Define the principal focus of a concave mirror.

The refractive index of diamond is $2.42$. What is the meaning of this statement?

$A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also,find the power of the lens.

The magnification produced by a plane mirror is $+1$. What does this mean?

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Find the focal length of a lens of power $-2.0 \, D$. What type of lens is this?