AQA AS Level Physics Paper 1, June 2019: Question 4

✅ Correct Answer

Any 2 valid points from:

• The refractive index of water is greater than air.
• The angle of incidence is greater than the critical angle.
• Total internal reflection (TIR) of the laser beam occurs.

💡 Key Knowledge

TIR requires two core conditions: light must travel from a medium of higher optical density (higher refractive index) to a lower one, and the angle of incidence in the denser medium must exceed the critical angle.

❌ Common Errors

Students often just state "reflection happens" without mentioning total internal reflection, or fail to explicitly link the higher refractive index of water compared to air.

📐 Step-by-Step Calculation

1. Identify formula: n = c / c_s
2. Rearrange for speed in medium ( c_s ): c_s = c / n
3. Substitute values: c_s = (3.00 × 10⁸) / 1.33
4. Calculate value: = 2.2556... × 10⁸ m s⁻¹
5. Apply correct sig figs: 2.26 × 10⁸ m s⁻¹ (3 sig figs, matching the given value of 1.33).

🧠 Exam Technique

Always state your base formula before plugging in numbers. Pay close attention to significant figures: because the refractive index (1.33) is given to 3 significant figures, your final answer should also be rounded to 3 sf to secure the final mark.

✅ Correct Answer

49° (Accept 48.8° or similar precise variations, but do not allow 1 sf answers like 50°).

💡 Key Knowledge

The critical angle formula is derived from Snell's Law: sin(C) = 1 / n . For water where n = 1.33 , C = sin⁻¹(1 / 1.33) = 48.75° .

💡 Key Knowledge Breakdown

• Parts: X is the core; Y is the cladding.
• Functions of X (Core): Transmits light via TIR; has a higher refractive index than cladding; low attenuation/absorption.
• Functions of Y (Cladding): Protects core from scratching/damage; provides clean boundary for TIR; prevents cross-talk between touching fibres.
• Material Dispersion: Caused by white light containing different wavelengths travelling at different speeds. Solution: Use monochromatic light.
• Modal Dispersion: Caused by light rays taking different paths/angles down the fibre, arriving at different times. Solution: Use a narrow core (single-mode fibre) to restrict paths.

🧠 Exam Technique (Level of Response)

This is a 6-mark extended response marked using levels (L1: 1-2 marks, L2: 3-4 marks, L3: 5-6 marks). To hit Level 3, you must comprehensively cover structural names, functions, both types of dispersion, and how to overcome them without significant errors or omissions.

✅ Correct Answer

Any 2 valid points:

• Light encounters impurities at different positions, angles, numbers, or sizes when bent.
• The angle of incidence at the impurity or core boundary may become less than the critical angle.
• Bending may cause micro-cracks in the core or cladding, altering reflection/refraction paths.

❌ Common Errors

Avoid saying "the critical angle changes" — the refractive index and material properties of the core do not change simply because the cable curves; rather, the angle of incidence of the light rays changes relative to boundaries and impurities.

✅ Correct Answer

• Transverse waves: Oscillations/displacements are at right angles (perpendicular) to the direction of energy transfer.
• Longitudinal waves: Oscillations/displacements are parallel (along) to the direction of energy transfer.

🧠 Exam Technique

Be precise with terminology. Always mention oscillations or particles/field vibrations relative to the direction of energy transfer (or wave propagation). Avoid vague phrases like "waves move up and down".