CIE iGCSE Co-Ordinated Science P3.2.3 Thin converging lens Exam Style Questions Paper 4
Question
A student investigates the motion of smoke particles in air using a microscope. The student shines a bright light on a transparent box containing a mixture of smoke and air and observes the smoke particles as bright dots of light.
(a) The student observes that the smoke particles move in straight lines between random changes of direction.
Fig. 9.1 shows the observed path of one smoke particle.
The motion shown in Fig. 9.1 is known as Brownian motion.
Describe what causes the motion of the smoke particles shown in Fig. 9.1.
▶️Answer/Explanation
Answer:
- Collisions with fast-moving air molecules
- Air molecules are invisible but transfer momentum during impacts
Explanation: Brownian motion provides evidence for:
- The existence of moving air molecules too small to see
- Random molecular motion in fluids
- Particle nature of matter
(b) The microscope uses a filament lamp to illuminate the smoke particles. Fig. 9.2 shows how current varies with potential difference (p.d.) for the filament lamp.
Use the shape of the graph in Fig. 9.2 to describe and explain what happens to the resistance of the filament lamp as the potential difference is increased.
▶️Answer/Explanation
Answer:
- Resistance increases with higher p.d.
- Shown by decreasing gradient (I-V curve bends toward voltage axis)
- Because filament heats up, increasing lattice vibrations
Explanation: In a filament lamp:
- Higher current → more heating (I²R effect)
- Hotter filament → more electron scattering
- Positive temperature coefficient of resistance
(c) The microscope uses a thin converging lens to produce an image. Fig. 9.3 shows a thin converging lens.
(i) Draw a ray diagram on Fig. 9.3 to show the formation of a real image. Label the image with the word image.
▶️Answer/Explanation
Answer: Diagram should show:
- Parallel ray refracting through focal point
- Ray through center continuing straight
- Intersection point marked “image”
Explanation: For converging lenses:
- Real images form when object is beyond focal length
- Image is inverted and can be projected
- Two construction rays locate the image position
(ii) Fig. 9.4 shows a single ray of light entering a thin glass block.
Calculate the refractive index of the thin glass block.
▶️Answer/Explanation
Answer: 1.63
Calculation:
n = sin(i)/sin(r) = sin(50°)/sin(28°) ≈ 1.63
Explanation: Refractive index is:
- Ratio of light speeds in two media
- Calculated using Snell’s Law
- Dimensionless number >1 for denser media