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?

$A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from the lens. Where should the needle be placed in front of the convex lens so that this image is of the same size as the object? Also, find the power of the lens.

$(a)$ Why does a ray of light passing through the centre of curvature of a concave mirror after reflection get reflected back along the same path?
$(b)$ Why are convex mirrors commonly used as rear-view mirrors in vehicles?
$(c)$ $A$ pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water. Name the phenomenon of light responsible for it.

What can be measured using a dioptre meter?

When a concave mirror produces a virtual and magnified image of an object,what is the object distance?

Which mirror is used by dentists?