G1 Science is Science studied at General 1 subject level in Singapore’s secondary schools. This guide explains a foundation in light: we see a luminous source when light from it enters our eyes, and we usually see a nonluminous object when light reflected from it enters our eyes. Looking towards something is not enough if no suitable light reaches the eye.
The quick reasoning is to follow a complete light path. For a book under a lamp, the useful route is lamp to book to eye. For a book seen in a plane mirror, add the mirror between the book and eye. The arrows show light travelling towards the observer; they do not show the eye sending out a searching beam.
Once that route is clear, reflection diagrams become easier. A mirror changes the direction of light according to a rule measured from the normal, while its image appears behind the mirror because of the directions from which the reflected rays enter the eye. The apparent location is different from a place where reflected rays actually meet.
The examples below are original paper based models, not instructions for using bright lamps or lasers. Required depth depends on the actual school course. Some later chapters are useful extensions rather than claims about compulsory G1 assessment. For the wider subject context, start with the G1 Science learning guide.
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Read from the beginning, or choose the question closest to your child’s learning needs.
Trace the seeing route · Chapters 1–5
Use reflection geometry · Chapters 6–10
Explain mirror images · Chapters 11–15
Compare and repair · Chapters 16–20
Review and enquire · Chapters 21–25
Contents
Trace the seeing route · Chapters 1–5
Use reflection geometry · Chapters 6–10
Explain mirror images · Chapters 11–15
Compare and repair · Chapters 16–20
Review and enquire · Chapters 21–25
CHAPTER 1 OF 25
1. Begin with light entering the eye
The first question in a seeing problem is whether light from the relevant source or object can enter the eye. A diagram may show the observer facing an object, but orientation alone does not establish a light path. Illumination, obstruction and reflection can all affect what reaches the observer.
Imagine a closed dark room containing a book and a person. If there is no suitable visible light source and no light enters from outside, the book cannot be seen in the ordinary school model. The book has not disappeared; the information carried by reflected visible light is missing.
Now turn on a lamp. Some light reaches the book, and some is reflected towards the eye. The person can then receive light associated with the book’s surface. This explanation names a mechanism rather than saying the eye becomes strong enough to see through darkness.
Ask your child to draw arrows before writing a process name. The simple act of choosing arrow directions often reveals whether the learner thinks the eye sends out light. Correcting that idea first gives later reflection and image questions a much more stable starting point.
CHAPTER 2 OF 25
2. Separate luminous from nonluminous objects
A luminous object emits visible light. A nonluminous object does not emit its own visible light in the situation considered and is normally seen through reflected illumination. The classification describes the relevant light behaviour, rather than whether the object looks bright in a photograph.
A lamp can be luminous when switched on but not when switched off. A pale wall can look bright under strong illumination while remaining nonluminous. A bright appearance therefore does not automatically identify a source; ask whether the object is emitting the light or reflecting light from elsewhere.
The Moon is a familiar example of an object seen mainly by reflected sunlight. Its brightness in the night sky does not make it a visible light source in the same sense as the Sun. The example helps separate observed brightness from the origin of the light.
For a school question, use the conditions supplied. Do not label every screen luminous if it is stated to be off, or every hot object nonluminous if it is visibly glowing. The useful habit is to describe what the object is doing in the particular situation, not attach a permanent label without context.
CHAPTER 3 OF 25
3. Trace the source object eye route
For a nonluminous object, show light travelling from a source to the object and then from the object to the eye. A single arrow from lamp to eye explains seeing the lamp, but does not by itself explain seeing the book beside it. The object must be part of the relevant route.
An arrow from eye to book reverses the physical account. The observer may direct attention towards the book, but the ray diagram represents light travel, not attention. Keep those ideas separate so the arrows retain a clear and consistent scientific meaning.
Not every ray from the source reaches the object, and not every reflected ray enters the eye. A diagram selects a few representative paths that explain the observation. It is not a complete drawing of all light in the room, so the absence of other rays does not imply they do not exist.
