Physics Assertion (A) & Reason (B) MCQ

2. Both A and R are true but R is NOT the correct explanation of A.

Explanation:

When intensity of light emerging from two slits is equal, the intensity at minima,

$\text{I}_\text{min}=\Big(\sqrt{\text{I}_\text{a}}-\sqrt{\text{I}_\text{b}}\Big)^2=0$, or absolute dark.

2. Both A and R are true but R is NOT the correct explanation of A.

Explanation:

When slit is wide $(\text{i.e., a}>>\lambda)$, bending of light becomes so small that it cannot be detected upto a certain distance of screen from the slit. Hence, practically, no diffraction occurs.

2. Both A and R are true but R is NOT the correct explanation of A.

Explanation:

If maximum intensity is observed at P then for maximum intensity to be also observed at Q, S₁ and S₂ must have phase difference of $2\text{ m}\pi$ (where m is an integer).

1. Both A and R are true and R is the correct explanation of A.

Explanation:

As given in the expression $\beta=\frac{\text{D}\lambda}{\text{d}}$, fringe width $\beta$ is independent of n (position of fringe), hence all the fringes are of same width.

2. Both A and R are true but R is NOT the correct explanation of A.

Explanation:

For case when dark fringe is observed opposite to one of the slit.

Here S₁P = D and S₂P $=\sqrt{\text{D}^2+\text{d}^2}=\text{D}\Big[1+\frac{\text{d}^2}{2\text{D}^2}\Big]$

Path difference $=\text{S}_2\text{P}-\text{S}_1\text{P}=\frac{\text{d}^2}{2\text{D}}=\frac{\lambda}{2}$ or, $\lambda=\frac{\text{d}^2}{\text{D}}$.

3. A is true but R is false.

Explanation:

Fringe width is given by, $\beta=\frac{\text{D}\lambda}{\text{d}}$

where, D = distance between slit and screen, d = distance between coherent sources of light and $\lambda$ = wavelength of incident light.

1. Both A and R are true and R is the correct explanation of A.

Explanation:

ln case of interference, intensity of maxima is $\text{I}_\text{max}=\Big(\sqrt{\text{I}_\text{a}}+\sqrt{\text{I}_\text{b}}\Big)^2$ and intensity of minima is $\text{I}_\text{min}=\Big(\sqrt{\text{I}_\text{a}}-\sqrt{\text{I}_\text{b}}\Big)^2$. Thus, whatever energy disappears at the minimum is actually appearing at the maximum. So, the law of conservation of energy holds good in the phenomenon of interference because in interference energy is neither created nor destroyed but is redistributed.

1. Both A and R are true and R is the correct explanation of A.

Explanation:

For reflected system of the film, the maxima or constructive interference is $2\mu\text{t}\cos\text{r}=$ while the maxima for transmitted system of film is given by equation

$2\mu\text{t}\cos\text{r}=\text{n}\lambda$

where t is thickness of the film and r is angle of refraction.

From these two equations we can see that condition for maxima in reflected system and transmitted system are just opposite.

3. A is true but R is false.

Explanation:

Fringe width $\beta=\frac{\lambda\text{D}}{\text{d}}$ shall remain the same as the waves travel in air only, after passing through the thin transparent sheet. Due to introduction of thin sheet, only path difference of the wave is changed due to which th ere is shift of position of fringes only, which is given as $\triangle\text{x}=\frac{\text{D}(\mu-1)\text{t}}{\text{d}}$, where $\mu$ is refractive index of thin sheet and tis its thickness.

1. Both A and R are true and R is the correct explanation of A.

Explanation:

The central spot of Newton's rings is dark when the medium between piano convex lens and plane glass is rarer than the medium oflens and glass. The central spot is dark because the phase change of $\pi$ is introduced between the rays reflected from surfaces of denser to rarer and rarer to denser media.