Physics MCQs for NEET — Practice Questions with Answers

The NCERT text states, 'Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding.' This ensures that light entering the core undergoes total internal reflection at the core-cladding interface.

The NCERT text explains, 'When light gets reflected by a surface, normally some fraction of it gets transmitted. The reflected ray, therefore, is always less intense than the incident ray, howsoever smooth the reflecting surface may be. In total internal reflection, on the other hand, no transmission of light takes place.' This implies that the intensity of the totally internally reflected ray is essentially the same as the incident ray, as there is no loss due to transmission.

From Snell's law, $\sin i_c / \sin 90^\circ = n_2 / n_1$. Since $n_{12}$ is the refractive index of medium 1 with respect to medium 2, $n_{12} = n_1/n_2$. Therefore, $\sin i_c = n_2/n_1 = 1/n_{12}$, which means $n_{12} = 1/\sin i_c$. The NCERT text also states 'The refractive index of denser medium 1 with respect to rarer medium 2 will be $n_{12} = 1/\sin i_c$.'

The NCERT text clarifies, 'Optical density should not be confused with mass density, which is mass per unit volume. It is possible that mass density of an optically denser medium may be less than that of an optically rarer medium... optical density is the ratio of the speed of light in two media.'

The NCERT text describes this exact experiment: 'Adjust the direction of laser beam until you find the angle for which the refraction above the water surface is totally absent and the beam is totally reflected back to water. This is total internal reflection at its simplest.'

The NCERT text says, 'The above equation implies that when a wave gets refracted into a denser medium ($v_1 > v_2$) the wavelength and the speed of propagation decrease but the frequency $\nu$ (= v/$\lambda$) remains the same.' This is relevant because total internal reflection involves refraction principles before the critical angle is reached.

The NCERT text explicitly states, 'For values of $i$ larger than $i_c$, Snell’s law of refraction cannot be satisfied, and hence no refraction is possible.'

According to the NCERT text, '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 NCERT states, 'for p-type semiconductor, the acceptor energy level $E_A$ is slightly above the top $E_V$ of the valence band as shown in Fig. 14.9(b).'

The NCERT text mentions that 'At room temperature, most of the donor atoms get ionised but very few (~10^12) atoms of Si get ionised. So the conduction band will have most electrons coming from the donor impurities...'