CBSE Class 12 Physics Semiconductor Electronics Worksheet Set A

Question. The refractive index is greater for
a) light of low frequency
b) light of shorter wavelength
c) lighter of greater wavelength
d) All the above
Answer. B

Question. When a ray is refracted ,which of the following does not change
a) frequency
b) wavelength
c) velocity
d) amplitude
Answer. A

Question. The focal length of an equi convex lens in air is equal to either of its radii of curvature. The refractive index of the material of the lens is
a) 4/3
b) 2.5
c) 1.25
d) 1.5
Answer. D

Question. If the critical angle for a material to air is 300 , then the refractive index of the material will be
a) 1
b) 1.5
c) 2
d) 2.5
Answer. C

Question. A ray of light travels from vacuum to a medium of refractive index n. If the angle of incidence is found to be twice angle of refraction, then angle of incidence is
a) cos-1 (n/2)
b) 2 cos-1 (n/2)
c) sin-1 (n/2)
d) 2 sin-1 (n/2)
Answer. B

Question. In a compound microscope the intermediate image is
a) virtual, inverted and magnified
b) real, inverted and diminished
c) virtual erect and magnified
d) real , inverted and magnified
Answer. D

Question. An endoscope is employed by a physician to view the internal parts of a body organ. It is based on the principle of
a) Refraction
b) Reflectio
c) TIR
d) Dispersion
Answer. C

Question. A prism of refractive index √2 has a refracting angle of 600. At what angle a ray must be incident on it so that it suffers a minimum deviation
a) 45
b) 60
c) 90
d) 180
Answer. A

Question. A lens acts as converging in air and diverging when immersed in water.
The refractive index of lens is ( nw=1.33 )
a) 1
b) below 1.33
c) greater than 1.33
d) less than unity
Answer. B

Question. A convex lens is used to form a real inverted image of an illuminated object. If the top half of the lens is covered with a black paper then the
a) bottom half of the image will vanish
b) top half of the image will vanish
c) brightness of the image will decrease
d) magnification of image will decrease
Answer. C

Question. When a ray of light incident normally on one refracting surface of an equilateral prism of refractive index 1.5 then
a) emergent ray is deviated by 300
b) emergent ray is deviated by 450
c) emergent ray just grazes the second refracting surface
d) the ray undergoes TIR at the second refracting surface
Answer. D

Assertion Reason type Question

Option A: Both Assertion and reason are right and reason is the correct explanation of assertion.
Option B: Both Assertion and reason are right and reason is not correct explanation of assertion.
Option C : Assertion is right but reason is wrong
Option D: Both Assertion and reason are false

Question. Assertion: Power of a lens is inversely proportional to its focal length
Reason: focal length is measured in metres whereas power of a lens in dioptre
Answer. B

Question. Assertion: focal length of lens varies when it is immersed in liquids
Reason: focal length of lens depends on refractive index of medium in which it is placed
Answer. A

Question. Assertion: A glass slab is placed over different colored dots ( violet to red) and viewed through it, then violet dot appeared to be raised more compared to red dot.
Reason: Red has least deviation than violet color
Answer. A

Question. Assertion: Magnifying power of a telescope depends on its focal lengths of objective and eyepiece
Reason: In telescope focal length of eyepiece is greater than that of objective
Answer. C

Question. Assertion: A convex can act as divergent lens
Reason: when a convex lens placed in medium of higher refractive index, it behaves like concave lens
Answer. A

1 MARK QUESTIONS

Question. The focal length of an equiconvex lens is equal to the radius of curvature of either face. What is the value of refractive index of the material of the lens?
Answer. N=sini/sinr=v1/v2,
nA/nB=sinrB/sinrA=𝑣𝐵/𝑣𝐴
𝑟_𝐴<𝑟_𝐵 , 𝑣_𝐴<𝑣_𝐵
N=1.5
1𝑓=(𝑛−1)2𝑅, f=R, (n-1)=1/2

