Probability Engine · PHY113

Physics (BSc CSIT, PHY113): the questions likely to come

68 analyzed questions from 7 past papers (2074-2081), grouped by syllabus unit — each with its probability, how often it's been asked, and where to study the answer.

Papers analyzed

2074-2081

Analyzed questions

across 8 syllabus units

Very likely units

high-probability topics

Units = 80% of marks

study these first

Model answers for this subject are being written. Every question links to its original paper so you can study from the source meanwhile.

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U6 · Q1/17 · 208010 marks

Laser and Fiber Optics

Explain the propagation of light through an optical fiber. Derive expressions for the acceptance angle and numerical aperture, and discuss types of fibers.

28%

Occasional to appearAppeared in 2 of the last 2 board papers

Seen in

How well do you know this?rating moves you on

MODEL ANSWERU6 · 10 marks

Light Propagation in an Optical Fiber

An optical fiber has a cylindrical core of refractive index $n_1$ surrounded by a cladding of slightly lower index $n_2$ ( $n_1 > n_2$ ). Light launched into the core travels by repeated total internal reflection (TIR) at the core–cladding boundary, since the ray strikes the interface at an angle greater than the critical angle $\theta_c$ where $\sin\theta_c = n_2/n_1$ . The signal is thus guided along the fiber with very low loss.

Acceptance angle

Let a ray enter from a medium of index $n_0$ (usually air, $n_0=1$ ) at angle $\theta_0$ to the fiber axis and refract into the core at angle $\theta_r$ . At the core–cladding interface it strikes at angle $(90^\circ-\theta_r)$ . For TIR this must equal or exceed $\theta_c$ :

\sin\theta_r \le \cos\theta_c = \sqrt{1-\sin^2\theta_c} = \sqrt{1-\frac{n_2^2}{n_1^2}}

At the input face, Snell's law gives $n_0\sin\theta_0 = n_1\sin\theta_r$ . Substituting the limiting condition,

n_0\sin\theta_{0,\max} = n_1\sqrt{1-\frac{n_2^2}{n_1^2}} = \sqrt{n_1^2 - n_2^2}

The acceptance angle is

\boxed{\theta_{0,\max} = \sin^{-1}\!\left(\frac{\sqrt{n_1^2-n_2^2}}{n_0}\right)}

Rays entering within the cone of half-angle $\theta_{0,\max}$ are guided.

Numerical aperture

\boxed{NA = n_0\sin\theta_{0,\max} = \sqrt{n_1^2 - n_2^2}}

In terms of the fractional index difference $\Delta = (n_1-n_2)/n_1$ , $NA \approx n_1\sqrt{2\Delta}$ . $NA$ measures the light-gathering capacity of the fiber.

Types of fibers

Step-index single-mode fiber: very small core (~8–10 µm); supports only one mode; negligible intermodal dispersion; used for long-haul, high-bandwidth links.
Step-index multimode fiber: larger core (~50–62.5 µm); supports many modes; suffers intermodal dispersion; used for short distances.
Graded-index multimode fiber: core index decreases gradually from axis to edge, so rays follow curved paths and arrive nearly together, greatly reducing intermodal dispersion.

AI-generated answer · unverifiedView in 2080 paper →

U6 · Question 1 of 17

Question Priority · U6ranked by appearance likelihood — study top-down

Laser and Fiber Optics

Analyzed next40%

★ TOP PICK

Explain the propagation of light through an optical fiber. Derive expressions for the acceptance angle and numerical aperture, and discuss types of fibers.

10 marksSEEN IN

28%

Explain the principle of laser action. Describe the construction and working of a semiconductor laser with a neat diagram.

10 marksSEEN IN

21%

Write short notes on the properties of laser light.

5 marksSEEN IN

40%

Define acceptance angle and numerical aperture of an optical fiber.

5 marksSEEN IN

34%

Explain the construction and working of a He-Ne laser. Compare it with a semiconductor (diode) laser.

10 marksSEEN IN

17%

Explain the construction and working of a Ruby laser with an energy-level diagram.

10 marksSEEN IN

14%

Distinguish between single-mode and multi-mode optical fibers.

5 marksSEEN IN

27%

What is a laser? Explain the principle of stimulated emission, population inversion and the construction and working of a He-Ne laser.

