How does the angular separation between fringes in single-slit diffraction experiment change when the distance of separation between the slit and screen is doubled?

The image of a candle formed by a convex lens is obtained on a screen. Will full size of the image be obtained if the lower half of the lens is painted completely opaque?

For the same angle of incidence, the angles of refraction in media P, Q and R are 35°, 25°, 15° respectively. In which medium will the velocity of light be minimum?

The image of an object formed by a lens on the screen is not in sharp focus. Suggest a method to get a dear focussing of the image on the screen without disturbing the positions of the object, the lens or the screen.

A lens whose radii of curvature are different is forming the image of an object placed on its axis. If the surfaces of the lens facing the object and image are interchanged will the position of the image change?