A laser beam of `10mW` power and wavelength `700nm` has aperture of `3mm`. If it is focused by a lens of focal length `5cm`, the intensity of image is

A beam of 8mW power and wavelength 6000Å has aperture 2 mm. If it is focused by a lens of focal length 6 cm the intensity of the image (xx10^(3)) is

Calculate the power of the lens of focal length 10 cm ?

A converging lens has a focal length of 50cm. The power of this lens is :

A lens having focal length f and aperture of diameter d//2 in the central region of the lens is covered by a black paper.Focal length of the lens and intensity of image image now will be respectively