Grade 10 Academic Science - Optics
Introductory Ideas about Light
Many forms of electromagnetic radiation originate in the sun. Visible light is only one component
of electromagnetic radiation…in fact, a very small component (see the diagram below). As
shown, the wavelengths of visible light ranges from 400-700 nm (violet to red).
Why do I only see visible light?
 My eyes are sensitive to light which
lies in a very small region of the
electromagnetic spectrum called
visible light. That is, the cones and
rods in my eyes only respond to /
react with this narrow range of
wavelengths.
 Do you know what cone and rods
detect?
 My cones only see the Primary
Colours of Light (i.e., red, green
and blue). How do I see the “other”
colours?
What is colour?
 White light is a mixture of the
colours of the visible spectrum.
Black is a total absence of light. If
part of the visible spectrum is
absorbed and missing, I see the
other parts of the visible spectrum.
Thus, I see colours.
 Describe that idea in your own
words…
We cannot see ultraviolet light energy (UV) because its wavelengths are too short for our eyes to
detect this wavelength range. How do I know it exists?
 UV causes sunburns. Our skin is sensitive to this kind of light because our skin absorbs
this energy. After the energy is absorbed, it can make our skin change colour (tan).
Why? The colour protects the cell. If too much UV is absorbed, it can damage the parts
of the cell.
We cannot see infrared light energy (IR) because its wavelengths are too long for our eyes to
detect this wavelength range. How do I know it exists?
 Energy whose wavelength is too long to see is "redder than red". Light with such long
wavelengths is called "Infrared" light. The term "Infra-" means "lower than". I can feel
energy with these wavelengths (e.g., sit in front of a campfire). Light beyond red makes
things hot.
What is the speed of light?
 The velocity of light (C) in a vacuum is 2.99793 x 1010cm/sec. Light cannot travel any
faster, but if it travels through a different substance (e.g., water, oil), its velocity will
decrease.
 What happens to light as it travels between different substances?