Page 208 - Physics_Form_2

Physics for Secondary Schools

Real-is-positive convention
To determine the values of u and v using the formula, a sign convention is adopted. The
convention is referred to as the real-is-positive convention. When using this convention,

all distances are measured from the optical centre. Distances of real objects and real
images are treated as positive whereas distances of virtual objects and images are taken
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to be negative. Because the principal focus of a concave lens is virtual, concave lenses
have negative values of focal length.

Activity 5.14

Aim: To verify the lens equation

Materials: an illuminated object, screen, metre rule, a convex lens of a known focal
length, lens holder, computer or tablet with graphing software

Procedure

1. Set up the apparatus as shown in the Figure 5.41.

Illuminated Screen
object position Lens

Ray
Box

u v
Metre rule
Figure 5.41

2. Place the object and the screen at a reasonable distance apart so that a real image
is formed on the screen.
3. Adjust the lens position along the metre rule until a sharp image is formed on the
screen.
4. Measure the object and the image distances from the lens. By changing the position
of the object, obtain several values of u and the corresponding values of v. Record

your results in a table like the one shown in Table 5.6.

202
Student’s Book Form Two

Physics Form 2 Final.indd 202 25/10/2025 10:28