Physics MCQs for NEET — Practice Questions with Answers

The context mentions: 'In the energy band diagram of n-type Si semiconductor, the donor energy level $E_D$ is slightly below the bottom $E_C$ of the conduction band and electrons from this level move into the conduction band with very small supply of energy.'

The text states: 'For C (diamond), Si and Ge, the energy gaps are 5.4 eV, 1.1 eV and 0.7 eV, respectively. Sn also is a group IV element but it is a metal because the energy gap in its case is 0 eV.' Therefore, $E_g > 3 \text{ eV}$ for Sn is incorrect.

The text defines: 'The energy band which includes the energy levels of the valence electrons is called the valence band.'

The context states: 'In extrinsic semiconductors, because of the abundance of majority current carriers, the minority carriers produced thermally have more chance of meeting majority carriers and thus getting destroyed. Hence, the dopant, by adding a large number of current carriers of one type, which become the majority carriers, indirectly helps to reduce the intrinsic concentration of minority carriers.'

The text clearly explains this for semiconductors: 'Here a finite but small band gap ($E_g < 3 \text{ eV}$) exists. Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band.' This implies easier transition for smaller $E_g$, leading to conduction.

The 'POINTS TO PONDER' section states: 'The energy bands ($E_C$ or $E_V$) in the semiconductors are space delocalised which means that these are not located in any specific place inside the solid. The energies are the overall averages. When you see a picture in which $E_C$ or $E_V$ are drawn as straight lines, then they should be respectively taken simply as the bottom of conduction band energy levels and top of valence band energy levels.'

According to the NCERT text, 'The normal in this case is to be taken as normal to the tangent to surface at the point of incidence. That is, the normal is along the radius, the line joining the centre of curvature of the mirror to the point of incidence.'

The NCERT text states, 'The reflected rays converge at a point F on the principal axis of a concave mirror [Fig. 9.3(a)].' This point F is defined as the principal focus for a concave mirror.

The NCERT text explicitly derives and states this relationship: 'We now show that $f = R/2$, where R is the radius of curvature of the mirror.' This derivation is based on the small angle approximation for paraxial rays.

The Cartesian Sign Convention usually dictates that distances measured against the direction of incident light (which typically comes from the left) are negative. While the provided excerpt specifically mentions 'principal axis (x-axis) of the mirror/lens are taken as positive', the standard convention (implied by Fig 9.2's left-to-right incident light and positive x-axis to the right) is that object distances for real objects placed to the left are negative, as per general physics conventions for optical instruments. The textbook also mentions 'Distances measured in the same direction as the incident light are positive; those measured in the opposite direction are negative.' If incident light is from left to right, and the pole is the origin, then an object to the left would have a negative distance.