Answers to Important & Expected Questions from Tamil Nadu 12th Physics

Lesson 6: Optics

2 Marks Questions

• State Snell’s Law: Snell’s Law states that the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant, given by the refractive indices of the two media:
  n_1 \sin i = n_2 \sin r
• What is a virtual image? Give an example: A virtual image is an image that cannot be projected on a screen as it is formed by the divergence of rays. Example: The image formed by a plane mirror.
• Define focal length: Focal length is the distance between the focal point and the lens or mirror. It is the point where parallel rays of light either converge or appear to diverge.
• What is refraction? Refraction is the bending of light rays as they pass from one medium to another due to a change in speed.
• Explain the term ‘optical center’: The optical center is the point in a lens where light rays passing through do not bend; it is located at the center of the lens.

5 Marks Questions

• Derive the lens formula for a convex lens: The lens formula is given by
  \frac{1}{f} = \frac{1}{v} – \frac{1}{u}
  where f is the focal length, v is the image distance, and u is the object distance.
• Explain total internal reflection with a diagram: Total internal reflection occurs when light travels from a denser medium to a rarer medium at an angle greater than the critical angle. The diagram illustrates this phenomenon.
• Discuss the formation of images by concave mirrors with ray diagrams: Concave mirrors can form real and virtual images depending on the position of the object. Diagrams show the paths of light rays and the resulting images.
• Describe the construction and working of a compound microscope: A compound microscope consists of two convex lenses, the objective and the eyepiece, which magnify the object viewed. The total magnification is given by
  M = M_o \times M_e
  where M_o is the magnification of the objective and M_e is the magnification of the eyepiece.
• Explain the principle of magnifying glass and derive its magnifying power: A magnifying glass works by forming a virtual image of an object that is closer than the focal length. The magnifying power is given by
  M = \frac{D}{f}
  where D is the near point distance and f is the focal length.

Lesson 7: Magnetism

2 Marks Questions

• Define magnetic field: A magnetic field is a vector field that describes the magnetic influence on charged particles, defined as the force experienced by a unit positive charge moving in the field.
• What is the right-hand thumb rule? The right-hand thumb rule states that if you curl the fingers of your right hand around a conductor in the direction of current flow, your thumb points in the direction of the magnetic field lines.
• State the formula for the magnetic field due to a long straight current-carrying conductor: The magnetic field B at a distance r from a long straight conductor carrying current I is given by
  B = \frac{\mu_0 I}{2 \pi r}
  where \mu_0 is the permeability of free space.
• What is magnetic flux? Magnetic flux is a measure of the quantity of magnetism, taking into account the strength and the extent of a magnetic field. It is given by
  dB = \frac{\mu_0}{4\pi} \frac{I \, dl \, \sin \theta}{r^2}
  By integrating over the circular loop, we find the total magnetic field at the center.
• Discuss Ampere’s circuital law with its applications: Ampere’s circuital law relates the circulating magnetic field in a closed loop to the electric current passing through it. It is given by
  B = \mu_0 n I
  where n is the number of turns per unit length and I is the current.
• What are diamagnetic, paramagnetic, and ferromagnetic materials? Explain:
  • Diamagnetic: Materials that are weakly repelled by a magnetic field (e.g., bismuth).
  • Paramagnetic: Materials that are weakly attracted to a magnetic field (e.g., aluminum).
  • Ferromagnetic: Materials that can be permanently magnetized (e.g., iron).

Lesson 8: Waves

2 Marks Questions

• What is wave speed? Wave speed is the distance traveled by a wave per unit time, given by the formula
  v = f \lambda
  where f is the frequency and \lambda is the wavelength.
• Define amplitude: Amplitude is the maximum displacement of a point on a wave from its rest position.
• State the principle of superposition of waves: The principle of superposition states that when two or more waves overlap, the resultant displacement is the sum of the individual displacements.
• What is the difference between transverse and longitudinal waves? Transverse waves have oscillations perpendicular to the direction of wave travel, while longitudinal waves have oscillations parallel to the direction of wave travel.
• Define wavelength: Wavelength is the distance between two consecutive points in phase on a wave, such as crest to crest or trough to trough.

5 Marks Questions

• Explain the phenomenon of standing waves with examples: Standing waves are formed by the interference of two waves traveling in opposite directions. Examples include strings fixed at both ends vibrating in harmonics.
• Derive the expression for the speed of a wave in a stretched string: The speed of a wave in a stretched string is given by
  Q = \Delta U + W
  where Q is heat added, U is the change in internal energy, and W is work done.
• What is heat capacity? Heat capacity is the amount of heat required to change the temperature of an object by one degree Celsius.
• Define adiabatic process: An adiabatic process is one that occurs without transfer of heat to or from the system.

5 Marks Questions

• Explain the Carnot cycle and derive the efficiency of a Carnot engine: The Carnot cycle consists of two isothermal processes and two adiabatic processes. The efficiency is given by
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