10 marksSEEN IN

11%

Explain the principle of light propagation through an optical fiber.

5 marksSEEN IN

19%

Define total internal reflection and critical angle.

5 marksSEEN IN

19%

Write short notes on population inversion.

5 marksSEEN IN

19%

Explain the V-number of an optical fiber.

5 marksSEEN IN

17%

Distinguish between step-index and graded-index fibers.

5 marksSEEN IN

17%

Distinguish between LED and LASER.

5 marksSEEN IN

15%

Define attenuation and dispersion in optical fibers.

5 marksSEEN IN

15%

Write short notes on spontaneous and stimulated emission.

5 marksSEEN IN

14%

Explain the working of a step-index fiber.

5 marksSEEN IN

12%

03The mock

Sit a probable paper

A full mock exam built from the most likely questions, mirroring the real paper's structure. Every slot is a real past question.

Most Probable Paper

Mirrors the real structure · 60 marks · based on 7 past papers

Section A: Long Answer QuestionsAttempt any TWO questions.

1.
Explain the propagation of light through an optical fiber. Derive expressions for the acceptance angle and numerical aperture, and discuss types of fibers.
[10 marks]
Laser and Fiber OpticsVery likelyfrom 2080 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Laser and Fiber Optics) appears in 100% of years.
2.
Discuss the theory of Newton's rings. Derive expressions for the radii of bright and dark rings and explain the experimental determination of the wavelength of light.
[10 marks]
Physical Optics — InterferenceVery likelyfrom 2080 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Physical Optics — Interference) appears in 100% of years.
3.
Derive Maxwell's electromagnetic wave equation in free space and show that electromagnetic waves travel with the speed of light.
[10 marks]
Electromagnetism and Electromagnetic WavesVery likelyfrom 2079 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Electromagnetism and Electromagnetic Waves) appears in 100% of years.

Section B: Short Answer QuestionsAttempt any EIGHT questions.

1.
Write short notes on the properties of laser light.
[5 marks]
Laser and Fiber OpticsVery likelyfrom 2081 paper →
This question has recurred in 3 of 7 years; so far only in internal assessments, not the board; and its topic (Laser and Fiber Optics) appears in 100% of years.
2.
Define acceptance angle and numerical aperture of an optical fiber.
[5 marks]
Laser and Fiber OpticsVery likelyfrom 2081 paper →
This question has recurred in 3 of 7 years; so far only in internal assessments, not the board; and its topic (Laser and Fiber Optics) appears in 100% of years.
3.
Explain Fraunhofer diffraction at a single slit.
[5 marks]
Physical Optics — Diffraction and PolarizationVery likelyfrom 2081 paper →
This question has recurred in 3 of 7 years; so far only in internal assessments, not the board; and its topic (Physical Optics — Diffraction and Polarization) appears in 100% of years.
4.
Distinguish between single-mode and multi-mode optical fibers.
[5 marks]
Laser and Fiber OpticsVery likelyfrom 2081 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Laser and Fiber Optics) appears in 100% of years.
5.
Explain the principle of light propagation through an optical fiber.
[5 marks]
Laser and Fiber OpticsVery likelyfrom 2077 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Laser and Fiber Optics) appears in 100% of years.
6.
State Coulomb's law and write it in vector form.
[5 marks]
ElectrostaticsVery likelyfrom 2081 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Electrostatics) appears in 100% of years.
7.
Derive the capacitance of a parallel plate capacitor with a dielectric slab inserted between the plates.
[5 marks]
ElectrostaticsVery likelyfrom 2081 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Electrostatics) appears in 100% of years.
8.
Explain the conditions for observing interference fringes.
[5 marks]
Physical Optics — InterferenceVery likelyfrom 2080 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Physical Optics — Interference) appears in 100% of years.
9.
State and explain the Biot-Savart law.
[5 marks]
Electromagnetism and Electromagnetic WavesVery likelyfrom 2079 paper →
This question has recurred in 2 of 7 years; so far only in internal assessments, not the board; and its topic (Electromagnetism and Electromagnetic Waves) appears in 100% of years.

04The receipts

Behind the numbers

The raw evidence the predictions are computed from: marks per unit per year, syllabus weights, trends, and coverage.

