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Primary 3 General Studies | Light and Shadows 備課指引（導師用）｜straight-line light, shadow length, mirror angles
Most pupils draw the shadow directly under the object, because they think the shadow falls out of it. This lesson explains every case with one cause: light travels in straight lines.
Objectives — By the end the pupil can: (1) explain shadows, eclipses and a pinhole image with one idea, straight-line light; (2) state that a lower light source gives a longer shadow, using a 1.5 m object with a 2 m shadow; (3) state that the angle of reflection equals the angle of incidence, 40 degrees in and 40 degrees out, which is 80 degrees between the two rays.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Light sources | Is the Moon a light source | No, it reflects sunlight |
| Transparent or not | Which lets light through, glass or wood | Glass lets light through |
| Position and direction | If the Sun is in the east, where is the shadow | To the west |
02 Lesson flow (40 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-5 min | Diagnose | Ask pupils to draw a shadow in sunshine | Check the side of the shadow |
| 5-13 min | Straight lines | Shine a torch through a hole in card | Say light travels straight |
| 13-23 min | Shadow length | Lower the torch and watch the shadow grow | Measure and record |
| 23-32 min | Mirrors | Measure the angle in and the angle out | Fill a table, two angles equal |
| 32-40 min | Exit check | Mark three items live | Three out of three |
03 Teaching script (read it out)
Scene 1: Teach straight-line light first, the shadow is a by-product
Pupils assume the shadow belongs to the object, so they draw it at the feet. Do not start with shadows.
Start with light: it travels in straight lines, an opaque object blocks it, and the space behind the object receives no light. That dark space is the shadow, so its outline copies the object and it always sits on the far side from the source.
Prove it with one move: shine a torch through a small hole in card and a small dot appears on the wall, because only light travelling along the straight line through the hole gets there.
Start with light: it travels in straight lines, an opaque object blocks it, and the space behind the object receives no light. That dark space is the shadow, so its outline copies the object and it always sits on the far side from the source.
Prove it with one move: shine a torch through a small hole in card and a small dot appears on the wall, because only light travelling along the straight line through the hole gets there.
Ask the pupil: why does the hole make one dot on the wall instead of a wide patch of light?
Scene 2: A lower light source makes a longer shadow
Pupils think shadow length depends on the size of the object and ignore the angle of the light.
Keep the object still and lower the torch from above: the tip of the shadow moves further away.
Close with numbers: a 1.5 m object with a 2 m shadow gives a ratio of 0.75, which means sunlight meets the ground at about 36.9 degrees. Early morning and late afternoon have low Sun and long shadows; noon has a high Sun and short shadows.
Make the pupil say the cause and effect: the shadow did not grow, the light became slanted.
Keep the object still and lower the torch from above: the tip of the shadow moves further away.
Close with numbers: a 1.5 m object with a 2 m shadow gives a ratio of 0.75, which means sunlight meets the ground at about 36.9 degrees. Early morning and late afternoon have low Sun and long shadows; noon has a high Sun and short shadows.
Make the pupil say the cause and effect: the shadow did not grow, the light became slanted.
Ask the pupil: lower the torch slowly and say which way the tip of the shadow moves and why.
Scene 3: The angle of reflection equals the angle of incidence
Pupils know a mirror throws light back but cannot state the angle rule, so they draw the reflected ray anywhere.
Draw the normal first, the dashed line perpendicular to the mirror. Measure both rays from that normal: 40 degrees in gives 40 degrees out, and the two rays are 80 degrees apart. Measured from the mirror surface instead, every answer is wrong.
Everyday evidence: move left in front of a mirror and the image moves left; a periscope uses two mirrors at 45 degrees to turn the light through two right angles.
Draw the normal first, the dashed line perpendicular to the mirror. Measure both rays from that normal: 40 degrees in gives 40 degrees out, and the two rays are 80 degrees apart. Measured from the mirror surface instead, every answer is wrong.
Everyday evidence: move left in front of a mirror and the image moves left; a periscope uses two mirrors at 45 degrees to turn the light through two right angles.
Ask the pupil: draw the normal first, then 40 degrees in and 40 degrees out, and measure the 80 degrees between.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Explain a phenomenon | Why is there a shadow | Straight light, blocked, no light behind | Must mention straight |
| Draw and label | Shadow when the light is upper right | Shadow lower left and shorter | Side and length both |
| Angle question | Incident 40 degrees, what is the reflected angle | Equal angles | 40 degrees, 80 between |
| Ordering | Rainbow colours from outside in | Red orange yellow green blue indigo violet | 7 colours |
Sanity check (teach this): Reasonableness check: the lesson is 40 minutes and the five blocks add to 40; the speed of light is background only and is not memorised at this level; the shadow is always opposite the source and always longer when the source is lower; the ratio 0.75 and the angle 36.9 degrees come from the same two numbers, so a pupil who says the shadow is long must also name the position of the source.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Draw three shadows | Light high, slanted and low | All three on the opposite side |
| Measure | Same pencil in three positions | Records the change in length |
| Mirror angles | Incident 30 and 60 degrees | Normal drawn before measuring |
| Match phenomena | Shadow, eclipse, mirror image | All explained with straight lines |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Shadow drawn under the object | Sun is slanted, shadow at the feet | Where is the Sun? The shadow goes on the other side. |
| Shadow on the same side as the light | Sun in the east, shadow in the east | Light comes from the east, so the dark side is the west. |
| Angle measured from the mirror | Incident 40 degrees, reflection drawn from the surface | Always measure from the normal, not the surface. |
| Moon treated as a light source | Writes that the Moon makes its own light | The Moon is a mirror, it borrows the Sun. |
07 Homework (paper, section and question numbers)
- Primary General Studies index — Other science chapters in this subject
- All prep guides — Comparable chapters in other subjects
- Redo this chapter — Items 1 to 3 again, label the source first
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Sun in the east, where is the shadow | To the west | Answer east means reteach act one |
| Lower source, longer or shorter shadow | Longer | Answer shorter means reteach act two |
| Incident 30 degrees, reflected angle | 30 degrees | Answer 60 means reteach the normal |
09 Tutor FAQ
Does a Primary 3 pupil need the speed of light?
No. It is used only to show that light arrives instantly at this scale; the idea to keep is straight-line travel.
Why does the shadow change length?
Because the height of the Sun changes. A low Sun sends slanted light and stretches the shadow, a high Sun keeps it short.
Why can a mirror show what is behind you?
It reflects light in an orderly way with equal angles, so your eye follows the reflected ray back to the object.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.