50278 + Interview Questions in Physics

1.	image of an object approaching a convex mirror of radius of curvature 20 m along its optical axis is observed to move from 25/3m to 50/7m in 30 seconds. What is the speed objective of in km per hour
Answer» image of an object approaching a convex mirror of radius of curvature 20 m along its optical axis is observed to move from 25/3m to 50/7m in 30 seconds. What is the speed objective of in km per hour

Discussion

2.	A conducting wire ab of length l and resistance R, is moving inside a magnetic field B on a pair of fixed vertical rails as shown in the figure. If the mass of the wire is m, then calculate the terminal velocity achieved by the loop.Resistance of the rails are negligible.
Answer» A conducting wire $a b$ of length $l$ and resistance $R$ , is moving inside a magnetic field $B$ on a pair of fixed vertical rails as shown in the figure. If the mass of the wire is $m$ , then calculate the terminal velocity achieved by the loop. Resistance of the rails are negligible.

Discussion

3.	A person brings a mass of 2 kg from A to B. The increase in kinetic energy of mass is 4 J and work done by the person on the mass is −10 J. The potential difference between B and A is (in J/kg)
Answer» A person brings a mass of $2 kg$ from $A$ to $B$ . The increase in kinetic energy of mass is $4 J$ and work done by the person on the mass is $- 10 J$ . The potential difference between $B$ and $A$ is $($ in $J/kg)$

Discussion

4.	A non conducting ring of radius r=10 cm and mass m=500 g which has a total charge Q=5 C distributed uniformly on it. The ring is rotated about its axis with a uniform speed v=0.2π m/s as shown. Find angular momentum of the ring using magnetic moment.
Answer» A non conducting ring of radius $r = 10 cm$ and mass $m = 500 g$ which has a total charge $Q = 5 C$ distributed uniformly on it. The ring is rotated about its axis with a uniform speed $v = 0.2 π m/s$ as shown. Find angular momentum of the ring using magnetic moment.

Discussion

5.	A balloon initially at rest start rising with constant acceleration of 10 m/s2. After 2 s a particle drops from the balloon. After further 2 s, match the following (Take g=10 m/s2 )Table-1Table-2(A) Height of particle from ground is(P) Zero(B) Speed of particle is(Q) 10 SI units(C) Displacement of particle is(R) 40 SI units(D) Acceleration of particle is (S) 20 SI units
Answer» A balloon initially at rest start rising with constant acceleration of $10 {m/s}^{2}$ . After $2 s$ a particle drops from the balloon. After further $2 s$ , match the following (Take $g = 10 {m/s}^{2}$ ) $\begin{matrix} Table-1 & Table-2 (A) Height of particle from ground is & (P) Zero (B) Speed of particle is & (Q) 10 SI units (C) Displacement of particle is & (R) 40 SI units (D) Acceleration of particle is & (S) 20 SI units \end{matrix}$

Discussion

6.	A composite wall having three layers of thickness 0.3 m, 0.2 m and 0.1 m and of thermal conductivity 0.6,0.4 and 0.1 W/m-K respectively, has a surface area of 1 m2. If the inner and outer temperatures of the composite wall are 1840 K and 340 K respectively, find the rate of heat transfer.
Answer» A composite wall having three layers of thickness $0.3 m$ , $0.2 m$ and $0.1 m$ and of thermal conductivity $0.6, 0.4$ and $0.1 W/m-K$ respectively, has a surface area of $1 m^{2} .$ If the inner and outer temperatures of the composite wall are $1840 K$ and $340 K$ respectively, find the rate of heat transfer.

Discussion

7.	What mass of a point charge kept above finite line charge at axial line can be balanced by the electric field produced by finite line charge.
Answer» What mass of a point charge kept above finite line charge at axial line can be balanced by the electric field produced by finite line charge.

Discussion

8.	A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the appparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km/h ? Speed of sound in air = 340 m/s.
Answer» A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the appparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km/h ? Speed of sound in air = 340 m/s.

Discussion

9.	E net is equal to external electric field divided by dielectric constant .proof
Answer» E net is equal to external electric field divided by dielectric constant .proof

Discussion

10.	A charge Q is placed at a dis†an ce a/2 above the center of a horizontal ,square surface of edge a .Find eectric flux through the sqaure surface.
Answer» A charge Q is placed at a dis†an ce a/2 above the center of a horizontal ,square surface of edge a .Find eectric flux through the sqaure surface.

Discussion

11.	The velocity acquired by a body when it falls through a height h is v. If it further falls through a very small height x, the increase in velocity is approximately
Answer» The velocity acquired by a body when it falls through a height $h$ is $v$ . If it further falls through a very small height $x$ , the increase in velocity is approximately

Discussion

12.	if momentum of an object is increased by 10% then its K.E. is increased by
Answer» if momentum of an object is increased by 10% then its K.E. is increased by

Discussion

13.	A bar magnet with magnetic moment M is placed in a space where a magnetic field B1 is acting in 300 anticlockwise to S-N direction and another magnetic field B2 is along 450 clockwise to S-N direction. The magnitude of B1 is √2×10−2T. What should be the magnitude of B2 for the equilibrium of the magnet?
Answer» A bar magnet with magnetic moment M is placed in a space where a magnetic field $B_{1}$ is acting in $30^{0}$ anticlockwise to S-N direction and another magnetic field $B_{2}$ is along $45^{0}$ clockwise to S-N direction. The magnitude of $B_{1}$ is $\sqrt{2} \times 10^{- 2} T$ . What should be the magnitude of $B_{2}$ for the equilibrium of the magnet?

Discussion

14.	If the energy stored in a condenser is increased by 300%, its potential difference increases by?
Answer» If the energy stored in a condenser is increased by 300%, its potential difference increases by?

Discussion

15.	In the equilibrium arrangement as shown, tension T2 is (g=10 m/s2)
Answer» In the equilibrium arrangement as shown, tension $T_{2}$ is $(g = 10 {m/s}^{2})$

Discussion

16.	A uniform cube is subjected to volume compression. If each side is decreased by 1%, then bulk strain is
Answer» A uniform cube is subjected to volume compression. If each side is decreased by 1%, then bulk strain is

Discussion

17.	what is the difference between Ferrimagnetic and paramagnetic?
Answer» what is the difference between Ferrimagnetic and paramagnetic?

Discussion

18.	The light waves from two coherent sources have the same intensity I1=I2=I0. In an interference pattern, the intensity of light at minima is zero. What will be the intensity of light at maxima?
Answer» The light waves from two coherent sources have the same intensity $I_{1} = I_{2} = I_{0}$ . In an interference pattern, the intensity of light at minima is zero. What will be the intensity of light at maxima?

Discussion

19.	The magnetic susceptibility of a medium is –0.08, what will be its relative permeability?
Answer» The magnetic susceptibility of a medium is $- 0.08$ , what will be its relative permeability?

Discussion

20.	Sound with intensity larger than 120 dB appears painful to a person. A small speaker delivers 2.0 W of audio output. How close can the person get to the speaker without hurting his ears ?
Answer» Sound with intensity larger than 120 dB appears painful to a person. A small speaker delivers 2.0 W of audio output. How close can the person get to the speaker without hurting his ears ?

Discussion

21.	Two balls are projected at different angles from the same place and with the same initial speed of 50 m/s.Both have the same range of 216 m. The difference in their times of flight is: (1)14.4 s (2)7.8 s (3)3.6 s (4)0 s
Answer» Two balls are projected at different angles from the same place and with the same initial speed of 50 m/s.Both have the same range of 216 m. The difference in their times of flight is: (1)14.4 s (2)7.8 s (3)3.6 s (4)0 s

Discussion

22.	Centre of mass of a system of three particles of masses 1 kg, 2 kg and 3 kg at the point (1 m,2 m,3 m) and centre of mass of another group of two particles of masses 2 kg and 3 kg is at point (−1 m,−2 m), where a 10 kg particle should be placed, so that the centre of mass of the system of all those six particles shifts to centre of mass of the first system?
Answer» Centre of mass of a system of three particles of masses $1 kg$ , $2 kg$ and $3 kg$ at the point $(1 m, 2 m, 3 m)$ and centre of mass of another group of two particles of masses $2 kg$ and $3 kg$ is at point $(- 1 m, - 2 m)$ , where a $10 kg$ particle should be placed, so that the centre of mass of the system of all those six particles shifts to centre of mass of the first system?

