Page 136 - Physics_Form_2

Physics for Secondary Schools

The type of shadow formed varies depending on the size of the source of light. Point
sources of light such as the ray box produce a sharp shadow (Figure 4.7 (a)). An
extended source of light produces a shadow that is not uniform. A shadow has two main
parts. One part is totally dark and is called total shadow or umbra, and the other part is
partially dark and is called partial shadow or penumbra (Figure 4.7 (b)). An example
of an extended source of light is the Sun. The shadow formed by an extended source
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of light is blurred at the edges. To explore the concept of light, perform Activity 4.3.

Screen Penumbra

Point source Opaque
of light object Umbra

(a) Shadow from point source of light (b) Shadow from an extended source of light

Figure 4.7: Shadow formation

Activity 4.3

Aim: To demonstrate the formation of a shadow from a point source of light
Materials: piece of cardboard, torch, white screen, and object hanging on a string

Procedure
1. Make a small hole (2 to 5 mm) in the cardboard (to simulate a point source).
2. Place the white screen about 30 cm from the cardboard.
3. Suspend the object between the cardboard and the white screen as in Figure 4.8.

Shadow

Torch

Screen

Cardboard
Hanging object
Figure 4.8
4. Place a light source close to the hole in the cardboard and observe the shadow of
the object that forms on the screen.

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Student’s Book Form Two

Physics Form 2 Final.indd 130 25/10/2025 10:27