AQA GCSE Physics Physics Paper 2 (Higher), June 2025: Question 4
13 marks · Standard Demand difficulty · Short Answer
Identify a lens type and complete its ray diagram, calculate the frequency of electromagnetic radiation given its wavelength, and answer questions about galactic red-shift and orbital velocity.
Practise this questionQuestion
Mark scheme
Show the mark scheme
How to answer it
Telescopes, Wave Speed, Red-Shift & Orbital Motion
What this question tests:
- Lenses (Spec 4.6.2.5): Recognising convex (converging) lens symbols and completing ray diagrams for parallel incident light.
- The Wave Equation (Spec 4.6.1.2): Calculating wave frequency using v = f × λ with metric unit conversions (micrometres, μm, to metres, m) and standard form.
- Red-Shift & Expanding Universe (Spec 4.8.2): Defining red-shift, explaining why galaxies are red-shifted, and relating red-shift magnitude to distance.
- Circular Motion & Orbits (Spec 4.8.1.3): Explaining why orbital motion involves continuous acceleration and changing velocity despite constant speed.
Lens Identification
Identifying the lens type from standard ray diagram symbols
✅ Correct Answer
Convex (or converging)
💡 Key Knowledge
In ray diagrams:
- A straight line with arrows pointing outwards represents a convex (converging) lens.
- Arrows pointing inwards represent a concave (diverging) lens.
Ray Diagram for a Convex Lens
Completing refracted ray paths for incident rays parallel to the principal axis
- Both parallel horizontal rays must travel straight to the vertical lens line.
- At the lens line, both rays must bend inward (converge) and continue as straight lines that pass directly through the marked principal focus F on the right side of the lens.
- Draw your rays continuing slightly past point F with a sharp pencil and ruler.
✅ Mark Scheme Requirements
- Mark 1: Both parallel incident rays refract so that they converge (bend towards each other).
- Mark 2: The rays meet and cross exactly at the principal focus F.
🧠 Exam Technique
- Always use a sharp pencil and a 30 cm clear ruler. Freehand lines instantly lose accuracy marks.
- Make sure rays refract at the central vertical line of the lens, not before or after it.
❌ Common Errors
- Refracting rays before reaching the lens line.
- Making the rays cross before or behind F instead of directly on it.
- Diverging the rays outward as if it were a concave lens.
Wave Equation & Unit Conversion
Calculating radiation frequency from speed and wavelength given in micrometres
📐 Step-by-Step Calculation
- Convert wavelength to standard units (metres, m):
Prefix micro (μ) = 10⁻⁶
λ = 0.60 μm = 0.60 × 10⁻⁶ m (or 6.0 × 10⁻⁷ m, or 0.000 000 6 m)[Mark 1] - Recall and substitute into the wave equation:
Wave speed = frequency × wavelength ( v = f × λ )
3.0 × 10⁸ = f × (0.60 × 10⁻⁶)[Mark 2] - Rearrange for frequency ( f ):
f = (3.0 × 10⁸) / (0.60 × 10⁻⁶)[Mark 3] - Calculate final numerical value:
f = 5.0 × 10¹⁴ Hz[Mark 4]
❌ Calculation Traps
- Forgetting the prefix conversion: Using 0.60 directly instead of converting to metres loses the conversion mark.
- Calculator syntax errors: When dividing by standard form, use brackets: 3.0 × 10⁸ / (0.60 × 10⁻⁶) or use the dedicated EXP / ×10ˣ button. Otherwise, your calculator might multiply by 10⁻⁶ at the end!
🧠 Exam Technique
Even if you make an error in converting 0.60 μm to metres, AQA awards "error carried forward" (ecf). All subsequent marks for substitution, rearrangement, and calculation can still be awarded if your method is clear.
Definition of Red-Shift
Completing the sentence about observed light
✅ Correct Answer
"This means that the electromagnetic radiation shows an increase in wavelength."
❌ Common Errors
Writing frequency or speed. Red-shift is a shift towards the red end of the spectrum, which corresponds to longer wavelength (and therefore reduced frequency). Wave speed remains constant at the speed of light.
Cause of Red-Shift
Multiple-choice question explaining galactic red-shift
✅ Correct Option
☑ Distant galaxies are moving away from our galaxy.
💡 Why This Happens
Because distant galaxies are receding from us due to the expansion of space, the light waves are stretched out as they travel across space towards Earth, increasing their observed wavelength (Doppler effect/cosmological red-shift).
Red-Shift and Distance Relationship
Deducing distance from the magnitude of red-shift
✅ Correct Answer
The distance to this galaxy is greater than any previously observed galaxy (or: it is the furthest / most distant galaxy yet observed).
💡 Hubble's Law Link
The further away a galaxy is from Earth, the faster it is moving away, and therefore the greater the observed red-shift.
Orbital Motion and Changing Velocity
Explaining why velocity changes when an object orbits at constant speed
✅ Correct 3-Step Answer
- A gravitational force (or centripetal / resultant force) acts on the satellite towards the Sun. [1]
- This force causes the satellite to accelerate towards the centre of the orbit. [1]
- Because acceleration changes the direction of motion, the velocity changes (velocity is a vector with both magnitude and direction). [1]
💡 Scalar vs. Vector Concept
- Speed is a scalar (magnitude only) → remains constant.
- Velocity is a vector (magnitude AND direction).
- As the satellite moves in a circular path, its direction changes continuously → therefore velocity must change continuously!
❌ Mark Scheme Exclusion
Do NOT write: "acceleration causes a change in speed." The question specifically states speed is constant. The acceleration causes a change in direction only.
🧠 Top-Level Exam Strategy
Whenever asked why an orbiting object changes velocity at constant speed, use the standard 3-link chain: Force (gravity) → causes Acceleration → changes Direction → changes Velocity.
Topics
Physics · P6: Waves · P8: Space Physics (physics only)
Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.