Discussion

23.	A spherical capacitor consists of a spherical shell of radius b. This is concentric with a smaller conducting sphere of radius a. If charge given to inner sphere is +Q then energy of system is U1. Now if - Q charge is given to outer sphere also, energy of system is U2. What will be the potential energy of the system if - Q charge is given to outer sphere only (i.e. charge on inner sphere is zero)?
Answer» A spherical capacitor consists of a spherical shell of radius b. This is concentric with a smaller conducting sphere of radius a. If charge given to inner sphere is +Q then energy of system is U1. Now if - Q charge is given to outer sphere also, energy of system is U2. What will be the potential energy of the system if - Q charge is given to outer sphere only (i.e. charge on inner sphere is zero)?

Discussion

24.	Which of the following sentences are incorrect about Light Fidelity? Code:
Answer» Which of the following sentences are incorrect about Light Fidelity? Code:

24.

Which of the following sentences are incorrect about Light Fidelity? Code:

Answer»

Which of the following sentences are incorrect about Light Fidelity?

Code:

Discussion

25.	A capillary tube of radius 1 mm is kept vertical with the lower end in water . (a) Find the height of water raised in the capillary. (b) If the length of the capillary tube is half the answer of part (a), find the angle θ made by the water surface in the capillary with the wall.
Answer» A capillary tube of radius 1 mm is kept vertical with the lower end in water . (a) Find the height of water raised in the capillary. (b) If the length of the capillary tube is half the answer of part (a), find the angle $θ$ made by the water surface in the capillary with the wall.

Discussion

26.	A circular platform is free to rotate in a horizontal plane about vertical axis passing through its centre. A tortoise is sitting at the edge of the platform. Now, the platform is given an angular velocity ω0. When the tortoise move along a chord of the platform with a constant velocity (with respect to the platform). The angular velocity of the platform ω(t) will vary with time t as
Answer» A circular platform is free to rotate in a horizontal plane about vertical axis passing through its centre. A tortoise is sitting at the edge of the platform. Now, the platform is given an angular velocity $ω_{0}$ . When the tortoise move along a chord of the platform with a constant velocity (with respect to the platform). The angular velocity of the platform $ω (t)$ will vary with time t as

Discussion

27.	A metal block of heat capcity 80 J/∘C placed in a room at 20∘C is heated electrically. The heater is switched off when the temperature reaches 30∘C. The temperature of the block rises at the rate of 2∘C/s when the heater is switched on and falls at the rate of 0.2∘C/s just after the heater is switched off. Assume Newton's law of cooling to be applicable. If the power radiated at 25∘C represents the average value in the heating process, the time for which the heater was kept on is:
Answer» A metal block of heat capcity $80 J /^{\circ} C$ placed in a room at $20^{\circ} C$ is heated electrically. The heater is switched off when the temperature reaches $30^{\circ} C$ . The temperature of the block rises at the rate of $2^{\circ} C/s$ when the heater is switched on and falls at the rate of ${0.2}^{\circ} C/s$ just after the heater is switched off. Assume Newton's law of cooling to be applicable. If the power radiated at $25^{\circ} C$ represents the average value in the heating process, the time for which the heater was kept on is:

Discussion

28.	a 0.5 kg ball moving with a speed of 12 m/s strikes a hard wall at an angle of 30 degree with the wall it is reflected with the same speed and at the same angle if the ball is in contact wit the wall for 0.25 sec the average force acting along the wall is
Answer» a 0.5 kg ball moving with a speed of 12 m/s strikes a hard wall at an angle of 30 degree with the wall it is reflected with the same speed and at the same angle if the ball is in contact wit the wall for 0.25 sec the average force acting along the wall is

Discussion

29.	A particle undergoing SHM has the equation x = A sin(2ωt+Φ),where x represents the displacement of the particle. The kineticenergy oscillates with time period
Answer» A particle undergoing SHM has the equation x = A sin(2ωt+Φ), where x represents the displacement of the particle. The kinetic energy oscillates with time period

Discussion

30.	Solar radiation of intensity 1.5 kW/m2 is incident on a perfectly black surface, making an angle of 30∘ with the surface. What is the pressure experienced by the surface?
Answer» Solar radiation of intensity $1.5 {kW/m}^{2}$ is incident on a perfectly black surface, making an angle of $30^{\circ}$ with the surface. What is the pressure experienced by the surface?