When explaining visibility, write the sequence in ordinary words after drawing it. “Light from the lamp is reflected by the book into the eye” identifies source, interaction and destination in one sentence. Extra vocabulary is unnecessary if the central path is missing or reversed.
CHAPTER 4 OF 25
4. Read straight rays as a model
In a uniform transparent medium, geometrical optics models light as travelling along straight paths. A ray is a line indicating a direction of propagation. It is not a physical wire or a solid strip, and its drawn thickness does not describe the thickness of an individual beam.
The model is useful for shadows, reflection and many introductory image questions. It does not represent every wave effect or interaction of light. A student can use it effectively without claiming that a straight line drawing explains all optical phenomena at every scale.
Use a ruler for a straight ray in a formal diagram, and put an arrow on it. A line without direction can leave the path ambiguous. Label a source point or object point if the question needs one, so the reader knows what the selected ray represents.
The earlier guide to scientific models explains this broader principle. A good model simplifies while preserving the relationship being tested. Here the essential information is direction, interaction point and the route by which light reaches the eye.
CHAPTER 5 OF 25
5. Explain obstruction without changing the object
An opaque screen can prevent light from following a particular route. If it blocks illumination reaching a nonluminous object, the object may no longer reflect suitable light towards the eye. If it blocks the reflected path from object to eye, it can also prevent the observer seeing the object along that route.
These are different places for the obstruction. Read where the screen is located before writing the explanation. “The screen blocks light” is a start, but the answer becomes clearer when it identifies whether the blocked path is source to object or object to observer.
A screen need not stop every possible route in a real room. Other reflections may provide some illumination. Introductory questions often state or imply a simplified arrangement with one relevant source and path. Use that model while avoiding an unsupported claim that every obstruction creates complete darkness everywhere.
This is a useful changed example for learning. Keep the book and lamp fixed, then move an imagined opaque card to different positions. Ask which part of the complete path is interrupted. The task tests a physical route rather than the memorised statement that opaque materials block light.
CHAPTER 6 OF 25
6. Separate transmission, reflection and absorption
When light reaches a material, some may be transmitted, reflected or absorbed, depending on the material and conditions. These describe different outcomes. Transmission carries light through; reflection changes its direction back into the original side; absorption transfers energy to the material rather than continuing that ray through it.
A transparent pane can transmit much of the incident visible light while also reflecting some. A mirror is designed to give substantial reflection, but it does not necessarily reflect every bit of incident energy. Simple diagrams may emphasise one outcome without claiming the others are impossible.
For a seeing question, identify which outcome creates the route to the eye. Seeing through a clear pane primarily involves transmitted light from what lies beyond it. Seeing a mirror image involves reflected light. A surface can contribute both under different viewing and lighting conditions.
Do not define an opaque material as one that reflects all light. Opaque means it does not transmit the relevant light through in the stated model; some incident light can be absorbed. This distinction prevents a common error in explanations of dark surfaces, screens and shadows.
CHAPTER 7 OF 25
7. Draw the normal at the reflection point
The normal is an imaginary line perpendicular to the reflecting surface at the point where the incident ray meets it. For a plane mirror in a two dimensional diagram, draw it at 90 degrees to the mirror line. It is a reference for measuring angles, not another light ray.
The normal belongs at the point of incidence. Drawing a perpendicular somewhere else on the mirror can make the angle labels misleading. Begin by locating the ray’s contact point, then construct the normal through that point before deciding where the reflected ray should go.
If the mirror is tilted, the normal tilts with it. It is not always vertical or horizontal on the page. The reference is the surface itself, not the paper edge. This is why a student should construct it rather than assume the familiar orientation from the previous diagram.
Give the normal a different line style where the course convention uses one, and label it if needed. The visual distinction helps keep reference geometry separate from light travel. Only actual ray lines need propagation arrows; the normal’s job is to provide the angle baseline.
CHAPTER 8 OF 25
8. Measure incidence and reflection from the normal
The angle of incidence is measured between the incoming ray and the normal. The angle of reflection is measured between the outgoing reflected ray and the normal. Under the law of reflection, these angles are equal. They are not measured from the mirror surface itself.
