A lens with one surface plane and the other surface convex is called a ___ lens.

Plano-convex

What type of lens has both surfaces curving inwards?

A double concave lens or simply a concave lens.

I am a lens that diverges light and is thicker at the periphery than at the middle. What am I?

Fill in the blank: A lens that converges light rays is called a ___ lens.

Convex

What are the three types of convex lenses?

The three types of convex lenses are bi-convex (double-convex or equi-convex), plano-convex, and concavo-convex.

What is the origin of the coordinate system used for measuring distances in optics?

Fill in the blank: The focal length of a convex lens is taken as ___ and that of a concave lens is taken as ___.

True or False: All lengths measured below the principal axis are considered positive.

False. Lengths measured below the principal axis are considered negative.

In a lens system, what convention is followed for distances measured above the principal axis?

All distances measured above the principal axis are taken as positive.

What is the significance of placing the object on the left of the lens in the sign convention?

Placing the object on the left of the lens aligns the measurements with the Cartesian coordinate system.

Fill in the blank: The point on the principal axis of a lens through which light rays pass undeviated is called the ___.

Optical centre.

True or False: A lens has only one principal focus located on one side of the optical centre.

False. A lens has two principal foci situated at equal distances from the optical centre, one on either side.

Riddle: I am the line that connects two centers, but I am not a road. I help in understanding how light flows through a lens. What am I?

The principal axis.

What happens to a ray of light passing through the optical center of a lens?

A ray of light passing through the optical center of the lens will emerge without any deviation.