Discussion

31.	Abrass boiler has a base area of 0.15 m2and thickness 1.0 cm. It boils waterat the rate of 6.0 kg/min when placed on a gas stove. Estimate thetemperature of the part of the flame in contact with the boiler.Thermal conductivity of brass = 109 J s –1m–1 K–1;Heat of vaporisation of water = 2256 × 103J kg–1.
Answer» A brass boiler has a base area of 0.15 m² and thickness 1.0 cm. It boils water at the rate of 6.0 kg/min when placed on a gas stove. Estimate the temperature of the part of the flame in contact with the boiler. Thermal conductivity of brass = 109 J s ^–1 m^–1K^–1; Heat of vaporisation of water = 2256 × 10³ J kg^–1.

Discussion

32.	Write the principal value of cos-1cos680°
Answer» Write the principal value of $\cos^{- 1} (\cos 680 °)$

Discussion

33.	Define velocity.
Answer» Define velocity.

Discussion

34.	If, in Exercise 5.21, the speed of the stone is increased beyond the maximum permissible value, and the string breaks suddenly, which of the following correctly describes the trajectory of the stone after the string breaks : (a) the stone moves radially outwards, (b) the stone flies off tangentially from the instant the string breaks, (c) the stone flies off at an angle with the tangent whose magnitude depends on the speed of the particle ?
Answer» If, in Exercise 5.21, the speed of the stone is increased beyond the maximum permissible value, and the string breaks suddenly, which of the following correctly describes the trajectory of the stone after the string breaks : (a) the stone moves radially outwards, (b) the stone flies off tangentially from the instant the string breaks, (c) the stone flies off at an angle with the tangent whose magnitude depends on the speed of the particle ?

Discussion

35.	A clean body of mass 100 g starts with a velocity of 2 ms on a smooth horizontal plane, accumulating dust at the rate of 5 gs. Find the velocity in m/s at the end of 20 seconds.
Answer» A clean body of mass 100 g starts with a velocity of 2 $\frac{m}{s}$ on a smooth horizontal plane, accumulating dust at the rate of 5 $\frac{g}{s}$ . Find the velocity in m/s at the end of 20 seconds.

Discussion

36.	A 100W power source emits green light with wavelength 5000 Armstrong how many photon per minute are emitted by source
Answer» A 100W power source emits green light with wavelength 5000 Armstrong how many photon per minute are emitted by source

Discussion

37.	Induced electric field due to change in magnetic field.
Answer» Induced electric field due to change in magnetic field.

Discussion

38.	What caused the lack of communication between the Englishman and the people at Gemini Studios?
Answer» What caused the lack of communication between the Englishman and the people at Gemini Studios?

Discussion

39.	9. The armature of a generator of resistance 1ohm is rotated at it's rated speed and produces 125 V Without load and 115 V with full load. The current in the armature coil is 1) 240 A 2) 10 A 3) 1 A 4) 2 A :
Answer» 9. The armature of a generator of resistance 1ohm is rotated at it's rated speed and produces 125 V Without load and 115 V with full load. The current in the armature coil is 1) 240 A 2) 10 A 3) 1 A 4) 2 A :

Discussion

40.	A loop of irregular shape carrying current is located in a external magnetic field.if wire is flexible it will take shape of 1. Will remain in same shape 2. Circle 3. Square 4.None of these
Answer» A loop of irregular shape carrying current is located in a external magnetic field.if wire is flexible it will take shape of 1. Will remain in same shape 2. Circle 3. Square 4.None of these

40.

