A beam of parallel rays is brought to a focus by a plano-convex lens. A thin concave lens of the same focal length is joined to the first lens. The effect of this is

Step-by-step solution

The combined focal length of plano-convex lens is 1 f = 1 f₁ + 1 f₂ where, f₁= for the plane surface and f₂=f array ll & 1 f = 1 + 1 f f & =f array Now, when concave lens of same focal length is joined to first lens, then combined focal length array ll & 1 F = 1 F₁ + 1 F₂ 1 F & = 1 f - 1 f & 1 F =0 F= array Thus, the image can be focused on infinity or focus shifts to infinity.