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Q1
short answer
2 marks
Explain the formation of depletion region in a p-n junction.
Solution
When a p-n junction is formed, holes diffuse from the p-side to the n-side, and electrons diffuse from the n-side to the p-side due to the concentration gradient. This leaves behind uncompensated ionized acceptors in the p-region and ionized donors in the n-region near the junction. These immobile ions create an electric field directed from n to p, which opposes further diffusion of charge carriers. The region depleted of free charge carriers near the junction is called the depletion region.
Q2
short answer
2 marks
(a) (i) Define impact parameter and distance of closest approach for an alpha-particle in Geiger-Marsden scattering experiment. (ii) What will be the value of the impact parameter for scattering angle (I) = 0 and (II) = 180 ?
Solution
(i) Impact parameter: The perpendicular distance of the velocity vector of the alpha-particle from the center of the nucleus when it is far away. Distance of closest approach: The minimum distance from the nucleus to which an alpha-particle, heading directly towards the center, can approach before coming to rest and reversing its direction. (ii) For scattering angle theta = 0, impact parameter b = infinity. For scattering angle theta = 180 degrees, impact parameter b = 0.
Q2
short answer
2 marks
OR: (b) Photoelectric emission occurs when a surface is irradiated with the radiation of frequency (i) v1, and (ii) v2. The maximum kinetic energy of the electrons emitted in the two cases are K and 2K respectively. Obtain the expression for the threshold frequency for the surface.
Solution
Using Einstein's photoelectric equation: K = h*v1 - Phi and 2K = h*v2 - Phi (where Phi is the work function). Subtracting the two gives K = h(v2 - v1). Substituting back: h(v2 - v1) = h*v1 - Phi, which simplifies to Phi = h(2*v1 - v2). Since the threshold frequency v0 = Phi/h, we get v0 = 2*v1 - v2.
Q3
short answer
2 marks
How is an emf generated by a solar cell due to the three basic processes involved ? Explain.
Solution
The three basic processes are: (1) Generation: Electron-hole pairs are created near the junction due to light absorption. (2) Separation: The electric field of the depletion region separates the electrons and holes, forcing electrons to the n-side and holes to the p-side. (3) Collection: The separated charges are collected by metal contacts, creating a potential difference (emf) across the device.
Q4
short answer
3 marks
(a) (i) Depict a plane electromagnetic wave propagating along the x-axis. Write the expressions for its oscillating electric and magnetic fields. (ii) Write three characteristics of electromagnetic waves.
Solution
(i) The electric field E = E0 sin(kx - wt)j and magnetic field B = B0 sin(kx - wt)k. (ii) Characteristics: 1. They do not require a material medium for propagation. 2. They travel at the speed of light in vacuum (c = 3x10^8 m/s). 3. They are transverse waves with oscillating E and B fields perpendicular to each other and the direction of propagation.
Q4
short answer
3 marks
OR: (b) Name the electromagnetic waves which are produced by the following : (i) Radioactive decays of nucleus (ii) Welding arcs (iii) Hot bodies. Write one use each of these waves.
Q5
short answer
3 marks
(a) State the conditions for total internal reflection to take place. (b) A coin is suspended by a thread in the liquid is gradually lowered till it touches the bottom. The apparent depth is determined corresponding to different positions of the coin. (i) Plot a graph showing variation of the apparent depth with the real depth of the coin. (ii) What is the physical significance of the slope of the graph?
Q6
long answer
3 marks
(a) Draw a labelled ray diagram showing the formation of an image by an astronomical refracting telescope in normal adjustment. Hence, obtain the expression for its magnifying power. OR (b) Light of wavelength lambda is incident normally on a slit of width 'a'. (i) Depict the intensity distribution in the diffraction pattern observed. (ii) Obtain the expression for the first secondary maximum from the central maximum.
Q7
short answer
3 marks
A converging lens made of glass (mu = 1.5) has its spherical faces of radii of curvature 10 cm and 20 cm. Find its focal length (a) in air, and (b) when it is immersed in a liquid of refractive index 1.25.
Q8
short answer
3 marks
The energy of a hydrogen atom in the first excited state is 3.4 eV. Find: (a) the radius of this orbit. (Take Bohr radius = 0.53 Angstrom), (b) the angular momentum of the electron in the orbit, (c) the kinetic and potential energy of the electron in the orbit.
Q9
short answer
3 marks
(a) Depict the variation of the potential energy of a pair of nucleons with the separation between them. (b) Imagine the fission of a Fe 56 26 into two equal fragments of Al28 13 nucleus. Is the fission energetically possible ? Justify your answer by working out Q value of the process. Given : m Fe 56 26 = 55·93494 u, m Al 28 13 = 27·98191 u.
Q10
short answer
3 marks
Find the ratio of the de Broglie wavelengths associated with an alpha particle and a proton, if both (a) have the same speeds, (b) have the same kinetic energy, (c) are accelerated through the same potential difference.
Q11
short answer
3 marks
With the help of a circuit diagram, explain the working of a p-n junction diode as a full-wave rectifier. Also draw its input and output waveforms.
Reading Passage
The British physicist Thomas Young explained the interference of light using the principle of superposition of waves. He observed the interference pattern on the screen, in his experimental set-double slit experiment. The two slits S1 and S2 were illuminated by light from a slit S. The interference pattern consists of dark and bright bands of light. Such bands are called fringes. The distance between two consecutive bright and dark fringes is called fringe width.
Q14
mcq
The British physicist Thomas Young explained the interference of light using the principle of superposition of waves. He observed the interference pattern on the screen, in his experimental set-double slit experiment. The two slits S1 and S2 were illuminated by light from a slit S. The interference pattern consists of dark and bright bands of light. Such bands are called fringes. The distance between two consecutive bright and dark fringes is called fringe width. If the screen is moved closer to the plane of slits S1 and S2, then the fringe width :
-
A.
will decrease, but the intensity of bright fringe remains the same.
-
B.
will increase, but the intensity of bright fringe decreases.
-
C.
will decrease, but the intensity of bright fringe increases.
-
D.
and the intensity both remain the same.
Q15
mcq
What will happen to the pattern on the screen, when the two slits S1 and S2 are replaced by two independent but identical sources?
-
A.
The intensity of pattern will increase
-
B.
The intensity of pattern will decrease
-
C.
The number of fringes will become double
-
D.
No pattern will be observed on the screen
Q16
mcq
Two sources of light are said to be coherent, when both emit light waves of :
-
A.
same amplitude and have a varying phase difference.
-
B.
same wavelength and a constant phase difference.
-
C.
different wavelengths and same intensity.
-
D.
different wavelengths and a constant phase difference.
Q17
mcq
The fringe width is beta. If the whole set-up is immersed in a liquid of refractive index n, then the new fringe width will be :
-
A.
beta/n
-
B.
beta*n
-
C.
n/beta
-
D.
2*beta
Q18
mcq
The total path difference between two waves meeting at points P1 and P2 on the screen are lambda/2 and 3 lambda/2 respectively. Then :
-
A.
bright fringes are formed at both points.
-
B.
dark fringes are formed at both points.
-
C.
a bright fringe at P1 and dark fringe at P2.
-
D.
a bright fringe at P2 and dark fringe at P1.