A loop of irregular shape carrying current is located in a external magnetic field.if wire is flexible it will take shape of 1. Will remain in same shape 2. Circle 3. Square 4.None of these

Answer»

A loop of irregular shape carrying current is located in a external magnetic field.if wire is flexible it will take shape of

1. Will remain in same shape

2. Circle

3. Square

4.None of these

Discussion

41.	Each side of regular polygon of n sides has a resistance R. The equivalent resistance between any two adjacent vertices is
Answer» Each side of regular polygon of $n$ sides has a resistance $R$ . The equivalent resistance between any two adjacent vertices is

Discussion

42.	The value of cos45°cos15° + sin 45° sin15° is
Answer» The value of cos45°cos15° + sin 45° sin15° is

Discussion

43.	19.) The ratio of maximum and minimum resistance thatcan be obtained by using three resistance of 202,50Ω and 100Ω is6851713345
Answer» 19.) The ratio of maximum and minimum resistance thatcan be obtained by using three resistance of 202,50Ω and 100Ω is6851713345

Discussion

44.	Position of a body changes with time asX (in m) = t^2- 4, where t is in seconds. Theaverage velocity of the body in first 2 s is
Answer» Position of a body changes with time asX (in m) = t^2- 4, where t is in seconds. Theaverage velocity of the body in first 2 s is

Discussion

45.	A point object O is placed in front of a plane mirror as shown in the figure. In order to see the image in mirror, the region where a person must place his eyes is best represented as:
Answer» A point object $O$ is placed in front of a plane mirror as shown in the figure. In order to see the image in mirror, the region where a person must place his eyes is best represented as:

Discussion

46.	A parallel plate capacitor of capacitance is being charged by a voltage source whose potential is changing at the rate of . The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively
Answer» A parallel plate capacitor of capacitance is being charged by a voltage source whose potential is changing at the rate of . The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively

Discussion

47.	Light of wavelength 2475Å is incident on barium. Photoelectrons emitted described a circle of radius 100 cm in a magnetic field of magnitude 1√17×10−5 Tesla. Work function of the barium is ( Given: em = 1.7×1011 Ckg−1
Answer» Light of wavelength 2475 $Å$ is incident on barium. Photoelectrons emitted described a circle of radius 100 $c m$ in a magnetic field of magnitude $\frac{1}{\sqrt{17}} \times 10^{- 5}$ Tesla. Work function of the barium is ( Given: $\frac{e}{m} = 1.7 \times 10^{11} C k g^{- 1}$

Discussion

48.	1.What is the net flux through a prism in which charge is kept at the corner and angle is 45^°
Answer» 1.What is the net flux through a prism in which charge is kept at the corner and angle is 45^°

Discussion

49.	Double-convexlenses are to be manufactured from a glass of refractive index 1.55,with both faces of the same radius of curvature. What is the radiusof curvature required if the focal length is to be 20cm?
Answer» Double-convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?

Discussion

50.	Can lenses can combine and produce zero power if yes explai
Answer» Can lenses can combine and produce zero power if yes explai

Discussion

Explore topic-wise InterviewSolutions in Current Affairs.

image of an object approaching a convex mirror of radius of curvature 20 m along its optical axis is observed to move from 25/3m to 50/7m in 30 seconds. What is the speed objective of in km per hour

A conducting wire ab of length l and resistance R, is moving inside a magnetic field B on a pair of fixed vertical rails as shown in the figure. If the mass of the wire is m, then calculate the terminal velocity achieved by the loop.Resistance of the rails are negligible.

A person brings a mass of 2 kg from A to B. The increase in kinetic energy of mass is 4 J and work done by the person on the mass is −10 J. The potential difference between B and A is (in J/kg)

A non conducting ring of radius r=10 cm and mass m=500 g which has a total charge Q=5 C distributed uniformly on it. The ring is rotated about its axis with a uniform speed v=0.2π m/s as shown. Find angular momentum of the ring using magnetic moment.

A composite wall having three layers of thickness 0.3 m, 0.2 m and 0.1 m and of thermal conductivity 0.6,0.4 and 0.1 W/m-K respectively, has a surface area of 1 m2. If the inner and outer temperatures of the composite wall are 1840 K and 340 K respectively, find the rate of heat transfer.

What mass of a point charge kept above finite line charge at axial line can be balanced by the electric field produced by finite line charge.

A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the appparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km/h ? Speed of sound in air = 340 m/s.

