CBSE Class 10 Science 2017 · Paper 31/2 · Question 16

Light - Reflection and Refraction 3 marks Medium

Draw ray diagrams to show the formation of three times magnified

(a)real, and
(b)virtual image of an object by a converging lens. Mark the positions of O, F and 2F in each diagram.

Answer

Real image ( $m = - 3$ ): object between $F_{1}$ and $2 F_{1}$ ; virtual image ( $m = + 3$ ): object between $O$ and $F_{1}$ .

(a) Real image, three times magnified (

m = - 3

): place the object between

F_{1}

and

2 F_{1}

, at

u = - \frac{4 f}{3}

. The ray parallel to the principal axis is refracted through

F_{2}

, and the ray through the optical centre

O

goes straight on; they meet beyond

2 F_{2}

, at

v = + 4 f

, forming a real, inverted image three times the height of the object (first ray diagram). (b) Virtual image, three times magnified (

m = + 3

): place the object between

O

and

F_{1}

, at

u = - \frac{2 f}{3}

. The refracted rays diverge; their backward extensions meet on the object's side at

v = - 2 f

, the position of

2 F_{1}

, forming a virtual, erect image three times the height of the object (second ray diagram).

O

F_{1}

F_{2}

2 F_{1}

and

2 F_{2}

are marked on the principal axis in both diagrams.

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