The power of a lens is $5$ dioptre. What is the focal length of the lens (in $m$)?

With the help of a ray diagram,show that the angle of incidence is equal to the angle of reflection when a ray is incident on a concave mirror.

An object is placed at a distance of $18 \ cm$ in front of a concave mirror of focal length $12 \ cm$. Where will the image be formed?

$(a)$ $A$ thin converging lens forms a:
$(i)$ Real magnified image.
$(ii)$ Virtual magnified image of an object placed in front of it.
Write the positions of the objects in each case.
$(b)$ Draw labelled ray diagrams to show the image formation in each case.
$(c)$ How will the following be affected on cutting this lens into two halves along the principal axis?
$(i)$ Focal length,
$(ii)$ Intensity of the image formed by half lens.

Define power of a lens. What is its unit? One student uses a lens of focal length $50\, cm$ and another of $-50\, cm$. What is the nature of the lens and its power used by each of them?

When light rays parallel to the principal axis fall on a convex lens,they refract and converge at a point on the principal axis. What is this point called?