Physics objectives

Physics: One-Liner Questions and Answers

Light: Reflection and Refraction

1. What is the angle of reflection if a light ray strikes a plane mirror at an angle of 30^\circ to the surface?
   • Answer: 60^\circ (since the angle of incidence with the normal is 90^\circ - 30^\circ = 60^\circ).
2. Which spherical mirror always forms a virtual, erect, and diminished image for all positions of the object?
   • Answer: Convex mirror.
3. What is the focal length of a plane mirror?
   • Answer: Infinity.
4. State the relation between radius of curvature (R) and focal length (f) of a spherical mirror.
   • Answer: R = 2f (or f = R/2).
5. Why does a pencil immersed in water appear bent at the interface?
   • Answer: Due to the refraction of light as it passes from water to air.
6. What is the SI unit of power of a lens?
   • Answer: Dioptre (D).
7. Which lens has a negative power?
   • Answer: Concave lens.

The Human Eye and the Colorful World

1. Name the part of the human eye that controls the amount of light entering it.
   • Answer: Iris.
2. Which defect of vision can be corrected by using a concave lens?
   • Answer: Myopia (Short-sightedness).
3. What is the phenomenon responsible for the twinkling of stars?
   • Answer: Atmospheric refraction.
4. Why does the sky appear dark to astronauts in space?
   • Answer: Due to the absence of an atmosphere for scattering light.
5. Which color of white light deviates the most while passing through a glass prism?
   • Answer: Violet.

Electricity

1. What is the SI unit of electric current?
   • Answer: Ampere (A).
2. Name the device used to measure electric current in a circuit.
   • Answer: Ammeter (connected in series).
3. State Ohm's Law mathematically.
   • Answer: V = IR (Voltage = Current \times Resistance).
4. What happens to the total resistance when resistors are connected in series?
   • Answer: Total resistance increases (R_s = R_1 + R_2 + \dots).
5. What is the commercial unit of electrical energy?
   • Answer: Kilowatt-hour (\text{kWh} or 'unit').
6. Why are alloys commonly used in electrical heating devices like irons and toasters?
   • Answer: They have high resistivity and do not oxidize easily at high temperatures.

Magnetic Effects of Electric Current

1. What is the shape of magnetic field lines around a straight current-carrying conductor?
   • Answer: Concentric circles.
2. Which rule is used to find the direction of force experienced by a current-carrying conductor in a magnetic field?
   • Answer: Fleming's Left-Hand Rule.
3. What is the frequency of alternating current (AC) supplied in India?
   • Answer: 50\text{ Hz}.
4. Name the two safety measures commonly used in domestic electric circuits.
   • Answer: Electric fuse and earthing.
5. What is the function of a split-ring (commutator) in a DC motor?
   • Answer: To reverse the direction of current in the coil after every half rotation.

Sources of Energy

1. Which component of biogas is its primary combustible constituent?
   • Answer: Methane (CH_4).
2. What is the ultimate source of tidal energy?
   • Answer: Gravitational pull of the moon on the earth.