Chapter 5.1 ± Light in Everyday Life
Energy and Power
x In science, the rate of energy flow is called power.
x The standard units of power are called watts.
x A power of one watt means an energy flow of joule per second: 1 watt = 1 joule/s.
o ie, for every second you leave a 100-watt light bulb turned on, 100 joules of
energy is used.
Light and Colour
x A prism that split light into the rainbow of light is called a spectrum.
x The basic colours in a rainbow-like spectrum are red, orange, yellow, green, blue, and
violet. White comes in when basic colours are mixed in roughly equal proportions.
x Light from the Sun or a light bulb is called white light, because it contains all the
colours of the rainbow
x Black is what we perceive when there is no light and hence no colour.
x Red, green, and blue are primary colours of vision, because they are the colours
directly detected by cells in your eyes.
x Because colours look different on paper, an alternate set of primary colours are red,
yellow, and blue.
x Spectrum can be produced either with a prism or a diffraction grating ± a piece of
plastic or glass etched with many closely spaced lines.
Interaction of Light and Matter
4 basic ways of interaction:
1. Emission: when you turn on a lamp, electric current heats the filament of the light bulb
to a point at which it emits visible light.
2. Absorption: if you place your hand near a lit light bulb, your hand absorbs some of the
light, and this absorbed energy warms your hand.
3. Transmission: Some forms of matter, such as glass or air, transmit light or allow light
to pass through.
4. Reflection/scattering: light can bounce off matter, leading to what we call reflection
(when the bouncing is all in the same general direction) or scattering (when the
bouncing is more random).
x Materials that transmit light are transparent, and materials that absorb light are
x Many materials are neither perfectly transparent nor perfectly opaque
o ie, dark sunglasses and clear eyeglasses are both at least partially
transparent, but the dark glasses absorb more and transmit less light.
x Many materials affect different colours of light differently.
o ie, red glass transmits red light but absorbs other colours, while a green
lawn reflects (scatters) green light but absorbs all other colours.
DIAGRAM 5.3, pg 148.