Core Unit III: Light
A. Characteristics of Light
1. Sources and Transmission of Light
Key Concepts
A careful examination of the behaviour of light is necessary
before trying to theorize about its nature.
Luminous bodies emit light. Nonluminous bodies do
not. Instead, nonluminous objects usually reflect light.
Changing the condition of a nonluminous object may make it
luminous.
Incandescent bodies emit light when they are heated.
Light travels in straight lines (except in a strong gravitational
field). This is called rectilinear propagation.
The path followed by light is represented by rays. (Rays
do not actually exist in nature. They are geometric constructs
used to help explain the behaviour of light.)
A beam can be thought of as a collection of rays.
Transparent objects transmit light (absorbing
and reflecting some as well).Translucent materials scatter
and transmit light, preventing objects from being seen clearly
through them. Opaque materials prevent light from passing
through them.
Shadows form when opaque objects are placed directly in the path
of light. A total shadow is called an umbra. A partial
shadow is a penumbra. During a solar eclipse, a total
eclipse can be observed within the umbra of the shadow.
The intensity of light radiating equally in all directions from a source varies inversely with the square of the distance between the source and the observer.
Learning Outcomes
Students will increase their abilities to:
- Define the following terms: luminous, nonluminous, rectilinear propagation, beam, incandescence, ray, transparent, translucent, opaque, penumbra, umbra, eclipse.
- Give examples of some common luminous and nonluminous objects.
- Explain that light usually travels in straight lines.
- Give some examples which illustrate the rectilinear propagation of light.
- Identify objects which are transparent, translucent, and opaque.
- Apply and understanding of the inverse square law relationship between the intensity of light and the distance from the source.
Teaching Suggestions, Activities and Demonstrations
- Design an experiment to determine if the inverse square law
relationship governing the intensity of light with the inverse of
the square of the distance from the source applies for a light
source contained within a reflector (e.g., flashlight) or
a diffuser (e.g., lamp shade).
- Using a photometer and different light sources, perform an
experiment to determine the luminous intensity and efficiency of
light bulbs. (A light meter can be used instead of a photometer.
Determining the efficiency of the light sources might be more
complicated. For several different light sources operating at the
same voltage, students could plot light intensity as a function
of current. Other methods, such as plotting light intensity as a
function of temperature could be considered. Pose this as a
Science Challenge activity for students.
)
- Distinguish between a partial eclipse and a total eclipse.
Set up a demonstration to allow students to visualize these two
phenomena.
- Perform an activity to observe the umbral and penumbral
regions of a shadow. Transfer these ideas to explain what happens
during an eclipse.