Fill in the Blanks

Principal axis is also called optical axis of the mirror or lens .

Benjamin Franklin, an American scientist, discovered that lightning and spark from your clothes during winters are same phenomena. Electric spark on large scale is known as lightning.

Electric spark is the electrical discharge that occurs when sufficiently high electric field creates an ionised channel.

Hydrometer is an instrument used for determining the density of a liquid. It usually consists of a glass float with a long thin stem which is graduated. The glass float is a large hollow bulb which increases the buoyancy so that the hydrometer floats. The narrow stem increases the sensitivity of the hydrometer. The bottom of the hydrometer is made heavier by loading it with lead shots so that it floats vertically.

A magnet can be easily turned into a permanent magnet by excessive heating.

Among the following options, magnetite is the only natural magnet.

tungsten has a high melting point and is able to withstand heat and corrosion. Tungsten is one of the most durable types of filaments used in light bulbs. It must be exposed to very high temperatures before melting, and only carbon arc has a higher heat resistance level than tungsten.

Potential energy is stored energy or the energy of position. The potential energy stored in a body of water held at a given height is converted to kinetic energy (movement energy) which is used to turn a turbine and create electricity.
Hence, water stored in the reservoirs of hydro-electric dams has potential energy.

Due to lateral inversion by mirror, right appears on left and left appears on right.

Planet Venus which is often visible before sunrise is called Morningstar. Likewise, John Wycliffe, who lived hundred years before Martin Luther was born, is heralded as the Morning Star of the Reformation. Wycliffe made sincere efforts to take the Bible to every hand at a time when translating Bible to a common language was forbidden by Roman Catholic Law. It was a precursor to the upcoming protests which eventually destabilized the entire set up of Roman Catholic Church.

A compass is an extremely simple device. A magnetic compass consists of a small, lightweight magnet balanced on a nearly frictionless pivot point. No matter where you stand on Earth, you can hold a compass in your hand and it will point toward the north and south pole.

Collision is a short time event in which two or more bodies apply forces on each other for a relatively short time. 

The above written document is known as circular. Circular can be defined as a type of written document which are mainly used in business organization in order to spread any information or message to all the employees working within the organization.

Ultrasound has higher frequency than the audible range sound, and thus it has shorter wavelength. Due to this short wavelength, it becomes easy for it to interact(reflect, disperse) from small objects in its path.