If the incident ray makes 30 degrees with the normal, the reflected ray also makes 30 degrees with the normal on the appropriate side. If it makes 30 degrees with the mirror surface instead, its angle to the normal is 60 degrees. Read which reference the question supplies.
The incident ray, reflected ray and normal lie in the same plane in the standard reflection description. A school page represents that plane. The diagram should show an incoming arrow towards the mirror and an outgoing arrow away from it, not two rays both approaching the surface.
Before using a protractor, point to the two lines forming the required angle. This small check often prevents the main mistake. A neatly measured number can still be wrong if it is the complement measured from the surface rather than the angle defined by the question.
CHAPTER 9 OF 25
9. Handle an angle given to the surface
Suppose a ray meets a plane mirror at 25 degrees to the surface. The normal is perpendicular to the surface, so the incidence angle is 90 minus 25, giving 65 degrees. The reflection angle measured from the normal is therefore also 65 degrees.
The reflected ray makes 25 degrees with the mirror surface in that same simple arrangement. These two angle descriptions are compatible because they use different baselines. Do not present 25 and 65 as contradictory answers without checking which angle the task asks for.
Now change the incident ray to 70 degrees from the surface. Its angle of incidence is 20 degrees. This paired example checks whether the learner is actually converting from the surface or simply copying the originally supplied number into every angle label.
Use a quick sketch to show the right angle split. The calculation then has a geometric reason: the surface and normal form 90 degrees. A remembered subtraction without the sketch may work once, while a visible reference relationship is easier to recover in a tilted or unfamiliar diagram.
CHAPTER 10 OF 25
10. Explain smooth and rough reflection
A smooth surface can reflect parallel incident rays in a common orderly direction, producing regular reflection. A rough surface has different local surface orientations, so the reflected rays spread in different directions. This is diffuse reflection, which helps explain seeing many ordinary objects from several viewpoints.
The law of reflection still applies at each local surface point in the simple ray model. Roughness does not mean the light ignores the equal angle rule. The normals vary from place to place, so the outgoing directions vary even when the incoming directions are similar.
Do not call diffuse reflection the absence of reflection. A wall can reflect light towards several observers without forming a clear mirror image. The observer receives reflected light, but the surface does not preserve the same organised mapping that produces an ordinary sharp plane mirror image.
This distinction connects the everyday book example with mirror diagrams. Both involve light entering the eye after reflection. The surface behaviour changes the organisation of the reflected directions and therefore what kind of visual information is available, not whether reflection occurs at all.
CHAPTER 11 OF 25
11. Separate seeing a mirror from seeing its image
A mirror surface can be noticed because of its frame, marks, edges or reflected surroundings. Seeing a particular object in that mirror requires a reflected path from that object to the eye. The presence of a mirror in the diagram does not guarantee that every object appears in it from every position.
Think of the mirror as a directional part of the route. Light must reach an appropriate point on its reflecting surface and then leave towards the eye according to the reflection law. If that geometry is impossible within the actual mirror boundaries, the chosen object point will not be visible along that path.
This is why moving the eye can change what appears in a mirror without moving the object. The set of reflected paths reaching the eye changes. A diagram should account for the observer’s location, rather than treating the image as something every observer must see identically from anywhere.
The image’s geometric location for a fixed plane mirror and object can remain unchanged while visibility to a particular observer changes. Keep those questions separate: where is the image, and can this observer receive light from the relevant object point through the mirror?
CHAPTER 12 OF 25
12. Locate a plane mirror image geometrically
In the ideal plane mirror model, a point object’s image lies as far behind the mirror as the object lies in front, measured perpendicular to the mirror. The mirror plane is the reference. Do not use an arbitrary sloping distance from one corner of the object to a mirror edge.
For an object point 6 cm in front of the mirror, the image point is 6 cm behind it. The object and its image are then 12 cm apart along the perpendicular line through that point. The two distances answer different questions, so read whether the task asks image distance or object to image separation.
For an extended object, construct corresponding image points. The plane mirror image has the same size as the object in the ideal model. A picture appearing smaller because of viewing distance is a separate visual issue and should not replace the stated geometric image size property.
Some courses require image location only qualitatively, while others require ray construction. Use the depth taught for the actual course. The core relationship is accessible: equal perpendicular distances on opposite sides of the plane, with a clear distinction between real light paths and apparent image position.
CHAPTER 13 OF 25
13. Explain why the image is called virtual
Reflected rays reaching the eye diverge as though they came from a point behind the plane mirror. Extending those rays backwards locates the apparent image point. In the usual plane mirror arrangement, the reflected light does not actually travel behind the mirror to meet at that image point.
The image is called virtual because the actual rays do not converge there. Backward extensions are a geometric interpretation, not hidden light paths through the mirror. Use the line style taught for extensions so the diagram does not imply that all drawn lines represent actual propagation.
A virtual image can still be seen. Virtual does not mean imaginary in the everyday sense of a made up perception with no physical basis. The reflected rays entering the eye carry real optical information; the apparent origin differs from a point where those rays physically meet.
Do not draw arrows carrying reflected rays through the mirror towards the image. That would confuse the construction with the actual route. The useful explanation follows the reflected rays into the eye, then separately explains how their backward directions establish the apparent location.
CHAPTER 14 OF 25
14. Trace rays from an object point to an eye
For a point seen in a plane mirror, locate its virtual image behind the mirror. A line from the eye towards that image can help find the reflection point on the mirror. The actual light route then goes from the object point to that point on the mirror and from there to the eye.
Check the equal angles about the normal at the reflection point. The geometric construction and reflection rule should agree. If they do not, inspect the image location, the mirror plane and the chosen intersection rather than adding a second arbitrary bend to make the ray reach the observer.
The mirror must physically include the reflection point. A construction crossing an extended mirror line beyond the actual mirror does not prove visibility through the existing mirror. The finite size of the reflecting surface is part of the problem, not merely decoration around an infinite mathematical line.
Where this construction is outside the current course, retain the simpler complete route. The object supplies reflected illumination to the mirror, which redirects some into the eye. The extension explains exactly which mirror point is needed when a question asks about viewing position or mirror size.
CHAPTER 15 OF 25
15. Predict changes when the object moves
If a point object moves 2 cm farther from a fixed plane mirror along the perpendicular direction, its image also moves 2 cm farther behind the mirror. The object to image separation increases by 4 cm. This follows from the equal distance construction, not from light travelling through the mirror.
Suppose the object begins 5 cm in front and moves to 8 cm in front. The image changes from 5 cm behind to 8 cm behind. The object image separation changes from 10 cm to 16 cm. Label each distance to avoid treating the image movement and total separation change as the same quantity.
If the object moves parallel to the mirror, the corresponding image shifts parallel as well in the ideal model. Its perpendicular distance can remain unchanged. The direction of motion therefore matters, and a statement that the image always moves closer or farther would be incomplete.
These are optional geometric extensions where appropriate. Their main learning value is disciplined reference choice. Keep the mirror fixed in the description, identify perpendicular versus parallel motion and state which distance changes. The calculation becomes simple once the quantities have distinct names.
CHAPTER 16 OF 25
16. Predict changes when the observer moves
For a fixed object and plane mirror, moving the observer does not by itself move the image’s geometric location. It changes which rays can enter that observer’s eye and what region of the reflected scene is visible. Image location and individual visibility are different questions.
Imagine a small mirror and an object near one side. A person may need to move to receive a reflected ray from it. Another observer elsewhere may see a different object through the same mirror. The mirror does not create a separate physical object behind itself for each person.
A useful diagram includes the eye position explicitly. If a question asks whether an observer can see an object, draw or trace the relevant path rather than relying only on the image location. The finite mirror and any obstructions can prevent the required ray from reaching the eye.
This distinction also prevents an everyday shortcut from becoming a false rule. Looking at the place where an image is expected does not guarantee the eye receives light from it. The complete optical route still has to exist under the stated geometry and illumination.
