Primary 4 Science Light Quiz

Primary 4 Science Quiz - Light: Answer Key

Total Marks: 40

Section A: Multiple Choice (2 marks each)

1. (b) straight

Explanation: Light travels in straight lines. This is why we cannot see around corners without a mirror. Light does not bend or curve on its own — it needs something to reflect it or change its direction. If light travelled in curves, shadows would not have sharp edges.

2. (c) The Moon

Explanation: The Moon is NOT a source of light. The Moon shines because it reflects sunlight — it does not produce its own light. The Sun (a star), a candle flame (burning wax produces light), and a torch (electric bulb) are all luminous objects that produce their own light. Remember: if you take away the Sun, the Moon would be dark.

3. (b) reflects

Explanation: Light from a luminous source (like the Sun or a lamp) reaches an object, then reflects (bounces off) that object into our eyes. This is how we see non-luminous objects. The light "travels" to the object, but the key process that lets us see is reflection into our eyes. "Absorbs" would mean the light is taken in by the object, and "bends" involves refraction (not covered in P4).

4. (c) transparent

Explanation:

• Transparent materials let all light pass through — you can see clearly through them (e.g., clear glass, clean water).
• Translucent materials let some light through but scatter it — you cannot see clearly (e.g., frosted glass, tracing paper).
• Opaque materials block all light — no seeing through them (e.g., wood, metal, brick).
• Luminous means the object produces its own light, not about light passing through.

5. (b) The shadow becomes larger

Explanation: When an object moves closer to the light source, the light rays spread out more behind the object, creating a larger shadow. Imagine standing close to a torch — your shadow on the wall grows huge. Move further away, and your shadow shrinks. The light source's rays diverge (spread apart), so less area is blocked when you're further away.

6. (c) Thick black velvet

Explanation: To block out morning sunlight completely, you need an opaque material. Thick black velvet is opaque and also dark-coloured, which absorbs light rather than reflecting it. Clear plastic (transparent) and thin white cotton (translucent) let light through. Frosted glass is translucent — it diffuses light but still lets plenty through.

7. (a) The shadow becomes smaller and sharper

Explanation: Moving the ball further from the light source means:

• The light rays have spread out more before reaching the ball, so the ball blocks a smaller angle of light → smaller shadow
• The ball is closer to the screen, so edges are less blurred → sharper shadow

The shadow also becomes less blurry because the "fuzzy" penumbra region shrinks as the object nears the screen.

8. (c) Light will bounce off the mirror in a different direction

Explanation: Mirrors are smooth, shiny surfaces that reflect light according to the law of reflection (angle in = angle out, though P4 students need only know that light bounces off). The reflected light changes direction. If the mirror is angled correctly, you can see the torch light in a different location — this is how periscopes work.

9. (c) A shadow is formed when light is blocked by an opaque object

Explanation:

• (a) is false — shadows need a light source; no light, no shadow.
• (b) is false — shadows can be coloured if the light source is coloured (e.g., red light makes red-tinged shadows).
• (d) is false — transparent objects let light through, so they cast very faint or no shadows.
• (c) is correct: opaque objects block light, creating a shadow in the region behind where light cannot reach.

10. (b) Ali, because he is taller and closer to the light source

Explanation: Shadow length depends on both height and distance from the light source. Ali is taller (150 cm vs 120 cm) AND closer to the light source (3 m vs 5 m from edge, closer to Sun direction). Both factors make his shadow longer:

• Taller objects cast longer shadows
• Objects closer to the light source have shadows that spread out more

Ben's shorter height and greater distance from the light source both reduce his shadow length.

Section B: Fill in the Blanks (1 mark each)

11. Objects that give out their own light are called luminous objects. Objects that do not give out their own light are called non-luminous objects.

Key idea: The Sun, stars, flames, and light bulbs are luminous. The Moon, tables, and people are non-luminous — we see them by reflected light.

12. When light hits a smooth, shiny surface like a mirror, it reflects (regularly). When light hits a rough surface like a wall, it still reflects but in many different directions.

Key idea: Smooth surfaces give regular (specular) reflection, creating clear images. Rough surfaces give diffuse reflection — light scatters, so no clear image forms, but the surface is still visible because light reaches our eyes from all over it.

