AQA AS Level Physics Paper 2, November 2021: Question 5
11 marks · Hard difficulty · Extended Answer
Calculate the power output of a blue LED from its wavelength and photon emission rate, deduce whether a red LED can achieve twice this power using graph data, and explain the fluorescence process and wavelengths produced by a coating on both LEDs.
Practise this questionQuestion
Question text
05.1 A light emitting diode (LED) emits blue light with a wavelength of 440 nm.
The rate of photon emission is 3.0 × 1016 s−1.
Show that the power output of the LED is approximately 0.014 W.
[2 marks]
05.2 A different LED emits red light with a wavelength of 660 nm.
Figure 12 shows how the rate of photon emission varies with current up to the
maximum operating current of this LED.
Figure 12
A student claims that the red LED can have twice the power output of the blue LED.
Deduce whether the student’s claim is correct.
[3 marks]
05.3 The student has paint that fluoresces when light of any wavelength is incident on it.
She coats the blue LED and the red LED with the paint.
Compare the wavelengths of light emitted by the paint on each LED.
*15* In your answer you should also explain the processes that cause the paint to
fluoresce.
[6 marks]
END OF SECTION B
Section C
Each of Questions 06 to 35 is followed by four responses, A, B, C and D.
For each question select the best response.
Only one answer per question is allowed.
For each question, completely fill in the circle alongside the appropriate answer.
CORRECT METHOD WRONG METHODS
If you want to change your answer you must cross out your original answer as shown.
If you wish to return to an answer previously crossed out, ring the answer you now wish to select
as shown.
You may do your working in the blank space around each question but this will not be marked.
Do not use additional sheets for this working.
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
05.1 substitution into E = hc/λ Condone POT error on MP1 2 AO2.1-f
multiplies E by 3.0 × 1016 to give 0.0136 (W)
Red photon energy calculated (3.0 × 10−19 J) and
05.2 considers the effect of wavelength on power or emission rate 3 AO3.1-a
used with P = Ephoton × rate of emission
Alternative for MP1: red photon energy is times
smaller (than blue photon energy)
considers the maximum possible, or required, emission rate maximum emission rate is 6.9 × 1016 s−1
OR
evaluates required emission rate as 9.0 × 1016 s−1
combining MP1 and MP2 with reference to graph to reach the not possible as:
conclusion that it is not possible max emission rate is 6.9 × 1016 s−1, and required is
9.0 × 1016 s−1
max power is 0.021 W, and required is 0.028 W
max current is 60 mA, and required is > 60 mA
– SICS – – JUNE 2021
05.3 The mark scheme for this question includes an overall assessment for Area A - Wavelength comparison: 6 4
the quality of written communication (QWC). There are no discrete
marks for the assessment of QWC but the candidate’s QWC in this AO1.1-a
• Red LED will emit longer wavelengths than 660 nm
answer will be one of the criteria used to assign a level and award the
(accept “longer than red light).
marks for this question.
• Blue LED will emit wavelengths longer than 440 nm 2
Mark Criteria QWC (accept “longer than blue light). AO2.1-a
6 All 3 areas A, B and C covered The student presents • Blue LED will emit visible light. Accept named colours.
Only allow minor omissions the relevant information
52 complete descriptions with one coherently, employing Area B - Excitation process:
partial from A, B and C. structure, style and
SP&G to render • Excitation mentioned (as first step of fluoresence)
meaning clear. The text • Photons are absorbed by atoms in coating
is legible. • Atoms are excited/gain energy;
4 Full description of one area, with The student presents
• Atomic electrons move to higher energy levels (than n
partial description of two other. relevant information in a
OR way which assists the = 2)
Full description of two areas with communication of • Photons have sufficient energy to promote electrons to
very little on third or nothing at all. meaning. The text is high enough levels
3 A full description of one area and legible. SP&G are
a partial description of one area. sufficiently accurate not Area C - De-excitation process:
OR to obscure meaning.
A partial discussion of all three • De-excitation or relaxation mentioned (as subsequent
areas. step)
2 A full description of one area. The student presents • Photons are emitted by atoms in coating
OR some relevant
A partial discussion of two areas. information in a simple • Atoms de-excite/lose energy
form. The text is usually • Atomic electrons move to lower energy levels
1 Only one area covered, and that legible. SP&G allow • Electrons move to ground state via other energy levels
partially. meaning to be derived • Emitted radiation consists of (a range of) lower photon
although errors are energies/frequencies or longer wavelengths
sometimes obstructive.
0 No relevant information
Total 11
How to answer it
LEDs, Photon Emission and Fluorescence Study Guide
What this question tests
This question assesses your understanding of quantum physics concepts, specifically photon energy calculations (E = hc/λ), interpreting graphical data relating emission rates to current, and explaining atomic processes such as excitation and de-excitation during fluorescence. You will need to combine multi-step calculations with structured written communication.
Calculating Power Output from Photon Emission Rate
📐 Step-by-Step Calculation
- Recall the photon energy equation: E = hc / λ
- Substitute values:
h = 6.63 × 10⁻³⁴ J s
c = 3.00 × 10⁸ m s⁻¹
λ = 440 nm = 440 × 10⁻⁹ m
E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / (440 × 10⁻⁹) = 4.52 × 10⁻¹⁹ J - Calculate total power output:
Power = Energy per photon × Rate of emission
P = 4.52 × 10⁻¹⁹ × 3.0 × 10¹⁶ = 0.01356 W (rounds to 0.014 W)
❌ Common Errors & Traps
- Unit Prefixes: Forgetting to convert nanometres (nm) to metres using × 10⁻⁹.
- Power of Ten Errors: Mismanaging indices when multiplying the per-photon energy by the emission rate. (Note: Examiners condoned minor POT errors on the first mark if carried forward logically, but final answers must match).
Evaluating the Student's Claim Using Graph Analysis
💡 Key Knowledge & Method
To determine if the red LED can have twice the power output of the blue LED (0.028 W):
- Red light has a longer wavelength (660 nm) than blue light (440 nm), meaning red photons carry less energy.
- Calculate the required emission rate for the red LED to achieve twice the blue LED's power, or calculate the maximum possible power of the red LED using Figure 12.
🧠 Exam Technique
This is a synoptic "deduce whether" question. You must explicitly reference calculations and data from the graph to earn full marks.
- Max emission rate from Figure 12 = 6.9 × 10¹⁶ s⁻¹ at 60 mA.
- Required emission rate for double power = 9.0 × 10¹⁶ s⁻¹.
- Conclusion: Claim is incorrect because the maximum emission rate/current required exceeds the limits shown on the graph.
Comparing Wavelengths and Explaining Fluorescence
✅ Core Marking Areas (A, B, and C)
Top-level responses must cover three distinct areas clearly and logically:
- Area A (Wavelength Comparison): The paint emits longer wavelengths than the incident light (both red and blue LEDs will produce longer wavelengths/lower frequency light upon re-emission).
- Area B (Excitation Process): Photons from the LEDs are absorbed by atoms in the coating. Atomic electrons gain energy and are promoted to higher energy levels (excitation).
- Area C (De-excitation Process): Electrons drop back down to lower energy levels (de-excitation), releasing energy as emitted photons. Because some energy is lost through intermediate steps, the emitted photons have lower energy, resulting in longer wavelengths.
❌ Quality of Written Communication (QWC) Traps
- Confusing absorption/emission directions (stating electrons move down to absorb or up to emit).
- Vague descriptions of fluorescence without mentioning energy level transitions or discrete photon packets.
- Failing to explicitly compare the emitted wavelengths against both starting LED wavelengths.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.