Primary 4 Science Light Quiz

Primary 4 Science Quiz - Light (Answer Key)

Total Marks: 40

Section A: Multiple-Choice Questions (10 × 1 mark = 10 marks)

1. Answer: (3) Sun
Explanation: The Sun is a natural light source that produces its own light. The Moon and mirrors only reflect light; white paper reflects light but does not produce it.

2. Answer: (2) straight line
Explanation: Light travels in straight lines. This is a fundamental property of light and explains why shadows have sharp edges when formed by a point source.

3. Answer: (3) an opaque object blocks light
Explanation: A shadow is a dark area formed when an opaque object blocks light from a source. Transparent objects allow light to pass through, so they do not form shadows.

4. Answer: (2) Clear glass
Explanation: Transparent materials allow almost all light to pass through so objects can be seen clearly through them. Wood and aluminium foil are opaque; clear glass is transparent.

5. Answer: (3) The shadow is larger than the ball.
Explanation: Because light rays diverge (spread out) from the torch, the shadow on the screen is magnified. The closer the object is to the light source, the larger the shadow. In the diagram, the ball is closer to the torch than to the screen, so the shadow is larger than the ball.

6. Answer: (2) B and D
Explanation: Frosted glass and tracing paper are translucent — they allow some light to pass through but scatter it, so you cannot see clearly through them. Clear plastic (A) is transparent; wood (C) is opaque.

7. Answer: (2) The shadow becomes larger.
Explanation: When an object moves closer to the light source, it blocks more of the diverging light rays, casting a larger shadow on the wall. This is the same principle as in Q5.

8. Answer: (2) Light from the surroundings is reflected by the book into our eyes.
Explanation: Most objects (like books) are non-luminous — they do not produce light. We see them because light from a source (e.g., room lights, sunlight) hits them and reflects into our eyes.

9. Answer: (2) 30°
Explanation: Law of reflection: Angle of incidence = Angle of reflection. If the angle of incidence is 30°, the angle of reflection is also 30°.

10. Answer: (1) A clear glass cup
Explanation: A clear glass cup is transparent — it allows light to pass through, so it does not block light to form a shadow. The other objects (metal spoon, wooden block, ceramic mug) are opaque and will form shadows.

Section B: Structured Questions (6 × 2 marks = 12 marks)

11.
(a) Object A [1]
(b) Object A is closest to the torch. Light rays diverge from the torch, so the object nearest to the light source blocks a wider spread of rays, forming a larger shadow on the screen. [1]
Marking note: Must mention "closest to torch" and "diverging rays" or "blocks more light" for full mark.

12. [2 marks total; ½ mark per correct placement]

Common mistake: Placing "clean water" as translucent. Water is transparent.

13.
(a) Angle of incidence (i) labelled between incident ray and normal; Angle of reflection (r) labelled between reflected ray and normal. Both angles equal (40°). [1]
(b) The angle of incidence is equal to the angle of reflection. [1]
Alternative acceptable: "The incident ray, reflected ray, and normal all lie in the same plane." But the angle equality is the key P4 concept.

14.
(a) Distance between the card (object) and the screen. [1]
(b) Size (height/area) of the shadow on the screen. [1]
Marking note: "Size of shadow" alone is acceptable; "height of shadow" is more precise.

15.
(a) (3) bends [1]
(b) Refraction [1]
Explanation: Light changes speed when it passes from one medium (water) to another (air) of different optical density, causing it to bend. This bending is called refraction.

16.
(a) Red light [1]
(b) The other colours (blue, green, etc.) are absorbed by the apple. [1]
Explanation: A red apple appears red because it reflects red light and absorbs most other colours of the visible spectrum.

Section C: Open-Ended Questions (3 × 6 marks = 18 marks)

17.
(a) Object X: Opaque [1]
Object Y: Translucent [1]
Object Z: Transparent [1]
Reasoning: Opaque objects block all light → sharp dark shadow. Translucent objects allow some light through but scatter it → faint blurry shadow. Transparent objects allow most light through → no shadow.

(b) Object Y is translucent. It allows some light to pass through but scatters the light in different directions, so only a small amount of light reaches the screen, forming a faint shadow. The scattering also makes the edges blurry. [2]
Mark breakdown: 1 mark for "translucent / allows some light through", 1 mark for "scatters light / blurry edges".

(c) Move Object X closer to the screen (or further from the torch). [1]
Explanation: Moving the object away from the light source (towards the screen) reduces the spread of blocked rays, making the shadow smaller.

18.
(a) Light ray drawn from object → top mirror (45° incidence) → reflects 90° down tube → bottom mirror (45° incidence) → reflects 90° horizontally to eye. Arrows show direction. [2]
Mark breakdown: 1 mark for correct path with 90° turns at each mirror, 1 mark for arrows showing direction from object to eye.

(b) Light from the object hits the top mirror, reflects down the tube to the bottom mirror, then reflects horizontally to the observer's eye. This allows the observer to see over the wall because the light path is bent by the mirrors. [2]
Mark breakdown: 1 mark for describing the two reflections, 1 mark for explaining how it sees over the wall (light path bent).

(c) The image would appear smaller and upright (virtual), and the field of view would be wider. A convex mirror diverges light rays, producing a diminished image. [2]
Mark breakdown: 1 mark for "smaller/diminished", 1 mark for "wider field of view" or "upright virtual image". At P4 level, "smaller and wider view" is sufficient.

19.
(a) As the distance between the torch and the object increases, the height of the shadow decreases. [1]
Or: "The further the torch from the object, the smaller the shadow."

(b) 6 cm (or approximately 6–7 cm) [1]
Reasoning: The pattern shows shadow height decreasing by 5, 3, 2, 2 cm. The decrease is slowing. Extrapolating: 8 cm → ~6 cm. Accept 6–7 cm with valid reasoning.

(c) When the torch is further from the object, the light rays reaching the object are more parallel (less divergent). The object blocks a narrower beam of light, so the shadow cast on the fixed screen is smaller. [2]
Mark breakdown: 1 mark for "rays more parallel / less divergent", 1 mark for "blocks narrower beam / smaller shadow".

(d) All shadow heights in the table would be larger (greater) because a larger object blocks more light, casting a bigger shadow at each distance. [2]
Mark breakdown: 1 mark for "larger shadows", 1 mark for "blocks more light / bigger object".

20.
(a) The light ray bends towards the normal. [1]
(b) Light slows down when it enters the denser medium (glass) from air, causing it to change direction (refract). [1]
Key concept: Change in speed → change in direction.

(c) The emergent ray is parallel to the incident ray but laterally displaced (shifted sideways). [1]
(d) The bending would be less (smaller angle of refraction) because water is less optically dense than glass, so light slows down less in water than in glass. [1]
At P4 level: "Less bending in water" is acceptable.

(e) Faster than [1]
Explanation: Light travels faster in water than in glass because water has a lower refractive index (less optically dense).

(f) The ray bends towards the normal when entering the glass, travels straight through, then bends away from the normal when exiting. The two bends are in opposite directions but the parallel faces cause the emergent ray to be parallel to the incident ray, shifted sideways. [1]
Key point: Parallel faces of the block cause the two refractions to cancel in direction but not in position.

End of Answer Key