Physics MCQs for NEET — Practice Questions with Answers

Combination of lenses helps to obtain diverging or converging lenses of desired magnification. It also enhances sharpness of the image. Such a system of combination of lenses is commonly used in designing lenses for cameras, microscopes, telescopes and other optical instruments.

Such a system of combination of lenses is commonly used in designing lenses for cameras, microscopes, telescopes and other optical instruments.

The SI unit for power of a lens is dioptre (D): 1D = 1m–1. The power of a lens of focal length of 1 metre is one dioptre.

This question tests comprehension of the conceptual simplification used in deriving the combined lens formula. 'It must, however, be borne in mind that formation of image by the first lens is presumed only to facilitate determination of the position of the final image. In fact, the direction of rays emerging from the first lens gets modified in accordance with the angle at which they strike the second lens.'

According to the NCERT text, 'The electric dipole potential falls off, at large distance, as $1/r^2$, not as $1/r$, characteristic of the potential due to a single charge.' Therefore, a point charge's potential decreases as $1/r$ and a dipole's potential decreases as $1/r^2$ at large distances.

The NCERT text states: '(i) The potential due to a dipole depends not just on $r$ but also on the angle between the position vector $\vec{r}$ and the dipole moment vector $\vec{p}$.' Equation (2.14) $V = \frac{1}{4\pi\epsilon_0} \frac{p \cos\theta}{r^2}$ clearly shows this dependence.

As per the NCERT text, 'The potential in the equatorial plane ($\theta = \pi/2$) is zero.' This is because for $\theta = \pi/2$, $\cos\theta = 0$, making the potential $V = \frac{1}{4\pi\epsilon_0} \frac{p \cos\theta}{r^2} = 0$.

The NCERT text states that the electric potential of a dipole is given by $V = \frac{1}{4\pi\epsilon_0} \frac{p \cdot \hat{r}}{r^2}$ or $V = \frac{1}{4\pi\epsilon_0} \frac{p \cos\theta}{r^2}$ for $r \gg a$. This formula (Equation 2.14 and 2.15) holds for large distances.

The NCERT text defines electrostatic potential as 'the work done in bringing a unit positive charge (without acceleration) from infinity to that point.' (Page 48).

The NCERT text states: 'For Q < 0, V < 0, i.e., work done (by the external force) per unit positive test charge in bringing it from infinity to the point is negative.' (Page 49, Example 2.1 note).