A periscope uses two plane mirrors inclined at 45°. If the tube of the periscope is filled with a complete transparent liquid of refractive index 1.5, then which one of the following is correct in respect of the image seen?

Plane mirrors form images by reflection, and reflection is governed by the law of reflection (angle of incidence equals angle of reflection), which does not depend on the medium. The angles within the periscope tube remain unchanged whether the medium is air or a transparent liquid, since light travels through a uniform medium throughout the tube. The image formed by each mirror retains the same size, orientation and position relative to the mirrors. Hence the image will remain unchanged.