CHAPTER 17 OF 25
17. Read shadows as missing illumination
A shadow forms where an opaque object prevents light from a source reaching a region. It is not a dark substance emitted by the object. The light path model explains the boundary by identifying which source paths are blocked and which remain available.
For a small source and simple geometry, a sharp shadow can be represented using straight edge rays. A larger extended source can produce regions receiving light from only part of the source, giving a partial shadow. Use the course’s required depth rather than assuming every shadow must have the same boundary quality.
An object in a shadow may still be partly visible if other illumination reaches it and reflects into the eye. Shadow does not always mean absolute darkness. The simplified question may remove those other routes, but a real setting can include scattered and reflected light from surroundings.
This chapter connects seeing and obstruction without replacing either with a new rule. Ask where light is prevented from reaching, then ask whether any suitable light from that region reaches the observer. The two questions explain both the illumination pattern and the observation.
CHAPTER 18 OF 25
18. Keep colour questions tied to illumination
A surface’s observed colour depends on the illuminating light, its reflection and absorption properties, and the observer’s visual system. In simple school models, a coloured surface reflects some visible wavelengths more strongly than others. It does not automatically produce that colour as its own emitted light.
A surface that looks red under white illumination may look different when the available illumination changes. The exact answer depends on the stated idealised reflection properties and light source. Do not assume the familiar daylight colour must remain unchanged under every coloured lamp.
For an introductory question, track which light is available, which is reflected and which can enter the eye. This uses the same source surface eye route as the opening example. The colour detail adds a selection of wavelengths without reversing the direction of light travel.
Keep this as an extension if the actual G1 topic does not require colour analysis. Its purpose is to show why visibility and appearance depend on conditions. A useful answer should not invent exact spectral behaviour from a simple colour word when the question supplies no such model.
CHAPTER 19 OF 25
19. Distinguish reflection from refraction
Reflection redirects light back into the original side of a boundary. Refraction concerns a change in direction associated with light passing between media under the relevant conditions. A ray entering a transparent material is not automatically following the reflection rule used for a mirror.
Both topics use a normal as an angle reference, which can make their diagrams look similar. Read whether the ray returns from the surface or crosses into another medium. The interaction and material conditions determine which relationship applies, not the mere presence of a perpendicular reference line.
For a mirror problem, do not add a refracted ray behind the mirror unless the question actually describes transmission through a relevant boundary. For a transparent block problem, do not force every outgoing ray to have equal incidence and reflection angles when it is the transmitted path being considered.
The current guide owns the seeing and reflection route. Refraction has its own additional relationships and conditions. Knowing where one explanation ends helps students use a familiar model correctly without stretching it to answer every diagram involving light and a surface.
CHAPTER 20 OF 25
20. Repair four common explanations
“The eye sends light to the object” needs the direction corrected. For a nonluminous object under illumination, light travels from the source to the object and then into the eye after reflection. Attention travels metaphorically towards a page; the ray arrows describe a different physical process.
“A rough wall does not reflect light” confuses lack of a clear image with absence of reflection. Diffuse reflection can send light towards many viewing directions. The reflection law still applies locally, with differing surface orientations producing different outgoing directions.
“The incidence angle is measured from the mirror” needs the reference changed to the normal. If an angle is given to the surface, use the perpendicular relationship to recover the defined angle. Label the normal before measuring rather than treating every printed angle as incidence.
“The image is behind the mirror because light meets there” confuses backward extensions with actual reflected rays. For the ordinary plane mirror image, the rays enter the eye as though from behind the mirror but do not physically converge there. The virtual image construction records an apparent origin.
CHAPTER 21 OF 25
21. Work through one complete mirror example
A point object lies 4 cm in front of a fixed plane mirror. A ray reaches the mirror at 35 degrees to the normal and is reflected towards an observer. Find the reflection angle, locate the image and state the object to image separation in the ideal model.
The reflection angle is 35 degrees from the normal. The image is 4 cm behind the mirror along the perpendicular from the object point. Object to image separation is 8 cm. Each number answers a different question, so keep angle and distance labels visible.
