Light: Shadows and Reflections · Lesson 6 of 6
Chapter Summary and Practice
“Connect sources, shadows, mirrors, and pinhole images through mixed visual reasoning.”
• Trace light from a source through transmission, blocking, or reflection to an observer or screen. • Distinguish luminous objects, shadows, mirror images, and pinhole images. • Use diagrams to predict how a change in position alters a shadow or reflected view. • Explain the different uses of periscopes and kaleidoscopes. • Apply the chapter’s concepts to mixed observations and unfamiliar setups.
The Chapter in One View
Every scene in this chapter begins with light from a source. What happens next depends on what the light meets: it may pass through a material, be blocked and leave a shadow, or change direction at a mirror. A small opening can select straight paths to form an image on a screen. The table gives one place to compare these outcomes before solving mixed questions.
| Chapter idea | What to look for | Example or result |
|---|---|---|
| Light source | Does the object emit its own light? | A star or lit lamp is luminous; the Moon reflects sunlight. |
| Straight path | Can the light follow an unobstructed route? | Aligned holes pass the beam; a shifted hole blocks it. |
| Material | How much light passes through? | Transparent: almost all; translucent: part; opaque: none in the test. |
| Shadow | Where does an object prevent light reaching a surface? | An opaque ball leaves a dark region on a wall. |
| Reflection | Where does a shiny surface redirect light? | Tilting a plane mirror moves the reflected spot. |
| Plane-mirror image | What appears behind a flat mirror? | Same size, erect, laterally inverted; not obtained on a screen. |
| Pinhole image | Where do straight paths cross at a small opening? | An inverted image appears on the camera screen. |
| Mirror devices | How many times is light redirected? | Two mirrors guide a periscope view; three make repeated kaleidoscope patterns. |
Follow the Light, Not Just the Object
If a luminous candle can be seen directly, light reaches the eye from the flame. Place opaque cardboard in that route and the direct view disappears. A screen behind the cardboard may show a shadow; add a tiny hole and a screen at a suitable distance and it may instead show an inverted image of the flame. A plane mirror can send light toward the eye along another route and make an upright image appear behind its surface.
This sequence helps separate four questions that sound similar: What produces the light? What path can it take? What reaches the eye or screen? What does the resulting bright or dark region tell us? Answering them in that order prevents a shadow from being mistaken for an image or a bright mirror from being called luminous.
Problem
A person sees the Moon, a lit lamp, and the lamp’s bright reflection in a mirror. Which objects emit light themselves?
- 1.The lit lamp emits light and is luminous.
- 2.The Moon is seen by reflected sunlight. The mirror redirects light from the lamp or other sources.
- 3.Of these three observed things, only the lit lamp is itself a light source.
Problem
A student sees a face in a plane mirror and the outline of a candle on a pinhole-camera screen. Which one can be collected on a screen, and how is it oriented?
- 1.The plane-mirror image appears behind the mirror, remains erect, and cannot be obtained on a screen.
- 2.Light through the pinhole reaches the camera screen, where paths from top and bottom cross.
- 3.The camera image can be seen on its screen and is inverted.
Work Through Position Changes
A shadow is affected by the geometry of source, object, and screen. Move a ball closer to a fixed torch and its shadow on a fixed wall generally grows; move it toward the wall and the shadow generally shrinks. Tilt a flat cutout and the shadow shape changes. Replace a red opaque cutout with an identical blue one and the dark region does not take on the cutout’s colour. A second torch introduces another blocked light route and may create another shadow.
Problem
A fixed torch points at a fixed wall. In position A a ball is near the torch; in position B it is near the wall. Compare the likely shadows.
- 1.Keep the torch and wall fixed; only the ball’s position changes.
- 2.Near the torch, the ball cuts off a wider spread of paths toward the wall. Near the wall, its outline is projected over a smaller region.
- 3.The shadow in A is generally larger than in B. Both require light and a receiving surface.
Problem
Why might a bird’s shadow be hard to see when it is high and easier to see when it flies low?
- 1.A shadow is the change in light reaching the ground because the bird blocks some paths.
- 2.When the bird is high, the shadow may be spread out and faint in the surrounding light. Near the ground it is usually more concentrated and recognisable.
