Answer in Brief

Numerical

List four characteristics of the image formed by a concave lens of focal length 20 cm when the object is placed at a distance of 40 cm from its optical centre.

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#### Solution

Characteristics of the image formed by a concave lens :

According to sign convensions :

`f = 20 "cm"` ; `u = -40 "cm"`

we know that ,

`-1/f = -1/"v" - 1/u`

`therefore 1/"v" = 1/f - 1/u = 1/20 - 1/40 = 20/800`

`therefore 1/"v" = 1/40`

`therefore "v" = 40 "cm"`

(i) image will be formed at 40 cm from the optical centre.

(ii) Radius of curvature of lens :

`R = -2f ∴ R = -2 xx 20 = -40 "cm"`

(iii) Since image distance is positive, therefore image is virtual.

(iv) Image will be erect.

(v) Image will form opposite side of the lens.

Concept: Refraction of Light

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