If a convex lens forms a real and magnified image of an object,where is the object placed?

$(a)$ Calculate the distance at which an object should be placed in front of a convex lens of focal length $10\, cm$ to obtain a virtual image of double its size.
$(b)$ In the above given case,find the magnification,if the image formed is real. Express it in terms of the relation between $v$ and $u$.

What changes; frequency or wavelength, when light passes from a rarer to a denser medium?

Two lenses of power $+3.5 D$ and $-2.5 D$ are placed in contact. Find the power and focal length of the lens combination.

Under which of the following conditions can a concave mirror form an image larger than the actual object?

What is the radius of curvature of a plane mirror?