Draw ray diagrams to show the image formed by a convex lens when the object is placed (i) between F₁ and 2F₁ and (ii) between F₁ and the optical centre. State the position, size and nature of the image in each case.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open with the two rays you will use. A ray parallel to the principal axis passes through F₂ after refraction. A ray through the optical centre goes straight.
2 · Diagram (i). [Diagram: AB between F₁ and 2F₁; parallel ray bent through F₂; ray through O straight; they meet beyond 2F₂ below the axis; A′B′ inverted and longer]
3 · Result (i). Image beyond 2F₂, enlarged, real and inverted.
4 · Diagram (ii). [Diagram: AB between F₁ and O; parallel ray bent through F₂; ray through O straight; dotted extensions meet on the object's side above the axis; A′B′ erect, larger and dotted]
5 · Result (ii). Image on the same side as the object, enlarged, virtual and erect.
6 · Daily-life use. Case (ii) is a magnifying glass, used to read small print or check gold jewellery.
7 · Close in one line. A convex lens gives a virtual image only when the object is inside F₁.
Scoring tip: Keep F₁, 2F₁, F₂ and 2F₂ evenly spaced and use a ruler. Draw the virtual image with dotted lines.
