entering a triangular prism leaves as a spread band running red, orange, yellow, green, blue and violet. The effect is called dispersion, and Isaac Newton settled the matter in 1666 by using a second prism to recombine the spectrum back into white light, proving the prism was separating colours already present rather than adding them.

A rainbow is the same process on a far larger scale, with raindrops as the prisms. Sunlight enters each drop, refracts, reflects once off the back surface and refracts again on the way out, emerging spread into colours at an angle of about 42 degrees from the direction of the incoming sunlight. That fixed angle is why a rainbow is always a circular arc, why it always appears opposite the Sun, and why it is low in the sky when the Sun is high. It also means no two people ever see exactly the same rainbow, since the drops sending light to your eye are not the ones sending light to anyone standing elsewhere.

Pause and explain

How did Newton show that a prism separates colours rather than adding them?

Check your reasoning after answering

By using a second prism to recombine the spectrum into white light again. If the glass were adding colour, passing the light through more glass should have added more, not removed it. Recovering the original white light shows the colours were present in it all along and the prism only spreads them apart by bending each one differently.

Part 4 · Why objects have colour

An object has no colour of its own in the dark. What it has is a particular pattern of absorbing and reflecting: a red tomato absorbs most of the blue, green and yellow light falling on it and reflects the red, which is the part that reaches your eye. A white page reflects all the colours roughly equally, a black surface absorbs nearly all of them, which is also why it warms up fastest in sunlight, as chapter 3 explained with radiation.

The test that proves this is striking and easy to do with coloured filters or a coloured lamp. Put a red tomato under pure green light and it looks black, because there is no red light present for it to reflect and it absorbs the green. A white page under the same green light looks green, since it reflects whatever arrives. This is why clothes bought under shop lighting sometimes look wrong outdoors, and why sodium street lamps, which give out almost pure yellow light, make a car park look like a black and white photograph. It is worth separating two things that are easily muddled. Mixing coloured light is not the same as mixing paint: red, green and blue light together make white, while red, green and blue paint together make a muddy brown close to black. Light adds, because each source contributes more light to the eye, while paint subtracts, since each pigment removes a further part of the spectrum from whatever is reflected. A screen uses the first, a painting uses the second.

Pause and explain

A red tomato looks black under green light. Explain why, and say what a white page would look like.

Check your reasoning after answering

Because the tomato reflects only red light and absorbs the rest, and under green light there is no red present to reflect, so almost nothing reaches the eye and it appears black. A white page reflects all colours roughly equally, so under green light it reflects green and looks green rather than white.

Apply the whole unit · Explain the illusion

A coin sits at the bottom of an opaque mug, hidden just below the rim when viewed from a fixed position. Water is poured in and the coin becomes visible without anything moving. A ruler is then stood in the mug and appears bent at the water line.

Your task

Explain both observations, and state the assumption the brain makes that produces them.

What your response should show

Explains the coin by refraction at the surface rather than by the water moving anything
Locates the ruler's apparent kink at the boundary and says why
Names the straight-line assumption the brain makes
Describes correctly what would happen if the speed did not change

Response guidance — after your attempt

Explain why the coin becomes visible once water is added.
Explain why the ruler appears bent at the surface and not anywhere else.
State the assumption the eye makes and say what would happen if light did not change speed in water.

Light leaving the coin travels up through the water at about 225,000 km/s and refracts as it crosses into air, bending away from the normal, which lets it clear the rim and reach an eye it could not previously reach. Nothing moved; the path changed. The ruler appears bent at the water line because refraction occurs only at the boundary: the submerged part is seen through rays that have been bent, while the part above the water is seen directly, so the two sections appear offset from one another and the kink falls exactly at the surface. The assumption behind both is that light always travels in straight lines, so the brain traces the rays entering the eye straight back and places the object wherever those lines cross. If light did not change speed on entering water, there would be no refraction, the coin would stay hidden, the ruler would look perfectly straight and a pool would appear exactly as deep as it is.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Light slows down when it enters glass or water, and because it slows it changes direction at the boundary. That single fact explains the broken-looking straw, why a pool is deeper than it looks, why a prism makes a spectrum and why a rainbow exists.

Worked examples

Example 1 · Compare a ray and a color model

Explain each outcome and one model limitation.

Compare the ray angles using the same reference.
Identify which color components reach the card.
Separate simplified components from all real materials.

Answer: The ray bends toward the normal because 22° is less than 35° in this air-to-water model. The card appears dark because the red-only source supplies no blue for it to reflect. These diagrams omit detailed wavelength response, intensity, and actual material variation; they are not universal measurements of every card or liquid.

Example 2 · Transfer to a different case

A learner says every transparent boundary must bend every ray. Give a counterexample.

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: A ray traveling directly along the normal can change speed without changing direction. Refraction’s speed change and oblique bending must be distinguished.

Example 3 · Distinguish apparent position from actual shape · a focused example

What evidence would challenge a claim that the spoon broke?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: An intact spoon outside the water and the modeled change in light paths support apparent displacement instead.

Avoid these mistakes

Misleading claim: Every ray must bend sideways at every transparent boundary.
Misleading claim: An object appears the same color under every possible light source.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Does a ray at 0° from the normal have to bend sideways?

Discuss the answer

No. Its speed can change while its direction remains along the normal.

What evidence would challenge a claim that the spoon broke?

Discuss the answer

An intact spoon outside the water and the modeled change in light paths support apparent displacement instead.

Does a prism make colors from new matter?

Discuss the answer

No. It separates components of the incoming light through wavelength-dependent propagation.

Why is source color part of the explanation?

Discuss the answer

The surface cannot reflect an absent green component in this idealized model; observed color depends on both source and material.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Refraction, Color & Transparent Materials · Model and explain

Assigned reading and preparation

Read A Reading Lamp Is More Than a Bright Object and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Refraction, Color & Transparent Materials, study Parts 3–4 and attempt the worked case: Explain each outcome and one model limitation. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

The ray bends toward the normal because 22° is less than 35° in this air-to-water model. The card appears dark because the red-only source supplies no blue for it to reflect. These diagrams omit detailed wavelength response, intensity, and actual material variation; they are not universal measurements of every card or liquid.

Activity 2 · Refraction, Color & Transparent Materials · Practice, revise and exit

Assigned reading and preparation

Read A Reading Lamp Is More Than a Bright Object and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Refraction, Color & Transparent Materials, complete practice questions 1–6 and both choice checks, then answer independently: A learner says every transparent boundary must bend every ray. Give a counterexample. Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

A ray traveling directly along the normal can change speed without changing direction. Refraction’s speed change and oblique bending must be distinguished.

Words to know

refraction: A change in light propagation speed at a medium boundary, commonly with oblique bending.
spectrum: A range of light components distinguished by wavelength.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

A learner says every transparent boundary must bend every ray. Give a counterexample.

Check your explanation after answering

A ray traveling directly along the normal can change speed without changing direction. Refraction’s speed change and oblique bending must be distinguished.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.