Question. A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens ?
Answer. Convex lens
1/𝑓_𝑚=(𝑛_𝑔/𝑛_𝑚−1)(−2/𝑅), nm>ng, so fm =+ve

Question. A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab which colour will emerge out first from it?
Answer. red

Question. Does the magnifying power of a microscope depend on the colour of the light used? Justify your answer.
Answer. Yes. M=𝐿/𝑓_𝑜 x(1+𝐷/𝑓_𝑒), fαλ

2 MARKS QUESTIONS

Question. Calculate the radius of curvature of an equi-concave lens of refractive index 1.5, when it is kept in a medium of refractive index 1.4, to have a power of –5D.
Answer. 1/𝑓 =(n-1)2/𝑅 , R=2.86cm

Question. An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and an eye piece is 36 cm and the final image is formed at infinity. Calculate the focal length of the objective and the focal length of the eye piece?
Answer. Magnification m = f0 / fe = 5
f0 = 5 fe length of the tube, L = f0 +fe=36cm
fe=6cm & fo=30cm

Question. The radii of curvature of both the surfaces of a lens are equal. If one of the surfaces is made plane by grinding, then will the focal length of lens change? Will the power change?
Answer. 1/𝑓 =(n-1)2/𝑅, f=R/2(n-1)
𝑓𝑜𝑟 𝑎 𝑝𝑙𝑎𝑛𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑅=𝛼
F’=R/(n-1)=2f, f is doubled
P=1/f , p’=p/2, power is halved

Question. When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency. Explain why?
Answer. Reflection and refraction arise through the interaction of incident light with the atomic constituents of matter. Atoms may be viewed as oscillators that take up the frequency of the external agency (light) causing forced oscillations. The frequency of light emitted by a charged oscillator equals its frequency of oscillation. Thus the frequency of scattered light equals the frequency of incident light.

Question. What is total internal reflection? Under what conditions does it occur? Find a relation between critical angle and refractive index and name one phenomenon which is based on total internal reflection.
Answer. Reflection of a light by a substance through which light can pass is called total internal reflection, it occurs when the angle made by the incidence ray is more than critical angle. critical angle is that angle which is made by incidence ray with normal above which the total internal reflection will take place. When the angle of refraction is equal to 90°, the angle of incidence is called the critical angle, at any angle of incidence greater than the critical angle, the light cannot pass through the surface - it is all reflected. Optical fiber works on total internal reflection

Question. Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.
Answer. Reflecting telescope is preferred over refracting telescope because
(a) No chromatic aberration, because mirror is used.
(b) Spherical aberration can be removed by using a parabolic mirror.
(c) Image is bright because no loss of energy due to reflection.
(d) Large mirror can provide easier mechanical support.

CASE BASED QUESTION: 

1.A prism is a portion of a transparent medium bounded by two plane faces inclined to each other at a suitable angle. A ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its original path. The angle of deviation of a prism is, δ = (μ- 1) A, through which a ray deviates on passing through a thin prism of small refracting angle A.
If μ is refractive index of the material of the prism, then prism formula is,μ= [sin (A+δm/2)]sinA/2

Question. For which colour, angle of deviation is minimum?
(a) Red
(b) Yellow
(c) Violet
(d) Blue
Answer. A

Question. When white light moves through vacuum
(a) all colours have same speed
(b) different colours have different speeds
(c) violet has more speed than red
(d) red has more speed than violet.
Answer. A

Question. The deviation through a prism is maximum when angle of incidence is
(a) 45°
(b) 70°
(c) 90°
(d) 60°
Answer. C

Question. A ray of light falling at an angle of 50° is refracted through a prism and suffers minimum deviation. If the angle of prism is 60°, then the angle of minimum deviation is
(a) 45°
(b) 75°
(c) 50°
(d) 40°
Answer. D

2.A compound microscope is an optical instrument used for observing highly magnified images of tiny objects. Magnifying power of a compound microscope is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object, when both the final image and the objects are situated at the least distance of distinct vision from the eye. It can be given that: M=MO x Me , where Me is the magnification produced by the eyepiece and Mo is the magnification produced by the objective lens. In the case of telescope , its magnification is given by the ratio of focal length of objective lens to eye piece for normal adjustment condition.

Question. A compound microscope with an objective of 1 cm focal length and an eyepiece of 2 cm focal length has a tube length of 20 cm. Then the magnifying power of the microscope if the final image is formed at the near point of eye
(a) 280
(b) 270
(c) 255
(d) 300
Answer. B

Question. The final image in an astronomical telescope with respect to object is
(a) virtual erect
(b) real erect
(c) real and inverted
(d) virtual and inverted
Answer. D

Question. The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. Then the focal lengths of lenses ?
(a) 10 cm, 10 cm
(b)15 cm, 5 cm
(c) 18 cm , 2 cm
(d) 11 cm , 9 cm
Answer. C

Question. The intermediate image formed by the objective of a compound microscope is
(a) real, inverted and magnified
(b) real, erect and magnified
(c) virtual, erect and magnified
(d) virtual, inverted and magnified
Answer. A

CBSE Class 12 Physics Semiconductor Electronics Worksheet Set A 

Very Short Answer

Question. What are energy bands?
Answer. Different energy levels with the continuous energy variations form what are called energy bands.

Question. Where we can use the diodes?
Answer. Diodes can be used for rectifying an ac voltage

Question. What is the use of cathode ray tubes?
Answer. Cathode ray tubes used in television and computer monitors.

Question. What is valence band?
Answer. The energy band which includes the energy levels of the valence electrons is called valence band.

Question. What is conduction band?
Answer. The energy band above the valence band is called conduction band.

Short Answer

Question. What are the advantages of light emitting diode?
Answer. Advantages of the light emitting diode are:
1. Low operational voltage and less power.
2. Long life and ruggedness.
3. Fast on-off switching capability.
4. Fast action and no warm-up time required.

Question. What do you understand by the band theory of solids?
Answer. Band theory in solid-state states that the theoretical model describing the states of electrons, in solid materials, that can have values of energy only within certain specific ranges. The behaviour of an electron in a solid is related to the behaviour of all other particles around it.

Question. Define p-n junction?
Answer. p-n junction is the key to all the semiconductor devices. When such a junction is made a depletion layer is formed consisting of the immobile ion-cores devoid of their electrons or holes. p-n junctions have been used to obtain many photonic or optoelectronics devices.

Question. How half wave rectifier is different from full wave rectifier?
Answer. Half wave rectifier is an electronic circuit which converts only one-half of the AC cycle into pulsating DC. It utilizes only half of AC cycle for the conversion process whereas full wave rectifier is an rectifier is an electronic circuit which converts entire cycle of AC into pulsating DC.

Question. What do you mean by the zener diode?
Answer. A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. Zener diode consists of two terminals: cathode and anode.In zener diode, electric current flows from both anode to cathode and cathode to anode.The symbol of zener diode is similar to the normal p-n junction diode, but with bend edges on the vertical bar.

