Question

What is the focal length of a
lens.​

Answer 1

Explanation:

The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm).

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Answer 2

Attachment fig. of Answer

$\sf{\huge{\underline{\purple{Answer :-}}}}$

The focal length of a lens is defined as the distance between the first principal focus & optical centre.

$\sf{\huge{\underline{\pink{About\:It :-}}}}$

A lens is a piece of a refracting medium bounded by two surfaces, at least one of which is a curved surface.

The commonly used lenses are the spherical lenses.These lenses have either both surfaces spherical or one spherical and the other a plane one.

Lenses can be divided into two categories :-

(i) Convex or converging lenses

(ii) Concave or diverging lenses.

A Convex lens :-

It is thicker at the centre than at the edges. It converges a parallel beam of light on refraction through it. It has a real focus.

Types of convex lenses :-

(a) Double convex or biconvex lens is the lens, having both surfaces are convex.

(b) Planoconcave lens is the lens, where one side is convex and the other is plane

(c) Concavoconvex is the lens,in which one side is convex and the other is concave.

Concave lens :-

It is thinner at the centre than at the edges. It diverges a parallel beam of light refraction through it. It has a virtual focus.

Types of concave lenses :-

• Double concave is the lens, both sides are concave.

• Planoconcave lens where, one side is plane and the other is concave.

• Convexoconcave lens where, one side is convex and the other is concave.

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