Light, Shadows & Reflection · Grade 6 Science
Chapter: Electricity, Magnetism & LightLight, Shadows & Reflection
Learning goals
- I can use straight-line travel to explain the size and sharpness of a shadow.
- I can apply the law of reflection to predict where a reflected ray goes.
- I can explain why a mirror shows an image and a matt wall does not.
Readings for this unit
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, which is 300,000,000 m/s, and nothing in the universe travels faster. Sunlight takes 8 minutes 20 seconds to cross the 150 million km to Earth, so the Sun you see has already been that bright for eight minutes before the light arrives.
Over the distances in a classroom that speed makes light seem instantaneous, and what matters instead is the direction. Light travels in straight lines until something changes its path, which is why you cannot see round a corner and why a shadow has the shape of the object casting it.
You see almost nothing directly. A lamp and a flame emit their own light; everything else in this room is visible only because light has bounced off it and reached your eye. This lesson is about what happens at that bounce.
Build your understanding
Where this fits
This lesson and the next complete the chapter. The straight-line rule here is what makes shadows and mirrors predictable, and it is also what the next lesson breaks on purpose when light crosses from air into glass or water. The shadow geometry in the second section is used again in chapter 8 to explain eclipses.
Use three flexible 40-45-minute sessions: straight-line travel and how we see; shadows and why some are sharp; the law of reflection, mirrors and matt surfaces, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · Straight lines, and how seeing works
The evidence that light travels in straight lines is everywhere once you look for it. Sunbeams through a gap in curtains are straight, a torch beam in dusty air is straight, and a laser pointer makes a straight line of scattered light. Line up three cards with a small hole in each and you can only see a lamp through all three when the holes are exactly in line. A ray drawn on paper is a model of this, a line with an arrow showing the direction light travels, and like every model from chapter 1 it is a simplification: real light spreads out from its source in all directions and a ray picks out one path through that spreading.
How we see follows directly, and it is worth stating because the ancient Greeks got it backwards for centuries, believing the eye sent something out. Light leaves a source such as the Sun or a lamp, travels in straight lines, strikes an object, bounces off it in many directions, and a small fraction of that scattered light enters your eye. The test that settles it is darkness: in a genuinely dark room you see nothing at all, no matter how good your eyes are or how long you wait, because there is no light to bounce. Your eye is a receiver and not a projector. The straight-line rule has one limit worth naming now, since the next lesson is built on it. Light goes straight only while it stays in the same material. Crossing from air into water or glass it changes speed and bends at the boundary, which is why a straw in a glass of water looks broken at the surface. Within any one material it is straight, and that is enough for everything in this lesson.
Pause and explain
What observation shows that the eye receives light rather than sending something out?
Check your reasoning after answering
That you see nothing in a completely dark room however long you wait. If the eye emitted something that made seeing possible, darkness would be no obstacle, since the emitted beam would still reach the object. Seeing stops the moment there is no light to bounce off objects and enter the eye, which is what a receiver would do.
Part 2 · Shadows and what makes them sharp
A shadow is the region a straight-travelling light cannot reach because an opaque object is in the way, and its size depends on geometry you can calculate. Move a torch closer to an object and the shadow grows, because the light spreading from a nearby source diverges more steeply past the edges. Hold an object 10 cm from a wall with a torch 50 cm away and the shadow is noticeably larger than the object; slide the object against the wall and the shadow shrinks to almost the object's own size.
Sharpness depends on the size of the source rather than its brightness. A very small source, close to a point, casts a crisp shadow with a sharp edge, because from any position behind the object the source is either fully visible or fully blocked. A large source such as a window or a fluorescent tube casts a shadow with a dark middle and a fuzzy grey border, because at the border the source is only partly blocked; those two regions have names, the umbra and the penumbra. This is why a shadow in direct sunlight on a clear day has a reasonably sharp edge while the same object on an overcast day casts almost no shadow at all, since the whole sky has become the source. The same geometry produces eclipses on an enormous scale, which chapter 8 returns to. The Moon casts a shadow whose umbra is only about 100 km wide where it touches Earth, which is why a total solar eclipse is visible from such a narrow strip while a far larger surrounding region sees a partial one from within the penumbra. The Sun is a large source rather than a point, and that single fact makes the difference between the two.
