ISC 12th Physics Exam 2025 : Important Questions & Answers for Last-Minute Revision

ISC Class 12 exams are going on and you have very little time left for the Physics Paper-1 exam. Therefore, we are providing important questions in this article. You can study these thoroughly and score good marks in your exams.

To help you make your preparation effective, we have compiled a list of important questions with detailed explanations.

These questions for ISC Class 12 Physics Paper-1 exam cover various topics from the syllabus.

ISC Class 12th Physics Paper-1 Exam 2025 Most Important Questions

Multiple Choice Questions

Que 1. Figure below shows a circuit containing an ammeter A, a galvanometer G, and a plug key K. When the key is closed:

(a) both G and A show deflections.
(b) neither G nor A shows deflection.
(c) G shows deflection but A does not.
(d) A shows deflection but G does not.

Ans. (d) A shows deflection but G does not.

Que 2.  A circular coil having N turns of radius R carrying a current I is used to produce a magnetic field B at its centre O. If this coil is opened and rewound such that the radius of the newly formed coil is 2R, carrying the same current I, what will be the magnetic field at the centre O?

(a) 2B
(b) B
(c) B/2
(d) B/4

Ans. (d) B/4

Que 3.  Magnetic susceptibility of a diamagnetic substance: 

(a) decreases with the increase in its temperature.
(b) is not affected by the change in its temperature.
(c) increases with increase in its temperature.
(d) first increases then decreases with increase in its with temperature.

Ans. (b) is not affected by the change in its temperature.

Que 4. The de Broglie wavelength of an electron in the first Bohr’s orbit of hydrogen atom is equal to

(a) diameter of the first orbit.
(b) circumference of the first orbit.
(c) radius of the second orbit.
(d) h/2π.

Ans. (b) circumference of the first orbit.

Que 5. An ideal PN junction diode offers: 

(a) zero resistance in forward as well as reverse bias.
(b) infinite resistance in forward as well as reverse bias.
(c) zero resistance in forward, but infinite resistance in reverse bias.
(d) infinite resistance in forward, but zero resistance in reverse bias.

Ans. (c) zero resistance in forward, but infinite resistance in reverse bias.

ISC Class 12 Study Material

Que 6. Given below are two statements marked Assertion and Reason. Read the two statements and choose the correct option.

Assertion: An astronomical telescope has an objective lens having large focal length.
Reason: Magnifying power of an astronomical telescope varies directly with focal length of the objective lens. 

(a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.
(c) Assertion is false and Reason is true.
(d) Both Assertion and Reason are false.

Ans. (a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

Que 7. Given below are two statements marked, Assertion and Reason. Read the two statements and choose the correct option.

Assertion: If critical angle of glass-air pair (μg = 3/2) is θ₁ and that of water-air pair (μw = 4/3) is θ₂, then the critical angle for the water-glass pair will lie between θ₁ and θ₂.
Reason: A medium is optically denser if its refractive index is greater.

(a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.
(c) Assertion is false and Reason is true.
(d) Both Assertion and Reason are false.

Ans. (c) Assertion is false and Reason is true.

Short Answer Questions

Que 8. Current I flowing through a metallic wire is gradually increased. Show graphically how heating power (P) developed in it varies with the current (I).

Ans.

Que 9. Explain why core of a transformer is always laminated

Ans. to reduce the loss of power/energy due to eddy currents

Que 10. Why does a car driver use a convex mirror as a rear-view mirror?

Ans. since it forms diminished images of objects behind and hence provides a wide field of view.

Que 11. Represent diagrammatically how the incident planar wavefronts of wavelength λ pass through an aperture of size d, when d is approximately equal to λ.

Ans. diagram showing diverging spherical wavefront

Que 12. What is meant by “Dual nature of matter”?

Ans. Matter exhibits both particle and wave nature.

Que 13. What happens when an electron collides with a positron?

Ans. They annihilate each other producing gamma ray photons.

Que 14. What energy conversion takes place in a solar cell?

Ans. Solar/ light energy to electrical energy

Que 15. (i) The focal length of a double convex lens is equal to the radius of curvature of either surface. What is the refractive index of its material?

Ans. 1/f = (n-1) (1/r1 + 1/r2)
1/f = (n-1) (1/f + 1/f)
(n-1) = ½
So, n = 1·5 

(ii) What is meant by a thin prism?

Ans. It is that prism whose refracting angle is small i.e., less than 4°

Que 16. (i) Name the electromagnetic radiation that has been used in obtaining the image below. 

Ans. X rays

(ii) What is the wavelength range of electromagnetic radiation used in radio broadcast?

Ans. Greater than 1m.

Que 17. How does stopping potential in photoelectric emission vary if

(i) the intensity of the incident radiation increases? (Recall)

Ans. The stopping potential is independent of the intensity of the incident photon.

(ii) the frequency of incident radiation decreases?

Ans. The stopping potential will change with the frequency of the incident photon.

Long answer questions

Que 18. A uniformly charged conducting spherical shell for the points outside the shell behaves as if the entire charge of the shell is concentrated at its centre”. Show this with the help of a proper diagram and verify this statement.

Ans. A shell of radius R is carrying uniformly distributed charge of charge density σ.

Electric field E at point P at a distance r from the centre of a uniformly charged spherical shell:

Consider a Gaussian surface to be a sphere of radius r >R and with centre O, passing through P.
Using the principles of spherical symmetry, E and area vector ΔS at every point are parallel, flux through each of the area element.
ΔΦ = E·ΔS

Summing over all
ΔS, Φ = E × 4 π R 2 .

The charge enclosed is
σ × 4 π R 2 . By Gauss’ law,

This formula is exactly similar to the field produced by a point charge q placed at the centre O, that is, as per Coulomb’s law.

Que 19. (i) A virologist studies details of a virus with the help of an instrument. Name the instrument used by him.

Ans. Compound microscope.

(ii) Draw a labelled ray diagram of an image formed by this instrument, assuming 

(a) a small upright object.
(b) image lies at least distance of distinct vision

Ans. 

Que 20. (i) The magnetic field through a single loop of wire, 12 cm in radius and 8·5 Ω resistance, changes with time as shown in graph below (Figure 9). The magnetic field is perpendicular to the plane of the loop.

(a) Find the induced emf for the time intervals 0 to 2·0 s, 2·0 to 4·0 s, and 4·0 to 6·0 s. 

Ans. 

(b) Hence, plot induced current as a function of time.

Ans. 

Que 21. Three students, X, Y, and Z performed an experiment for studying the variation of a.c. with frequency in a series LCR circuit and obtained the graphs as shown below. They all used

• an AC source of the same emf and
• inductance of the same value

(a) Who used minimum resistance? 

Ans. Resistance used by X is the least and resistance used by Z is the maximum.

(b) In which case will the quality Q factor be maximum?

Ans. Q maximum for X

(c) What did the students conclude about the nature of impedance at resonant frequency (f0)? 

Ans. at resonance impedance is equal to ohmic resistance.

(d) An ideal capacitor is connected across 220V, 50Hz, and 220V, 100Hz supplies. Find the ratio of current flowing through it in the two cases.

Ans. In a capacitor, the current is dependent directly on the frequency