is10_sb_unit_d

Shadows
translucent
opaque
transparent
Figure 10.45 Ray diagrams show how light is affected by transparent, opaque, and translucent
materials.
You can use ray diagrams to help explain the size and location of
shadows and why some shadows are sharp and well defined while other
shadows have less distinct edges. A shadow occurs when an opaque
object blocks the direct light from a light source. A ray diagram
illustrates how the size of a shadow depends on the size of the object
blocking the light and its distance from the light source (Figure 10.46).
shadow
shadow
objects
light source
Figure 10.46 The shadow is larger if the object is closer to the light source.
A small light source casts shadows that are sharp and well defined.
If the light source is large compared to the object blocking the light,
then the shadows will not have a sharp edge, because the object only
partly blocks the light. The wider the light source is, the more blurred
the shadows will be. The umbra is the part of the shadow in which all
light rays from the light source are blocked. The area of partial shadow
from a non-point light source is called the penumbra. A ray diagram
can show why some shadows form with a penumbra (Figure 10.47).
point source
of light
Light is part of the electromagnetic spectrum and travels in waves.
A
large source
of light
B
opaque
object
penumbra
umbra
Figure 10.47 A ray diagram illustrates
how the umbra and penumbra form
in a shadow when the light source is
not a point.
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