Pause and explain
Why does an object cast a sharp shadow in direct sunlight but almost none on an overcast day?
Check your reasoning after answering
Because the size of the source decides the sharpness. The Sun is small in the sky, so from most points behind an object it is either fully visible or fully blocked, giving a crisp edge. On an overcast day the whole sky scatters light and acts as an enormous source, so light reaches almost everywhere behind the object and the shadow nearly disappears.
Part 3 · The law of reflection
When light strikes a surface it bounces off, and the rule is exact: the angle of incidence equals the angle of reflection, with both angles measured from a line drawn perpendicular to the surface, called the normal. Send a ray in at 30 degrees to the normal and it leaves at 30 degrees on the other side. This holds for every surface, whether polished or rough, which is the part that surprises people, because a matt wall does not look like it is reflecting anything.
The difference between a mirror and a wall is the surface, not the law. A mirror is flat and smooth to within a fraction of the wavelength of light, so parallel rays arriving together leave together, still parallel, and arrive at your eye carrying the pattern they started with: that is specular reflection and it produces an image. A painted wall is rough at a microscopic scale, so each tiny patch faces a slightly different way and obeys the law individually, scattering rays in all directions. That is diffuse reflection, and it is precisely why you can see a wall from anywhere in the room while a mirror shows you a reflection only from certain positions. The distinction has practical consequences. Cinema and projector screens are deliberately matt so that every seat receives scattered light, whereas a glossy screen would show a bright reflection of the lamp to a few seats and almost nothing to the rest. Road signs do something different again, using tiny glass beads that send light back towards wherever it came from, so a sign is bright to a driver whose own headlights are lighting it and dim to everyone else.
Pause and explain
Both a mirror and a matt wall obey the law of reflection. Why does only one show an image?
Check your reasoning after answering
Because of surface roughness rather than the law. A mirror is smooth enough that parallel rays leave parallel and keep the pattern they arrived with, producing an image. A wall is microscopically rough, so each patch faces a slightly different direction and scatters rays everywhere; the law still holds at each patch, but the pattern is destroyed, which is also why the wall is visible from any angle.
Part 4 · The image in a flat mirror
Stand 60 cm in front of a flat mirror and your image appears to be 60 cm behind it, making the apparent distance between you and your image 120 cm. The image is the same size as you, the same way up, and it is virtual, meaning the light does not actually come from behind the mirror and no image would form on a screen placed there. Your brain assumes light has travelled in a straight line, traces the rays back behind the glass and places the image where those extended lines meet.
The image is also reversed in a particular way that is usually described carelessly. A mirror does not swap left and right; it reverses front and back, along the direction you are facing. Your image's nose points towards you while yours points away, and because you interpret that as a person turned round to face you, the swap looks like left and right. The proof is that writing held up to a mirror reads backwards horizontally but is not turned upside down, which is what a true left-right swap would have to do to text rotated on its side. A flat mirror is also the only optical device in this course that produces an image the same size as the object. A curved mirror changes the size: the inside of a spoon makes a magnified upside-down image of your face at arm's length, while the outside makes a small upright one with a much wider field of view. That is why shop security mirrors and the passenger wing mirror of a car bulge outwards, and why that mirror carries a warning that objects are closer than they appear.
Pause and explain
Why would a screen held behind a mirror show nothing at all?
Check your reasoning after answering
Because no light ever reaches that position. Every ray bounces off the front surface and travels back into the room, and none passes through to the far side. The eye assumes light has arrived in a straight line and traces those rays backwards, placing what it sees where the extended lines cross, which is a location the light never actually occupied.
