Question

The centre of the lens is called:
A. Principal focus
B. Principal axis
C. Optical centre
D. None

Answer 1

Hint: It is the point through which light always passes without any deviation. The centre of a lens always lies within the lens and not away the lens. The centre of the lens lies on the principal axis only for all the lenses. It can be easily found by projecting some light over the lens.

Complete answer:
A lens is a transmissive optical device that focuses a light beam by following the phenomenon called refraction. A simple lens consists of a single piece of transparent material, while a compound lens contains different simple lenses together. A lens can be used to focus light on an object. A lens consists of an imaginary line called principal axis on which the optical centre is present. We can find out the centre of a lens by projecting a beam of light on it. If the ray through any point goes through it without any bending, deviation and reflection from its path, that point can be considered as an optical centre.

Principal focus is the point where the beam parallel to the principal axis passes through or appears to pass through that point after passing through the lens. Principal axis is an imaginary line which is chosen as a reference line for all the problematic calculations. The line passing through the optical centre and centres of curvature of the face of lenses is the principal axis. The principal focus, optical centre etc. Points are present on the principal axis.

So, the correct answer is “Option A”.

Additional Information:
A lens is a tool used to bring the light to a fixed focal point. One of the primary characteristics of a lens is focal length. It is the distance between optical centre and camera’s Image sensor when focused at infinity. The lens bends light rays as they pass through it so they change its direction. The rays seem to come from a point that’s closer or further away from where it actually originates, this makes the image look bigger or smaller accordingly.

Note:
There’s a lot of difference between principal focus and principal axis. Principal axis is the imaginary line on which the principal focus, optical centre, is present. Any ray passing through the optical centre will not get deflected at all.