1. Unit 4: Optics

2. 10.1 Sources and Nature of Light There are many different types of light. Some light is due to natural sources like the sun. Other forms of light are due to artificial sources like light bulbs. These light sources are emitted from hot objects. This type of light is a mixture of many different colours and is referred to as “white light”. But light doesn’t have to be emitted from hot objects. Some microorganisms such as algae naturally emit light. These sources of light are usually only composed of one colour of light. If we examine light on an atomic level, we will notice that in all cases, light is created after atoms absorb a certain amount of energy. When atoms absorb energy they become “excited”. But after a while, the atoms stop becoming “excited” and revert back to their normal state. When this occurs, the energy is released as light.

3. Light from the Sun: The most abundance source of light is the Sun. Energy from the sun powers all life on Earth. The tremendous amount of energy produced by the sun is due to many fusion reactions occurring inside the sun. During a fusion reaction, hydrogen atoms have so much energy that they combine or fuse together to form helium. This processes releases extremely large amounts of energy which gets transferred to the gases on the outer layers of the sun. The atoms of these gases absorb this energy, become excited and subsequently release this energy as light.

4. Light from Incandescence: Incandescence means that light is emitted by a very hot object. This occurs with an incandescent light bulb. You will notice that if you touch a light bulb that has been left on for a period of time, it will feel very hot. This is due to an electrical current running through the metal wire inside the bulb. This metal wire gets very hot and releases light. Although incandescent bulbs are an extremely common form of producing light, they are incredibly inefficient at doing so. With an incandescent bulb, only about 5% of the energy that passes through it gets converted to light. The remaining 95% is lost as heat.

5. Light from Electric Discharge: Some streetlamps are not incandescent lights. Instead they produce light by another means. In these lamps, a gas like sodium vapour is heated up. An electric current is passed through this vapour and excites the gas atoms. These atoms heat up, become excited and release light. Fluorescent Lights: A fluorescent bulb is very similar to an electric discharge lamp. These bulbs contain mercury vapour and some other inert gas. The inside of the bulb is also coated with powdery phosphor. Just like electric discharge lights, an electric current is passed through the gas to excite these atoms. These particular gas atoms release ultraviolet light which we normally cannot see. The ultraviolet light is absorbed by the phosphor coating the inner surface which emits visible light. Fluorescent light bulbs are much more efficient than incandescent light bulbs. They are about 20% efficient at converting electrical energy into light energy. These bulbs also last much longer than incandescent bulbs. Luminescence is light which is generated without heating up an object. There are several types of luminescence. Fluorescence is one type of luminescence. Another type is phosphorescence. Phosphorescence is similar to fluorescence except that the phosphorescent material retains light for minutes to hours. Phosphorescence is used in things like glow-in-the-dark objects. Chemiluminescence is light that is generated by energy released in a chemical reaction. - Bioluminescence is light that is produced by living organisms.

6. What is Light? Light is one of the most important tools we have to learn about our surroundings. It allows us to see as it carries information from the world around us to our eyes and brain. Everything you see is due to the reflection of light on objects and things. Light is a form of energy. It is the only form of energy which travels like a wave through empty space and through some materials. Because light behaves like a wave, it is referred to as an electromagnetic wave. Just like water, light travels up and down in a wave-like fashion. The wavelength is defined as the distance from one crest (or trough) to the next. Electromagnetic waves can travel through a vacuum. They do not need particles in order to travel. As shown in Figure 10.11, the electromagnetic spectrum is a diagram that shows the entire range of activity of light. All the different colours are really just different wavelengths of light. For example, red has the longest wavelength of visible light whereas violet has the shortest wavelength of visible light.