Apply the whole unit · Work out the shadow
A small bright lamp is 90 cm from a wall. A card 8.0 cm tall is held upright between them, 30 cm from the lamp. The lamp is then replaced by a large frosted panel of the same brightness.
Your task
Predict the shadow's size and appearance in both cases, and explain the difference.
What your response should show
- Uses the ratio of distances to calculate a shadow height of 24 cm
- Describes umbra and penumbra when the source becomes large
- Separates what controls size from what controls sharpness
- Relies on straight-line travel rather than asserting the result
Response guidance — after your attempt
Use straight-line travel to work out the height of the shadow cast by the small lamp.
Describe how the shadow's edge changes when the large panel replaces the lamp.
Explain which property of the source controls size and which controls sharpness.
With the small lamp, rays travel in straight lines from a near-point source past the top and bottom edges of the card and continue to the wall. The card is 30 cm from the lamp and the wall is 90 cm away, three times as far, so the shadow is three times the card's height: 24 cm. With the large frosted panel, the shadow's centre stays about the same size but the edges become a broad fuzzy grey band, because points near the edge of the shadow can see part of the panel and not the rest, giving a penumbra between the fully dark umbra and the fully lit wall. If the panel is wide enough the umbra may disappear altogether. So the distances control the size of the shadow, through simple straight-line geometry, while the size of the source controls the sharpness of its edge, and the two are independent of each other.
Scientific references for this unit
- NASA: the speed of light and how long sunlight takes to reach Earth
- National Institute of Standards and Technology: the definition of the metre and the speed of light
- Institute of Physics: light, shadows and reflection in the classroom
- NASA: umbra and penumbra in solar and lunar eclipses
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 travels in straight lines at about 300,000 km/s, and almost everything you see is light that left a source, bounced off an object and entered your eye. Shadows, mirrors and the simple fact that you can see a wall from any angle all follow from those two statements.
Worked examples
Example 1 · Check the mirror angle
- Identify the perpendicular normal.
- Use equality of angles measured from the normal.
- Convert to the complementary surface angle.
Example 2 · Transfer to a different case
- 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.
Example 3 · Explain shadow geometry · a focused example
- 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.
Avoid these mistakes
- Misleading claim: Reflection angles are defined from the surface rather than the normal.
- Misleading claim: A visible object must generate its own light.
- 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 seeing a book require the eye to emit a ray?
Discuss the answer
No. Light from the source reaches the page and some reflected light enters the eye.
Why can shadow size change without card growth?
Discuss the answer
The geometry changes which diverging light paths are blocked before reaching the screen.
Which angle should be compared with 30°?
Discuss the answer
The reflected angle measured from the same normal, not the angle from the mirror surface.
Why can a rough card be visible without a mirror image?
Discuss the answer
Its surface scatters reflected light toward many directions rather than preserving one sharp organized image.
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 · Light, Shadows & Reflection · Model and explain
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 Light, Shadows & Reflection, study Parts 3–4 and attempt the worked case: Find the correct normal and surface angles, and explain the drawing error. 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 correct reflected angle is 40° from the normal and 50° from the surface. Drawing 40° from the surface makes it 50° from the normal, so the mismatch comes from the reference line. These are ideal ray-model angles, not a record of an actual mirror trial.
Activity 2 · Light, Shadows & Reflection · Practice, revise and exit
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 Light, Shadows & Reflection, complete practice questions 1–6 and both choice checks, then answer independently: A larger shadow is observed after moving a card. Must the card itself have become larger? 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
No. Source–object–screen distances and source size affect shadow geometry. Check those conditions before claiming a change in the object.
Words to know
- normal
- A line perpendicular to a surface at the point being considered.
- reflection
- Light being redirected from a surface.
- 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 larger shadow is observed after moving a card. Must the card itself have become larger?
Check your explanation after answering
No. Source–object–screen distances and source size affect shadow geometry. Check those conditions before claiming a change in the object.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.