Sources of Light
- Luminous objects emit their own light: the Sun, a candle flame, a light bulb.
- Non-luminous objects are only visible because they reflect light: the Moon, this book.
Shadows
Light travels in straight lines, so an opaque object blocks light and casts a shadow.
- Umbra — the central, darkest part of a shadow where no light reaches.
- Penumbra — the outer, lighter region where only some light is blocked (requires an extended light source).
Transparent, Translucent, Opaque
| Property | Light passage | Example |
|---|---|---|
| Transparent | Nearly all light passes | Clear glass, water |
| Translucent | Some light passes, scattered | Frosted glass, tissue |
| Opaque | No light passes | Wood, metal |
The Speed of Light
Light is the fastest thing in the universe:
Speed of Light in Vacuum
Approximately 300 000 km/s. In other media, light is slower: v = c / n.
It takes light about 8 minutes to travel from the Sun to Earth (≈ 150 million km).
Pinhole Camera
A small hole in an opaque box lets light from each point of a scene through in a straight line, forming an inverted image on the opposite wall. This is the basic principle behind cameras.
- 1
Identify the light source
Is it a point source or an extended source? - 2
Draw rays in straight lines
From the source past the edges of the object. - 3
Mark umbra and penumbra
Umbra = fully blocked; penumbra = partly blocked.
A point light source illuminates a ball. Describe the shadow.
A point source creates only an umbra — a sharp, well-defined shadow with no penumbra.
Real world: sundials
A sundial uses the shadow cast by its gnomon (pointer) to indicate the time. As the Sun moves across the sky (apparent motion), the shadow rotates around the dial. The shadow is sharp because the Sun acts approximately as a point source — light travels in straight lines, creating a well-defined umbra at the base of the gnomon.
Real world: solar eclipse
During a total solar eclipse, the Moon passes directly between Earth and the Sun. The Moon's umbra (darkest shadow) sweeps a narrow path across Earth's surface — people in this path experience total darkness for up to 7 minutes. Those in the penumbra see a partial eclipse. Understanding light as straight-line rays makes eclipse geometry predictable centuries in advance.
Real world: photography exposure
A camera shutter controls how long light is allowed onto the sensor. The pinhole camera principle — light from each scene point enters through a small aperture in a straight line — is the foundation of all photography. Photographers balance aperture size and shutter speed to control sharpness and exposure.
Practice Problems
How long does it take light to travel 300 000 km?
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What type of material must a window pane be?
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A solar eclipse occurs when the Moon's shadow falls on Earth. Which part of the shadow do observers in the total eclipse zone stand in?