- 3.A hard-to-see shadow does not mean the bird became transparent or ceased to block light.
Check Three Easily Confused Images
A dark patch cast by a ball is a shadow: it shows where light was blocked. A plane-mirror image is an apparent view behind the mirror, the same size and erect, with perceived left-right reversal. A pinhole image is a picture of a lit scene formed on a screen and upside down. Make a sketch showing source, object, optical part, eye, and screen before deciding which description applies.
| Observation | Where seen | Orientation and key reason |
|---|---|---|
| Shadow of an opaque cutout | On a wall or other surface | Its outline depends on the blocked paths and object position. |
| Plane-mirror image of a pen | Apparently behind the mirror | Erect and same-size; reflected light reaches the eye. |
| Pinhole image of a tree | On the tracing-paper screen | Inverted; top and bottom paths cross at the pinhole. |
Both can appear on a surface, but a pinhole image contains information from different parts of the scene; a shadow is a region that receives less light because something blocked it. A plane-mirror image is seen but is not obtained on a screen.
Put Reflection to Work
The periscope directs incoming light toward an eye through two reflections, letting someone see around an obstacle. The kaleidoscope uses three inward-facing mirrors to produce repeated views; rotating it changes the pattern. Neither device changes the rule that light travels along straight stretches between interactions with the mirrors.
You can extend the investigation by angling two mirrors and observing multiple images, or by testing whether a small mirror can show part or all of a distant large tree from your viewing position. These are questions about available light paths, not about a mirror producing its own light. For an outdoor shadow inquiry, compare observations at morning, around noon, and afternoon while recording the position and conditions carefully.
Quiz
Which listed object is luminous?
What happens to the spot behind three small holes if their openings cease to align?
Which material typically transmits part of the torch light but does not give a clear view?
Which change usually makes the ball’s shadow larger on a fixed wall with a fixed torch?
Which property belongs to a plane-mirror image?
Why is a tree inverted in a pinhole-camera image?
What does a simple periscope use to redirect light to an eye?
Practice Problems
- Identify the luminous objects among Mars, the Moon, the Pole Star, the Sun, Venus, and an unlit mirror, explaining one choice.
- Match each outcome to its cause: light passes almost fully, light passes partly, light is blocked, and light changes direction at a mirror.
- A candle is viewed through a straight pipe and then a bent pipe. Predict the two observations and explain them.
- Sketch source, object, and screen for a shadow of a standing person. How might changing the light position change the shadow?
- With torch and wall fixed, compare a ball near the torch with the same ball near the wall. Include a reason for the difference.
- One torch is replaced with two torches aimed at the same ball. Predict what could appear on the screen and explain the light paths.
- Sketch how the top and bottom of a tree are placed in a pinhole image. Label the crossing paths at the opening.
- Write your name on paper and inspect its plane-mirror image. Which letters look changed or unchanged? Explain left-right symmetry and lateral inversion.
- Explain why a plane-mirror image stays erect whereas a pinhole-camera screen image is upside down.
- Plan measurements of your shadow in the morning, around noon, and afternoon. What would you record, and why could its length change?
- Design a two-mirror light route for a shaped periscope tube. Mark where each mirror must redirect the light.
- A bird flies high overhead and then close to the ground. Explain why its shadow may be easier to notice when it is near the ground.
- Describe how changing the angle between two plane mirrors might affect the repeated images you observe, without assuming a fixed number.
- Can a small mirror show a distant large tree? Explain what the object, mirror, and observer positions determine.
- Find out whether fireflies used to be common where you live. Propose how to distinguish reduced habitat from excessive night lighting in a local investigation.
Key Takeaways
• Identify the source first, then trace the path of light to an eye or screen. • Blocking light makes a shadow; reflection redirects light; a pinhole forms an inverted image on a screen. • A plane-mirror image is erect, same-size, apparently behind the mirror, laterally inverted, and not caught on a screen. • Periscopes and kaleidoscopes arrange mirrors for different uses of repeated reflection. • A careful explanation states what changed in the setup and what the observation shows.
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Periscopes and Kaleidoscopes
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