Two light sources are said to be coherent when their:

1.	amplitudes are equal and have a constant phase difference.
2.	wavelengths are equal.
3.	intensities are equal.
4.	frequencies are equal and have a constant phase difference.

Which statement is true for interference?

Light waves of intensities \(I\) and \(9I\) interfere to produce a fringe pattern on a screen. The phase difference between the waves at point P is $\frac{3\mathrm{\pi }}{2}$ and 2$\mathrm{\pi }$ at other point Q. The ratio of intensities at P and Q is:

1.  8: 5

2.  5: 8

3.  1: 4

4.  9: 1

In Young's double-slit experiment sources of equal intensities are used. The distance between the slits is d and the wavelength of light used is $\lambda$($\lambda$<<d). The angular separation of nearest points on either side of central maximum where intensities become half of the maximum value is:

1.  $\frac{\lambda }{d}$

2.  $\frac{\lambda }{2d}$

3.  $\frac{\lambda }{4d}$

4.  $\frac{\lambda }{6d}$

Four coherent sources of intensity \(I\) are superimposed constructively at a point. The intensity at that point is:

1. \(4I\)
2. \(8I\)
3. \(16I\)
4. \(24I\)

If the 5th order maxima of wavelength 4000 $\stackrel{\mathrm{o}}{\mathrm{A}}$ in Young's double-slit experiment coincides with the nth order maxima of wavelength 5000 $\stackrel{\mathrm{o}}{\mathrm{A}}$, then n is equal to:

1.  5

2.  8

3.  4

4.  10

Young's double-slit experiment is performed in a liquid. The 10^th bright fringe in the liquid lies where the 8th dark fringe lies in a vacuum. Refractive index of the liquid is approximately:

1.  1.81

2.  1.67

3.  1.54

4.  1.33

If the ratio of amplitudes of two coherent sources producing an interference pattern is 3 : 4, the ratio of intensities at maxima and minima is:

1.  3 : 4

2.  9 : 16

3.  49 : 1

4.  25 : 7

Two coherent sources are 0.3 mm apart. They are 1 m away from the screen. The second dark fringe is at a distance of 0.3 cm from the center. The distance of the fourth bright fringe from the centre is:

1.  0.6 cm

2.  0.8 cm

3.  1.2 cm

4.  0.12 cm

Huygens' wave theory allows us to know the: