Find the focal length of a convex lens,if its power is $+2.0 \ D$. (in $m$)

The compound microscope consists of two convex lenses of $5 \, cm$ and $20 \, cm$ focal length. Which of them will be the objective and the eyepiece?

Where is the image formed when an object is placed between the principal focus $(F)$ and the center of curvature $(C)$ of a concave mirror?

What does negative magnification mean in the context of spherical mirrors?

An object is placed at a distance of $12 \ cm$ in front of a concave mirror. It forms a real image four times larger than the object. Calculate the distance of the image from the mirror. (in $cm$)

$(a)$ State the relation between object distance,image distance and the focal length of a spherical mirror.
$(b)$ Draw a ray diagram to show the image formed by a concave mirror when an object is placed between the pole and focus of the mirror.
$(c)$ $A$ concave mirror of focal length $15\, cm$ forms an image of an object kept at a distance of $10\, cm$ from the mirror. Find the position,nature and size of the image formed by it.