Question

L₁ is a convex lens of focal length 3f and L₂ is a concave lens of focal length f. The two are placed a distance 2f apart. The principle axis of two lens are parallel, but a distance h apart, as shown in Fig. A parallel beam is incident on L₁ . The emergent beam, after refraction at both L₁ and L₂ , is

(1) A parallel beam parallel to axis of L₁ and L₂

(2) A parallel beam, making an angle of tan^-1 (h/y), in the anticlockwise direction, with the principal axis

(3) A parallel beam, making an angle of tan^-1 (h/y) , in the clockwise direction, with the principle axis

(4) A divergent beam

Answer 1

(2) A parallel beam, making an angle of tan^-1 (h/y), in the anticlockwise direction, with the principal axis

Fig shows the ray diagram. The incident parallel beam, after refraction at L₁ would have meet its principle axis at point P. The point, P, acts as a virtual object for the concave lens, L₂ , and is lying off the axis of L₂ , in its focal plane. The emergent beam is, therefore, a parallel beam, inclined to the principle axis of L₂ , at an angle θ . From right angled triangle O₂ PF₂ ;