(a) Two thin convex lenses L1 and L2 of focal lengths f1 and f2, respectively, are placed coaxially in contact. An object is placed at a point beyond the focus of lens L1. Draw a ray diagram to show the image formation by the combination and hence derive the expression for the focal length of the combined system. (b) A ray PQ incident on the face AB of a prism ABC, as shown in the figure, emerges from the face AC such that AQ = AR. Draw the ray diagram showing the passage of the ray through the prism. If the angle of the prism is 60° and refractive index of the material of prism is, determine the values of angle of incidence and angle of deviation.

CBSE Class 12-science Answered

(a) Two thin convex lenses L₁ and L₂ of focal lengths f₁ and f_2, respectively, are placed coaxially in contact. An object is placed at a point beyond the focus of lens L₁. Draw a ray diagram to show the image formation by the combination and hence derive the expression for the focal length of the combined system. (b) A ray PQ incident on the face AB of a prism ABC, as shown in the figure, emerges from the face AC such that AQ = AR. Draw the ray diagram showing the passage of the ray through the prism. If the angle of the prism is 60° and refractive index of the material of prism is, determine the values of angle of incidence and angle of deviation.

Asked by Topperlearning User | 10 Jul, 2015, 11:30: AM

Expert Answer

(a) Consider two thin lens L₁ and L₂ of focal length f₁ and f₂ held coaxially in contact with each other. Let P be the point where the optical centres of the lenses coincide (lenses being thin).

Let the object be placed at a point O beyond the focus of lens L₁ such that OP = u (object distance). Lens L₁ alone forms the image at I₁ where P I₁ = v₁(image distance). The image I₁ would serve as a virtual object for lens L₂ which forms a final image I at distance PI = v. The ray diagram showing the image formation by the combination of these two thin convex lenses will be as shown below: