A student wishes to study the cell structure under a light microscope having $10 \mathrm{X}$ eyepiece and $45 \mathrm{X}$ objective. He should illuminate the object by which one of the following colours of light so as to get the best possible resolution?

In order to increase the resolution, the specimen must be viewed using either shorter wavelength $(\lambda)$ light or through an imaging medium with a relatively high refractive index or with optical components which have a high numerical aperture (NA). thus, resolution of microscope in inversely proportional to the wavelength of the light used. $$ \Delta d=\frac{\lambda}{2 n \sin \theta} $$ Where, $$ \begin{aligned} & \lambda=\text { wavelength } \\ & n=\text { refractive index } \\ & \theta=\text { half angle subtended } \\ & \Delta d=\text { shortest distance. } \end{aligned} $$