# CBSE Class 12 Physics Sample Papers

Marks: 25
Duration: 1 hour

##### Section A: Each question carries 1 mark

Question 1: A Concave mirror is held in water. What should be the change in focal length of the mirror?

Question 2: To which part of the electromagnetic spectrum does a wave of wavelength 3*10^(-10) m belong?

Question 3: Define the term “stopping potential” in relation to photoelectric effect.

##### Section B: Each question carries 2 marks

Question 1: Sky waves are not used in transmitting TV signals, why? state two factors by which the range of transmission of TV signals can be increased.

Question 2: An alternating current in ampere is given by I = 50 sin (400 πt + Ф). Find the frequency and the rms value of current.

Question 3: With the help of a circuit diagram to explain how a photodiode is biased. Draw its characteristics curves for different illumination intensities.

Question 4: Draw a neat labeled ray diagram of an astronomical telescope. Write the expression for magnifying power.

Question 5: A semiconductor has equal electron and hole concentration of 5 x 10^8 m^(-3). On doping with certain impurity, electron concentration increases to 8 x 10^(12) m^3.
(i) Identify the new semiconductor obtained after doping.
(ii) Calculate the new hole concentration.

##### Section C: Each question carries 3 marks

Question 1: Define the term
(i) cut-off voltage and
(ii) threshold frequency in relation to the phenomenon of photoelectric effect. Using Einstein’s photoelectric equation show how the Planck’s constant and threshold frequency for a given photosensitive material can be determined with the help of a suitable plot/graph.

Question 2: Give reason for the following.
(i) Why is ground wave transmission of signals restricted to a frequency of 1500 kHz?
(ii) How does the length of a dipole antenna vary with increase of the frequency of the Carrier wave?
(iii) How does the effective power radiated by an antenna vary with wavelength?

##### Section D: Question carries 6 marks

Question: Define the term resolving power of an astronomical telescope. How does it get affected on
(i) increasing the aperture of the objective lens?
(ii) increasing the wavelength of light used?
(iii) increasing the focal length of the objective lens?