3.2.4 Lenses & Dispersion: The Examiner
Authentic Cambridge-Style Questions • Papers 2, 4, and 6
📋 PAPER 2: Multiple Choice Questions
5 questions testing conceptual understanding and common misconceptions
Q1An object is placed between F and 2F in front of a converging lens. Which statement correctly describes the image formed?
Q2A person with short-sightedness (myopia) cannot see distant objects clearly. Which type of lens should be used to correct this condition?
Q3What does the term "monochromatic light" mean?
Q4When white light passes through a prism, which color is refracted (bent) the most?
Q5A ray of light passes through the optical center of a converging lens. What happens to this ray?
📝 PAPER 4: Structured Theory Questions
Deep conceptual questions with strict mark scheme keywords
Question 1: Ray Diagrams and Image Formation [8 marks]
(a) Draw a ray diagram to show how a converging lens forms an image when the object is placed exactly at the focal point F. [3]
Mark Scheme:
- Object drawn as upright arrow at F [1]
- Ray parallel to axis refracts through opposite F [1]
- Ray through optical center continues straight [1]
- Key observation: Rays emerge parallel (no image forms)
(b) State TWO differences between the image formed by a converging lens when the object is at 2F compared to when it is between F and 2F. [2]
Mark Scheme (Any TWO):
- At 2F: image is same size / Between F and 2F: image is magnified [1]
- At 2F: image forms at 2F / Between F and 2F: image forms beyond 2F [1]
- Both are real and inverted (no marks for stating this - same for both)
(c) Explain why a diverging lens can never form a real image. [3]
Mark Scheme Keywords:
- Diverging lens refracts rays away from the principal axis [1]
- Rays never actually converge / meet [1]
- Real images require rays to actually cross / only virtual images form [1]
⚠️ Examiner's Trap:
Do NOT say "diverging lens spreads light" without explaining that rays never cross.
Question 2: Dispersion and the Visible Spectrum [7 marks]
(a) White light is shone through a glass prism. Explain why the light is dispersed into a spectrum of colors. [3]
Mark Scheme Keywords:
- White light contains different wavelengths / frequencies [1]
- Different wavelengths travel at different speeds in glass [1]
- Different wavelengths are refracted by different amounts / violet most, red least [1]
⚠️ Examiner's Trap:
Saying "colors bend differently" is too vague. Must mention speed difference in the medium.
(b) State the order of colors in the visible spectrum from lowest to highest frequency. [2]
Mark Scheme:
- Red, Orange, Yellow, Green, Blue, Indigo, Violet [2]
- (Award 1 mark if only first 3 and last 3 are correct)
💡 Memory Aid: "Roy G. Biv"
(c) A student claims that red light travels faster than violet light in air. State whether this claim is correct and explain your answer. [2]
Mark Scheme:
- Incorrect [1]
- All electromagnetic waves travel at the same speed in air / vacuum (3 × 10⁸ m/s) [1]
⚠️ Examiner's Trap:
Different speeds only occur in glass/water. In air, all light travels at c.
🔬 PAPER 6: Alternative to Practical
Data analysis and experimental technique
Question: Investigating Focal Length [10 marks]
A student investigates the focal length of a converging lens using the following method:
- Place the lens in a holder on the bench
- Use a lamp as a distant object
- Move a white screen until a sharp image of the lamp forms
- Measure the distance from the lens to the screen
The student measures the focal length as 15.4 cm.
(a) The reading on the ruler is shown below. State the focal length to the nearest millimeter. [1]
Mark Scheme: 15.5 cm or 155 mm [1]
⚠️ Common Error: Reading 15.4 cm (not reading to nearest mm)
(b) Suggest why the student should repeat the measurement and calculate a mean. [2]
Mark Scheme:
- To identify anomalous results / outliers [1]
- To improve accuracy / reliability of the result [1]
(c) The student notices the image on the screen is blurred. Suggest TWO ways to improve the sharpness of the image. [2]
Mark Scheme (Any TWO):
- Adjust the position of the screen slightly [1]
- Use a brighter lamp / light source [1]
- Darken the room / reduce background light [1]
- Clean the lens [1]
(d) State ONE safety precaution the student should take during this experiment. [1]
Mark Scheme (Any ONE):
- Do not look directly at the lamp / bright light source [1]
- Ensure the lamp is stable / cannot fall [1]
- Allow the lamp to cool before touching [1]
(e) The student wants to find the power of the lens. The equation for power is:
where P is power in dioptres (D) and f is focal length in metres.
Calculate the power of this lens. Show your working. [3]
Mark Scheme:
- f = 15.5 cm = 0.155 m (or 0.154 m) [1] for unit conversion
- P = 1 / 0.155 [1] for correct substitution
- P = 6.45 D (or 6.5 D) [1] for answer with unit
⚠️ Examiner's Trap:
Forgetting to convert cm to m is the most common error. Always check units!
(f) Explain why the image formed on the screen is inverted (upside down). [1]
Mark Scheme:
- Light rays cross over at the lens / through the focal point [1]
Alternative: Rays from top of object converge at bottom of image [1]
📊 Assessment Summary
Paper 2 (MCQ)
5
questions
Paper 4 (Theory)
15
marks
Paper 6 (Practical)
10
marks
🎯 Key Exam Skills Tested:
- ✓ Image characteristic identification (real/virtual, magnified/diminished)
- ✓ Lens correction for eye defects
- ✓ Ray diagram construction rules
- ✓ Dispersion mechanism and spectrum order
- ✓ Practical measurement and ruler reading
- ✓ Power calculation with unit conversion
- ✓ Mark scheme keyword precision
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