Date: 20/05/2022

SET No. 3

Question Paper Code

55/5/3

Time: 2 hrs.

Class-XII

Max. Marks: 35

PHYSICS (Theory)

Term-II (CBSE-2022)

GENERAL INSTRUCTIONS

Please read the following instructions carefully and follow them:

1. This question paper contains 12 questions. All questions are compulsory.
2. This question paper is divided into THREE sections, Section A, B and C.
3. Section A - Question number 1 to 3 are of 2 marks each.
4. Section B - Question number 4 to 11 are of 3 marks each.
5. Section C - Question number 12 is a case study based question of 5 marks.
6. There is no overall choice in the question paper. However, internal choice has been provided in some of the questions. Attempt any one of the alternatives in such questions.
7. Use of log tables is permitted, if necessary, but use of calculators is not permitted.

SECTION-A

1. Give two differences between a half wave rectifier and a full wave rectifier.

[2] Sol. Two differences are:

1. Centre – tapping of the secondary windings is needed in a full-wave rectifier while no such tapping is needed in a half-wave rectifier.
2. For a full-wave rectifier, voltage rectified by each diode (of the total two diodes) is only half the total secondary voltage while for a half-wave rectifier, the total secondary voltage is rectified.
3. Why a photo-diode is operated in reverse bias whereas current in the forward bias is much larger than that in the reverse bias? Explain.

Mention its two uses. [2]

Sol. A photodiode is operated in reverse bias because it is easier to observe the change in current with change in the light intensity.

Uses:

1. Photodetector to detect optical signals.
2. Measurement of the intensity of light.
3. (a) What results do you expect if <math>\alpha</math>-particle scattering experiment is repeated using a thin sheet of hydrogen in place of a gold foil? Explain. (Hydrogen is a solid at temperature below 14 K) [2]

OR

1. Why it is the frequency and not the intensity of light source that determines whether emission of

photoelectrons will occur or not? Explain. [2]

Sol. (a) If <math>\alpha</math>-particle scattering experiment is repeated using a thin sheet of hydrogen in place of a gold foil, the scattering angle would not be large enough. This will happen because mass of hydrogen is less than the mass of incident <math>\alpha</math>-particle (around 4 times less).

OR

1. Frequency decides the energy of a photon while intensity decides the number of photons incident. So, for emission of photoelectrons where some minimum energy is required, the deciding factor is frequency and not intensity.

SECTION-B

1. (a) (i) Define SI unit of power of a lens.

[3]

1. A plano convex lens is made of glass of refractive index 1.5.

The radius of curvature of the convex surface is 25 cm.

(ii.i) Calculate the focal length of the lens.

(ii.ii) If an object is placed 50 cm in front of the lens, find the nature and position of the image formed.

OR

1. A slit of width 0.6 mm is illuminated by a beam of light consisting of two wavelengths 600 nm and 480 nm. The diffraction pattern is observed on a screen 1.0 m from the slit. Find [3]
2. The distance of the second bright fringe from the central maximum pertaining to light of 600 nm.
3. The least distance from the central maximum at which bright fringes due to both the wavelengths coincide.