Now suppose the given 35 degrees had instead been measured from the mirror surface. The incidence and reflection angles would each be 55 degrees from the normal. The image distances would remain governed by the object’s location, not by replacing the reflection angle in the equal distance rule.
Finally, ask whether the observer can necessarily see the point through any small mirror at that location. No: the required reflection point must lie on the actual mirror and the path must be unobstructed. Image location alone does not establish visibility from every eye position.
CHAPTER 22 OF 25
22. Build a lesson around the missing route
If a child draws arrows from the eye, begin with seeing a lamp and then a nonluminous book. If angles fail, construct the normal before using a protractor. If images fail, separate actual rays from backward extensions. These difficulties need different starting points rather than one longer set of copied diagrams.
A focused lesson can keep one object and mirror while changing the observer position or the angle wording. Ask the learner to explain what remains unchanged. A small variation reveals whether the child understands the model or has memorised the appearance of a single diagram.
Small group teaching can help when each student draws and explains a route before receiving specific feedback. The useful mechanism is visible reasoning followed by a correction and an independent check. Class size alone cannot show whether the normal, arrows and image location have been understood.
Parents can look for concrete progress: the child names source, object and eye in order, measures angles from the normal and distinguishes image distance from object image separation. These are observable skills, making a short conversation about a recent question more useful than a general claim that light has been revised.
CHAPTER 23 OF 25
23. Try three independent checks
First, a book is illuminated by a lamp and seen directly. Describe the route. Light travels from the lamp to the book and is reflected into the eye. A line directly from lamp to eye describes seeing the lamp, not the complete reason the book is visible.
Second, a ray makes 20 degrees with a plane mirror surface. Its angle to the normal is 70 degrees, so the reflection angle is 70 degrees from the normal. The reference line determines the number. A neat 20 degree answer is wrong if the requested angle is reflection as conventionally defined.
Third, an object point is 7 cm in front of a plane mirror. Its image is 7 cm behind, giving a 14 cm object image separation along the perpendicular. The image is virtual in the ordinary model because actual reflected rays do not meet at that apparent location.
Change one condition after each answer. Move an obstruction, tilt the mirror or move the object farther away. Ask for an explanation before a new number. A method becomes independent when the learner updates it correctly for a changed situation rather than repeating the original wording.
CHAPTER 24 OF 25
24. Answer the questions readers often ask
Can a nonluminous object look bright? Yes. Strong reflected illumination can make it appear bright without turning it into a visible light source. The relevant question is whether it emits the light in the situation or reflects light supplied by another source.
Does a virtual image mean the observer sees nothing real? No. Real reflected light reaches the eye. Virtual describes the apparent image location relative to actual ray convergence, not the absence of an optical observation. Use the technical definition rather than the everyday implication of imaginary.
Does the observer moving always move a plane mirror image? For a fixed mirror and object, its geometric image location remains fixed in the ideal model. The observer’s available reflected paths and visible scene can change. Keep image position and visibility as separate questions.
Must every G1 student learn all the extensions in this guide? No. The actual school syllabus sets the required depth. Begin with seeing routes and the reflection law where taught, then add image constructions, colour or refraction distinctions only as relevant. A broad guide should support learning without inventing uniform requirements.
CHAPTER 25 OF 25
25. Choose the next useful learning step
If seeing explanations are weak, trace source to object to eye. If diagrams are weak, add arrows and construct a normal at the correct point. If angle calculations are weak, identify the baseline before subtracting. If images are confusing, distinguish actual rays, backward extensions and apparent location.
For the wider reasoning framework, visit How Science Works. The OpenStax reflection chapter provides further background. Read it selectively alongside current school materials, because its full discussion extends beyond the requirements of some introductory courses.
Keep practice varied but small enough to inspect: one illuminated object, one mirror angle and one image position. Ask the child to explain the path before calculating. A correct complete route often does more to reveal understanding than another page of angle numbers with no light direction shown.
For individual support, bring a recent light question and the student’s own diagram to an eduKate consultation. A useful review identifies whether the difficulty lies in observation, geometry or model meaning, then checks the repaired reasoning on a fresh example.
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