$A$ circular disc of which $2/3$ part is coated with yellow and $1/3$ part is with blue. It is rotated about its central axis with high velocity,then it will be seen as

Red light differs from blue light as they have :

Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of $60^o$,by what fraction will the illuminance change?

$A$ laser beam is used for carrying out surgery because it

Light of different colours propagates through air:

Two point monochromatic and coherent sources of light of wavelength $\lambda$ are placed on the dotted line in front of a large screen. The sources emit waves in phase with each other. The distance between $S_1$ and $S_2$ is $d$,while their distance from the screen is much larger. Then,
$(1) \rightarrow$ If $d = 7\lambda / 2$,$O$ will be a minima.
$(2) \rightarrow$ If $d = 4.3\lambda$,there will be a total of $8$ minima on the $y$-axis.
$(3) \rightarrow$ If $d = 7\lambda$,$O$ will be a maxima.
$(4) \rightarrow$ If $d = \lambda$,there will be only one maxima on the screen.
Which is the set of correct statements?