E net is equal to external electric field divided by dielectric constant .proof

A charge Q is placed at a dis†an ce a/2 above the center of a horizontal ,square surface of edge a .Find eectric flux through the sqaure surface.

The velocity acquired by a body when it falls through a height h is v. If it further falls through a very small height x, the increase in velocity is approximately

if momentum of an object is increased by 10% then its K.E. is increased by

If the energy stored in a condenser is increased by 300%, its potential difference increases by?

In the equilibrium arrangement as shown, tension T2 is (g=10 m/s2)

A uniform cube is subjected to volume compression. If each side is decreased by 1%, then bulk strain is

what is the difference between Ferrimagnetic and paramagnetic?

The light waves from two coherent sources have the same intensity I1=I2=I0. In an interference pattern, the intensity of light at minima is zero. What will be the intensity of light at maxima?

The magnetic susceptibility of a medium is –0.08, what will be its relative permeability?

Sound with intensity larger than 120 dB appears painful to a person. A small speaker delivers 2.0 W of audio output. How close can the person get to the speaker without hurting his ears ?

Two balls are projected at different angles from the same place and with the same initial speed of 50 m/s.Both have the same range of 216 m. The difference in their times of flight is: (1)14.4 s (2)7.8 s (3)3.6 s (4)0 s

Which of the following sentences are incorrect about Light Fidelity? Code:

A capillary tube of radius 1 mm is kept vertical with the lower end in water . (a) Find the height of water raised in the capillary. (b) If the length of the capillary tube is half the answer of part (a), find the angle θ made by the water surface in the capillary with the wall.

a 0.5 kg ball moving with a speed of 12 m/s strikes a hard wall at an angle of 30 degree with the wall it is reflected with the same speed and at the same angle if the ball is in contact wit the wall for 0.25 sec the average force acting along the wall is

A particle undergoing SHM has the equation x = A sin(2ωt+Φ),where x represents the displacement of the particle. The kineticenergy oscillates with time period

Solar radiation of intensity 1.5 kW/m2 is incident on a perfectly black surface, making an angle of 30∘ with the surface. What is the pressure experienced by the surface?

Write the principal value of cos-1cos680°

Define velocity.

A clean body of mass 100 g starts with a velocity of 2 ms on a smooth horizontal plane, accumulating dust at the rate of 5 gs. Find the velocity in m/s at the end of 20 seconds.

A 100W power source emits green light with wavelength 5000 Armstrong how many photon per minute are emitted by source

Induced electric field due to change in magnetic field.

What caused the lack of communication between the Englishman and the people at Gemini Studios?

9. The armature of a generator of resistance 1ohm is rotated at it's rated speed and produces 125 V Without load and 115 V with full load. The current in the armature coil is 1) 240 A 2) 10 A 3) 1 A 4) 2 A :

A loop of irregular shape carrying current is located in a external magnetic field.if wire is flexible it will take shape of 1. Will remain in same shape 2. Circle 3. Square 4.None of these

Each side of regular polygon of n sides has a resistance R. The equivalent resistance between any two adjacent vertices is

The value of cos45°cos15° + sin 45° sin15° is

19.) The ratio of maximum and minimum resistance thatcan be obtained by using three resistance of 202,50Ω and 100Ω is6851713345

Position of a body changes with time asX (in m) = t^2- 4, where t is in seconds. Theaverage velocity of the body in first 2 s is

A point object O is placed in front of a plane mirror as shown in the figure. In order to see the image in mirror, the region where a person must place his eyes is best represented as:

A parallel plate capacitor of capacitance is being charged by a voltage source whose potential is changing at the rate of . The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively

Light of wavelength 2475Å is incident on barium. Photoelectrons emitted described a circle of radius 100 cm in a magnetic field of magnitude 1√17×10−5 Tesla. Work function of the barium is ( Given: em = 1.7×1011 Ckg−1

1.What is the net flux through a prism in which charge is kept at the corner and angle is 45^°

Double-convexlenses are to be manufactured from a glass of refractive index 1.55,with both faces of the same radius of curvature. What is the radiusof curvature required if the focal length is to be 20cm?

Can lenses can combine and produce zero power if yes explai