Long Answer

Question. Explain digital electronics and logic gates?
Answer. Digital electronics or digital circuits are electronics that operate on digital signals. Digital techniques are helpful because it is a lot easier to get an electronic device to switch into one of a number of known states than to accurately reproduce a continuous range of values. A digital circuit is typically constructed from small electronic circuits called logic gates that can be used to create combinational logic. Each logic gate is designed to perform a function of Boolean logic when acting on logic signals. The output of a logic gate can, in turn, control or feed into more logic gates. A logic gate is an electronic component that can be used to conduct electricity based on a rule. The output of the gate is the result of applying this rule to one or more inputs. These logic gates can be realised using semiconductor devices. There are five common types of logic gates are used NOT, AND, OR, NAND, NOR.
1. NOT gate: this is the most basic gate with one input and one output. A NOT gate performs logical negation on its input. In other words, if the input is true, then the output will be false. The standard NOT gate is given a symbol whose shape is of a triangle pointing to the right with a circle at its end. This circle is known as an inversion bubble.
2. OR gate: The OR gate is a digital logic gate that implements logical disjunction it behaves according to the adjacent truth table. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is high, a LOW output (0) results.
3. AND gate: The AND gate is a basic digital logic gate that implements logical conjunction it behaves according to the truth table to the right. A HIGH output (1) results only if all the inputs to the AND gate are HIGH (1). If none or not all inputs to the AND gate are HIGH, a LOW output results.
4. NAND gate: A NAND gate (NOT-AND) is a logic gate which produces an output which is false only if all its inputs are true; thus its output is complement to that of an AND gate. A LOW (0) output results only if all the inputs to the gate are HIGH (1), if any input is LOW (0), a HIGH (1) output results.
5. NOR gate: A NOR gate is a logic gate which gives a positive output only when both inputs are negative. Like NAND gates, NOR gates are so-called universal gates that can be combined to form any other kind of logic gate.

Question. State the difference between intrinsic semiconductor and extrinsic semiconductor?
Answer. 
1. Semiconductor in a pure form is called intrinsic semiconductor whereas semiconductor which are doped with the impurity is called extrinsic semiconductor.
2. Intrinsic semiconductors have low electrical conductivity whereas extrinsic conductors have high electrical conductivity.
3. Intrinsic semiconductors have low operating temperature whereas extrinsic semiconductors have high operating temperature.
4. In intrinsic semiconductor density of electron is equal to the density of holes whereas in extrinsic semiconductor density of electron is not equal to the density of the holes.
5. Intrinsic semiconductor dependence on temperature only whereas extrinsic semiconductor dependence on temperature as well as on the amount of impurity.

Question. Write short note on solar cell?
Answer. A solar cell is essential a p-n junction with a large surface area. The N-type material is kept thin to allow light to pass through to the p-n junction. Light travels in packets of energy called photons. The generation of electric current happens inside the depletion zone of the p-n junction. It generates electricity by using sunlight to make electrons hop across the junction between the different flavours of silicon: When sunlight shines on the cell, photons or light particles bombard the upper surface. The photons or yellow blobs carry their energy down through the cell. The solar cell is like an ordinary diode. It consist of silicon, germanium p-n junction with a glass windows on the top surface layer of p-type, the p-type material is made very thin and wide so that the incident light photon may easily reach to p-n junction. Solar cells are also called photovoltaic cells. The working principle of solar cells depends on the photovoltaic effects. Using the photovoltaic effect, a solar cell is a solid-state electrical device (p-n junction) that converts the energy of light directly into electricity (DC). Solar cells are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators and the production of low-cost photovoltaic cells for large scale solar energy is a topic for research.

Question. State the difference between semiconductor and insulator?
Answer. 
1. Semiconductor conducts the electric current less than a conductor whereas insulator does not conduct any current.
2. Semiconductor resistance is high whereas insulator resistance is very high.
3. Semiconductors are formed due to the covalent bonding whereas insulators are formed due to the ionic bonding.
4. A semiconductor has four valence electron in its outermost orbit whereas insulator has eight valence electron in its outermost orbit.
5. Silicon, germanium, boron etc. are the example of semiconductor whereas rubber, glass etc. are the example of insulator.
6. Semiconductor resistivity decreases with the increase in the temperature whereas in insulator it remains constant.

Question. Write short note on photodiodes and light emitting diodes?
Answer. 
Photodiode: A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. The working principle of a photodiode is, when a photon of ample energy strikes the diode, it makes a couple of an electron-hole. So, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Photodiodes are mainly made from gallium arsenide instead of silicon because silicon creates crystal lattice vibrations called phonons when photons are absorbed in order to create electronhole pairs.
Light emitting diode: A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do many different jobs in all kinds of devices. They form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. LEDs are used in applications as diverse as aviation lighting, automotive headlamps,