Question:
Fig. 7.1 is a full-scale diagram of a small nail N in front of a thin converging lens. The line L represents the lens.
The focal length of the lens is 3.0 cm.
(a) Rays of light, parallel to XY, are travelling towards the lens.
Describe what happens to the light after it passes through the lens.
Answer/Explanation
Ans: (all the light) meets (at a point) or is focused or intersects
(all the light) travels towards a point
it then diverges or spreads out (from that point) or point of convergence is on XY / at F / the focal point / principal focus / 3.0 cm from lens
(b) On Fig. 7.1, mark and label with an F each of the two principal focuses of the lens.
Answer/Explanation
Ans: two marked points on XY 3.0 cm from centre of lens and one on left and one on right and each labelled F
(c) The small nail N, of height 1.2 cm, is positioned 2.0 cm to the left of the lens.
(i) By drawing on Fig. 7.1, find the position of the image I of N and add image I to the diagram.
Answer/Explanation
Ans: two of these rays from tip of N drawn:
ray (that seems to come) from left-hand principal focus and emerges from lens paraxially
paraxial ray to lens and then towards right-hand principal focus
ray towards / through centre of lens
two rays traced back to intersection
and line from intersection to axis and line labelled I
(ii) State and explain whether I is a real or a virtual image.
Answer/Explanation
Ans: virtual and light / rays do not pass through I or
virtual and light / rays only seem to come from I or
virtual and produced by diverging rays
virtual and (real) rays do not meet
(iii) State the name given to a lens when it is used in this way.
Answer/Explanation
Ans: magnifying glass
Question
(a) Fig. 6.1 is a full scale diagram showing a converging lens, the two principal focuses F1 and F2 and an object PO.
On Fig. 6.1, draw two rays from point O of the object to determine the position of the image. Label the image IJ. Measure the length of the image.
image length = …………………………………………………..
(b) Ring three descriptions of the image.
diminished magnified real same size
same way up as object upside down compared to object virtual
(c) Fig. 6.2 shows three rays of green light passing through glass blocks. 
Three rays of red light approach the glass blocks on the same paths as the rays of green light.
On Fig. 6.2, draw the paths of these rays of red light to the right of the glass blocks.
Answer/Explanation
Answer:
(a)Any two correct rays from
• from O through optical centre (and beyond)
• from O parallel to principal axis to centre line of lens then through F1
• from F2 through O to centreline of lens then parallel to principal axis
M2 rays traced back to intersect AND 2.4 – 3.6 cm
(b) magnified
same way up as object
virtual
(c) one ray from each prism refracted towards principal axis
(rays) converge to the right of original convergence on the principal axis