Physics [5 Mark Question Answer]

(ii) The size of the image will be same as that of object.
(iii) The image formed will be real and inverted.
(iv) The distance of object O from optical lens will be equal to the focal length of the lens.
(v) The position of the mirror from lens does not affect the formation of image as long as the rays from the lens fall normally on the plane mirror M.

The approximate focal length of a convex lens can be determined by using the principle that a beam of parallel rays incident from a distant object is converged in the focal plane of the lens.
In an open space, against a white wall, a metre scale is placed horizontally with its 0 cm end touching the wall.



By moving the convex lens to and fro along the scale, focus a distant object on wall. The image which forms on the wall is very near to the focus of the lens and the distance of the lens from the image is read directly by the metre scale. This gives the approximate focal length of the lens.

(i) When object (AB) is situated at infinity then parallel rays from object appears to fall on concave lens. Due to which image forms at focus. This image is highly diminished in size and virtual and upright



(ii) When object (AB) is situated at any point between infinity and optical centre of the lens then image forms between focus and optical centre. This image is diminished in size and virtual and upright.

(i) When the object is situated at infinity, the position of image is at F2, it is very much diminished in size and it is real and inverted.



(ii) When the object (AB) is situated beyond 2F1, the position of image (A'B') is between F2 and 2F2, it is diminished in size and real and inverted.



(iii) When the object (AB) is situated at 2F1, the position of image (A'B') is at 2F2, it is of same size as the object and real and inverted.



(iv)When the object (AB) is situated between 2F1and F1, the position of image (A'B') is beyond 2F2, it is magnified in size and real and inverted.



(v) When the object (AB) is situated at F1, the position of image is at infinity; it is very much magnified in size and real and inverted.

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(vi) When the object (AB) is situated between lens and F1, the position of image (CD) is on the same side, behind the object; it is magnified in size and virtual and upright.

(i) A ray of light incident at the optical centre O of the lens passes undeviated through the lens.

(ii) A ray of light incident parallel to the principal axis of the lens, after refraction passes through the second focus F2 (in a convex lens) or appears to come from the second focus F2 (in a concave lens).

(iii) A ray of light passing through the first focus F1 (in a convex lens) or directed towards the first focus F1 (in a concave lens), emerges parallel to the principal axis after refraction.

The sun is at infinity so convex lens forms its image at second focal point which is real and very much diminished in size.



While using the convex lens as burning glass, the rays of light from the sun (at infinity) are brought to focus on a piece of paper kept at the second focal plane of the lens. Due to sufficient heat of the sun rays, the paper burns. Hence this lens is termed as 'burning glass'.

If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the incident ray downwards. The lower part bens the ray upwards while the central part passes the ray undeviated.

But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower part bends the ray downwards while the central part passes the ray undeviated.

Concave lens representing second focus

A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.
For a concave lens, the first focal point is a point F1 on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.

The second focal point for a concave lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.

A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.For a convex lens, the first focal point is a point F1 on the principal axis of the lens such that the rays of light starting from it or passing through it, after refraction through lens, become parallel to the principal axis of the lens

The second focal point for a convex lens is a point F2 on the principal axis such that the rays of light incident parallel to the principal axis, after refraction from the lens, pass through it.