If a medium has the same refractive index at every point in all directions, then the waveform obtained from a point source in such a medium is spherical since the wave travels in all directions at the same speed. Such a medium is known as isotropic medium. So, the assertion and reason both are true and the reason explain the assertion properly.

No interference pattern is detected when two coherent sources are too close to each other. The assertion is true.

Fringe width is proportional to 1/d. When d becomes too small, the fringe width becomes too large. So no pattern will be visible. So, the reason is also true. Reason also explains the assertion.

Fringes of interference pattern produced by blue light are narrower than that produced by red light. The assertion is true.

Fringe width = λD/d. Since blue light has a wavelength smaller than that of red light, blue light produces narrower fringes. So, reason is also true and explains the assertion.

According to Huygens theory each and every point on a wavefront is the source of secondary wavelets. Secondary wavelets do not proceed backward. So the assertion is true.

Kirchhoff’s explained that amplitude of secondary wavelets is proportional to (1+ cos q), where q is the angle between the ray at the point of consideration and direction of secondary wavelets. In the backward direction q = 180°; so 1 + cos q = 0; so their secondary wavelets do not proceed backward.

Hence assertion and both are true and the reason properly explains the assertion.

When a light wave travels from rarer to denser medium, its speed decreases. But this reduction of speed does not imply the loss of energy carried by the light wave. So the assertion is false.

Energy of a wave is proportional to the frequency of the wave which remains the same in every medium. Hence there is no loss of energy. So, the reason is true.

For destructive interference, a = a₁ ~ a₂ . When a₁ = a₂ , only the minima will be completely dark. This will create the best contrast. So the assertion is true.

For interference, the sources must be coherent. Reason is also true. But the reason does not explain the assertion.

Diffraction is the spreading of waves around obstacles. It takes place with all types of waves (mechanical, non-mechanical, transverse, longitudinal) and with very small moving particles (atom, neutron, electron etc.) which show wave-like properties. So, the assertion is true. Diffraction is perceptible when the wavelength of the wave is comparable to the dimension of the diffracting device. The reason is also true. But it does not explain the assertion.

When white light is incident on thin film, the film appears coloured. These colours are due to the interference between light waves reflected from the top and bottom surfaces of thin films. The colour depends on the thickness of the film and also the angle of incidence of the light.

For interference, the waves may be of unequal intensities.

If a film is introduced in the upper slit, the optical path traveled by ray from upper source is more and that traveled by that from lower source, hence some finite path difference exists at the point of earlier central maxima(point equidistant from both sources). Now to compensate for this extra path difference, ray from the upper source needs to travel less than ray from lower source. So the central maxima would now be obtained at a point of screen that is closer to the upper source. Hence central maxima shifts upwards, so does the pattern. If slit widths are different, intensity of light from both the sources is different. So the minima will have intensity (I₁ ​− I₂ )² > 0.

So the minima would not be dark. Therefore both the assertion and reason are incorrect.