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?