What is the power of a convex lens with a focal length of $20 \text{ cm}$?

What is the wavelength range of visible light?

$(a)$ 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.

Which of the following statements is true?

The magnification in a plane mirror is always ........

$A$ dice is placed with one of its edges parallel to the principal axis between the principal focus $(F)$ and the centre of curvature $(C)$ of a concave mirror. Then the final image has the shape of