The rays travelling parallel to the principal axis after reflecting from the concave mirror pass or appear to pass through the focus.

When an object is placed at the center of curvature $(C)$ in front of a concave mirror,what kind of image is formed?

What is the $SI$ unit of the power of a lens?

Where will the image be formed by a concave mirror when an object is placed between the pole and the focus point of the mirror?

$(a)$ Define the optical centre of a spherical lens.
$(b)$ You are given a convex lens of focal length $30 \, cm$. Where would you place an object to get a real,inverted,and highly enlarged image of the object? Draw a ray diagram showing the image formation.
$(c)$ $A$ concave lens has a focal length of $20 \, cm$. At what distance from the lens should an object be placed so that it forms an image at $15 \, cm$ away from the lens?

The power of a lens is $2.5$ dioptre. What is the focal length and the type of lens?