an object is held at the principal focus of a concave lens of focal length f where is the image formed explain it??
Answers
❤hola ❤
here,
u= -F,
therefore ,1/v = 1/F + 1/u
= 1/F - 1/F
= 0
✌ⓣⓗⓐⓝⓚⓢ
Explanation:
principal axis from which light rays parallel to the principal axis, appear to diverge after refraction by the lens. It is usually represented by the letter ‘F’. However a concave lens has two spherical surfaces and hence it has two principal foci or two focal points which are usually denoted by the letters ‘F1’ and ‘F2’ and are known as the first principal focus ‘F1’ and the second principal focus ‘F2’. The two foci of a lens are at equal distances from the optical centre, one an either side of the lens.
Since the light rays do not actually pass through the focus (or foci) of a concave lens, therefore, a concave lens has a virtual focus.
(i) First Principal Focus (F1) : The first principal focus (F1) of a concave lens is the virtual position of a point object on the principal axis of the lens, for which the image formed by the concave lens is at infinity. It is usually denoted by the letter ‘F1’, as shown in the figure (a)
(ii) Second Principal Focus (F2): The second principal focus (F2) of a concave lens is the position of image point on the principal axis of the lens, when the object is situated at infinity. It is generally denoted by the letter (F2), as shown in the figure (b).
FOCAL LENGTH:
The distance of the principal focus (F1 or F2) from the optical centre ‘O’ of a lens is called its focal length. It is usually denoted by the letter ‘f ‘. Since a concave lens has two principal foci, so it has two focal lengths, known as : first focal length (f1) and second focal length (f2).
(i) First Focal Length (f1): The distance of first principal focus (F1) of the lens from optical centre ‘O’ of the lens is called first focal length of concave lens. It is represented by ‘F1’. i.e. OF1 = f 1. [see (a) in above figure].
(ii) Second Focal Length (F2): The distance of second principal focus (F2) of the lens from the optical centre ‘O’ of the lens is called second focal length of concave lens. It is represented by ‘f2’. i.e. OF2 = f2 [see (b) in above figure].
Rules for the formation of images by Convex Lens:
The positions of the image formed by a convex lens can be found by considering two of the following rays (as explained below).
(i) A ray of light coming parallel to principal axis, after refraction through the lens, passes through the principal focus (F) as shown in the figure.
Convex Lens
(ii) A ray of light passing through the optical centre O of the lens goes straight without suffering any deviation as shown in the figure.
(iii) A ray of light coming from the object and passing through the principal focus of the lens after refraction through the lens, becomes parallel to the principal axis.
Image formed by Convex Lens:
The position, size and nature of the image formed by a convex lens depends upon the distance of the object from the optical centre of the lens. For a thin convex lens, the various case of image formation are explained below :
(i) When object at infinity:
When an object lies at infinity, the rays of light coming from the object may be regarded as a parallel beam of light. The ray of light BO passing through the optical centre O goes straight without any deviation. Another parallel ray AE coming from the object, after refraction, goes along EA’ Both the refracted rays meet at A’ in the focal plane of the lens. Hence, a real, inverted and highly diminished image is formed on the other side of the lens in its focal plane.
Real, Inverted and highly diminished image