13. A frosted glass window is translucent because it lets some light through but objects cannot be seen clearly through it. A brick wall is opaque because no light can pass through it.

14. The shape and size of a shadow depends on:

• the position/direction/size of the light source,
• the size/shape of the object, and
• the distance between the object and the screen/surface.

Key idea: A small light source (point source) gives sharp shadows. A large light source gives blurry-edged shadows with partial shadow regions. Moving the object closer to the screen makes the shadow sharper and smaller (relative to moving it toward the light).

Section C: Short Answer

15. (a)

(b) Different materials have different properties regarding light:

• Clear glass is transparent — its smooth, ordered structure lets light pass straight through without much scattering [1 mark]
• Frosted glass is translucent — its rough surface scatters light in many directions, so light passes through but images are blurred [0.5 mark]
• Cardboard is opaque — it absorbs or blocks light completely; no light can pass through its fibres [0.5 mark]

16. (a) As the distance from the light source increases, the height of the shadow decreases. [1 mark] The shadow gets smaller at a decreasing rate — from 10 to 20 cm distance, shadow drops 5 cm; from 20 to 30 cm, it drops only 3 cm. [1 mark for noting pattern detail or consistent trend description]

(b) Prediction: Approximately 5 cm (accept 4–6 cm) [1 mark] Explanation: Based on the pattern, each increase of 10 cm in distance gives a smaller decrease in shadow height. From 30 cm, the next decrease should be about 2 cm (following 5 cm, then 3 cm reduction trend), giving approximately 5 cm. Alternatively, following the decreasing trend, the shadow continues to get smaller. [1 mark for reasoning linked to data pattern]

17. (a) Tom will see a small, bright spot of light on the wall behind/beside him, corresponding to the hole in the card. [1 mark] Light travels in a straight line from the torch, through the hole in the card, to the mirror, then reflects off the mirror to the wall. [1 mark]

(b) The light spot will move upwards on the wall. [1 mark] When a mirror is tilted, the angle of reflection changes equally. Tilting the mirror upward means the reflected light beam angles upward, hitting a higher point on the wall. The law of reflection: if the mirror surface tilts by angle θ, the reflected ray changes direction by 2θ. [1 mark for explaining changed reflection direction]

Section D: Application and Thinking Questions

18. (a) Translucent material (e.g., frosted glass, textured glass). [1 mark] Reason: It lets some natural light into the bathroom for brightness during the day, but scatters the light so that people outside cannot see clearly in — providing privacy. [1 mark]

(b) Transparent material (e.g., clear glass). [1 mark] Reason: You need to see the view outside clearly, and transparent materials allow light to pass straight through without distortion, giving a clear image of the outside world. [1 mark]

(c) Opaque material (e.g., wood, laminate, metal). [1 mark] Reason: To keep clothes hidden from view, no light should pass through the wardrobe doors. Opaque materials block all light, ensuring privacy. [1 mark]

19. (a) People in the total eclipse zone are in the umbra — the darkest part of the Moon's shadow. [1 mark] The Moon completely blocks all direct sunlight from reaching these areas, so it becomes dark during daytime. The Moon's size and distance from Earth are just right for this complete blockage. [1 mark]

(b) People in nearby regions are in the penumbra — the partial shadow. [1 mark] In this region, the Moon only blocks part of the Sun, so some sunlight still reaches Earth. The Sun is much larger than the Moon, so light from the Sun's edges can still reach around the Moon, creating partial lighting. [1 mark]

20. (a) Light path description (with arrows on diagram): [3 marks total]

1. Light travels from the flower (object) upwards through the top opening to Mirror A [1 mark]
2. Light reflects off Mirror A (at 45°), travelling down the tube [1 mark]
3. Light hits Mirror B (at 45°) and reflects again, travelling horizontally to Jason's eye [1 mark]

The two reflections change the light's direction by 90° each, allowing Jason to see over the wall.

(b) The 45° angle is essential because:

• When light hits a mirror at 45°, it reflects at 45° on the other side, making a 90° turn [1 mark]
• This 90° turn directs light straight down the horizontal tube, then straight out to the eye. If angles were different, light would hit the tube walls and be absorbed, or miss the second mirror entirely [1 mark]

END OF ANSWER KEY