Show the heatmap, topic table and coverage analysis

The receipt: marks per unit, per year

Each row is a syllabus unit, each column an exam year, each cell the marks that unit earned that year. Click any cell to see the actual questions behind it.

Marks:nonefew → many

2074

2075

2077

2078

2079

2080

2081

Total

U6Laser and Fiber Optics

150

U7Electrostatics

110

U3Physical Optics — Interference

U8Electromagnetism and Electromagnetic Waves

U4Physical Optics — Diffraction and Polarization

U1Oscillation

U2Wave Motion and Acoustics

U5Geometrical Optics (Lenses)

#	Syllabus unit	Probability	Appeared	Avg marks	Syllabus weight	Exam vs syllabus	Trend	Questions
1	U6Laser and Fiber Optics	Very likely100%	2074 2075 2077 2078 2079 2080 2081	21.4	9%4 lecture hrs	Over-examinedexam 29% · syllabus 9%	Rising	5 recurring17 total
2	U7Electrostatics	Very likely100%	2074 2075 2077 2078 2079 2080 2081	15.7	13%6 lecture hrs	Over-examinedexam 21% · syllabus 13%	Steady	2 recurring14 total
3	U3Physical Optics — Interference	Very likely100%	2074 2075 2077 2078 2079 2080 2081	11.4	13%6 lecture hrs	Balancedexam 15% · syllabus 13%	Steady	2 recurring9 total
4	U8Electromagnetism and Electromagnetic Waves	Very likely100%	2074 2075 2077 2078 2079 2080 2081	10.7	13%6 lecture hrs	Balancedexam 14% · syllabus 13%	Steady	2 recurring12 total
5	U4Physical Optics — Diffraction and Polarization	Very likely100%	2074 2075 2077 2078 2079 2080 2081	10	16%7 lecture hrs	Balancedexam 13% · syllabus 16%	Steady	2 recurring8 total
6	U1Oscillation	Very likely100%	2074 2075 2077 2078 2079 2080 2081	5	16%7 lecture hrs	Under-examinedexam 7% · syllabus 16%	Steady	none repeat7 total
7	U2Wave Motion and Acoustics	Occasional14%	2075	5	11%5 lecture hrs	Under-examinedexam 1% · syllabus 11%	Fading	none repeat1 total
8	U5Geometrical Optics (Lenses)	Occasional0%		0	9%4 lecture hrs	Under-examinedexam 0% · syllabus 9%	Steady	None

Study smart, not hard

Drag the slider: studying the top 5 units in priority order covers ~92% of all observed marks.

Units to study5/8

~80% line

Lecture time vs exam marks

Where the exam pays more than the curriculum spends: ● lectures vs ● exam marks, as a share of the whole course. A long teal-leading bar = high-yield unit.

U6Laser and Fiber Optics

9% of lectures → 29% of markshigh yield

U7Electrostatics

13% of lectures → 21% of markshigh yield

U3Physical Optics — Interference

13% of lectures → 15% of marks

U8Electromagnetism and Electromagnetic Waves

13% of lectures → 14% of marks

U4Physical Optics — Diffraction and Polarization

16% of lectures → 13% of marks

U1Oscillation

16% of lectures → 7% of markslow yield

U2Wave Motion and Acoustics

11% of lectures → 1% of markslow yield

U5Geometrical Optics (Lenses)

9% of lectures → 0% of markslow yield

Physics (BSc CSIT, PHY113): the questions likely to come

Light Propagation in an Optical Fiber

Acceptance angle

Numerical aperture

Types of fibers

Laser and Fiber Optics

Sit a probable paper

Most Probable Paper

Behind the numbers

The receipt: marks per unit, per year

Study smart, not hard

Lecture time vs exam marks

2081 B.S.

Physics (BSc CSIT, PHY113)

2080 B.S.

Physics (BSc CSIT, PHY113)

2079 B.S.

Physics (BSc CSIT, PHY113)

2078 B.S.

Physics (BSc CSIT, PHY113)

2077 B.S.

Physics (BSc CSIT, PHY113)

2075 B.S.

Physics (BSc CSIT, PHY113)

2074 B.S.

Physics (BSc CSIT, PHY113)