Chapter 9: Light, Colour and Radiant Energy. Passed a beam of white light through a prism.

Transcription

1 Chapter 9: Light, Colour and Radiant Energy Where is the colour in sunlight? In the 17 th century (1600 s), Sir Isaac Newton conducted a famous experiment. Passed a beam of white light through a prism. A band of colours emerged each refracted (bent) at a different angle, producing a rainbow effect. Next, he passed these colours through a reversed prism. Only white light emerged. He proposed that white light, such as sunlight, is the result of mixing together all the different colours of light.

2 The Spectrum When white light is refracted into different colours, the resulting pattern is called a spectrum. For sunlight, this pattern is called a solar spectrum or a rainbow. The colours range from red, through orange, yellow, green, blue, indigo and violet. Remember ROY G. BIV. When light strikes an object it may be reflected off the object, refracted in the objects, absorbed by the object or transmitted through the object. If we see blue, then all colours are absorbed EXCEPT blue, which is reflected. SO: an object reflects the colour that it appears to be.

3 Additive Primary Colours Colour wheel Demonstration Additive Primary Colours red, green and blue Called this because if you ADD all three together in the proper amount you get white light. Television screens use additive colours. If all three glow brightly you see white light. All colours can be produced by varying the intensity of the three primary colours. The light of two additive colours will produce a Secondary Colour. Also called Subtractive Primary Colours because one of the primary colours is subtracted from white light to form it. Example: if you subtract green from the primary colours, you are left with red and blue, which form magenta. These are yellow, cyan and magenta.

4 These are the colours that are used in computer printers. The Colour Wheel: An Effective Tool Colours that are opposite each other have none of that colour in them. So: Cyan has absolutely no Red in it because it a combination of blue and green. They are opposite on the colour wheel. Colours are made up of the two colours that are on each side of it. RGB vs CMYK WHY DO WE USE RGB WITH TV S BUT CMYK WITH PRINTERS. (WHAT THE HECK IS THE K??) Red, green and blue use light to form the colours. All these will produce any other colour. Yet when we use ink, ink will not absorb all colours so that true black can be formed. So we have to add black ink (there s the K ) in a printer to get black on our paper.

5 How We See Colour Two types of cells in the retina of the eye that respond to light. 1. Rods cells that look like tiny cylinders. They detect light. 2. Cones cone shaped cells. They detect colour. Cones mainly respond to red, blue and green. This is why we THINK we see white when only those colours are used. Signals from the cone and rod cells travel along the optic nerve to the brain, which interprets the shape and colour of what you see. Defective cone cells cause people to have difficulty in detecting some colours. This is called colour blindness.

6 A New Way of Thinking about Light You have seen that light travels in straight lines. Some light seems to spread out around each side of a small opening. A Dutch scientist proposed that light travels as a wave. The Wave Model of Light To understand how light moves as a wave, we must study the anatomy of a wave. Resting Position Amplitude Parts of a Wave Wavelength distance from one crest to the next. Wave Height distance from the trough to the crest.

7 Crest Highest point in a wave. Trough Lowest point in a wave. Amplitude height of the crest or the depth of the trough from the resting position. Frequency the rate at which the waves move. It s the number of cycles completed by a vibrating object in a unit of time. (c/s cycles per second) Hertz unit in which frequency is measured. Example: if something vibrates 20 times in a second, its frequency is said to be 20 hertz (20 Hz). The Wave Model of Light Picture light traveling as a wave. Just as for water waves, the distance between the crests and troughs is called a wavelength. When light passes through small spaces it spreads out (radiates). Long wavelengths spread out more than short ones which explains the curved appearance of a rainbow.

8 Laser Light In 1966, Theodore H. Maiman became the first person to use a process called Or Laser Light Amplification by the Stimulated Emission of Radiation As you have seen, light consists of many colours. This means that it gives off waves of different lengths and frequencies. Waves are jumbled and work against each other. Called incoherent. Laser emits waves with only one frequency or wavelength. They all work together. Called coherent. (Incandescent light) ( (Laser light)

9 Uses of Laser Today Lasers have many uses in today s society. o CD and DVD technology. Lasers read tiny grooves in disks. o Used to perform delicate eye surgery. o Used to scan groceries in stores. o Used to cut metal in industry. o Used in surgery to reduce bleeding and scarring. o Used by police to measure speed of cars. o Used in fiber optics to transmit large quantities of information over long distances.

10 Beyond Light In water waves, water particles vibrate up and down as the wave passes. In a light wave, electrical and magnetic fields vibrate. Therefore, light is classified as electromagnetic radiation. Visible light energy, and all invisible forms of radiant energy, exists on the electromagnetic spectrum. Electromagnetic Spectrum

11 Different colours of light represent different frequencies and wavelengths of light. A nanometer is 1.0 x 10-9 m. This is equal to m ( would fit on the thickness of a piece of paper!) One Billionth of a meter!! Red light has a wavelength of 700 nanometers or 7.0x10-7 m As you go the red end of the visible spectrum the wavelengths get longer and the frequency decreases. The opposite is true at the violet end of the visible light spectrum. Gamma rays have a very short wavelength and a very high frequency. Visible Light Wavelengths in Nanometers

12 Infrared Radiation If you stretched red light to 1000 nanometers, it would no longer be light. It would be heat radiation - infrared radiation. You can feel infrared. Many uses include motion sensors, burglar alarms, lamps to keep food warm, remote controls and cameras for night vision. Radio Waves If you could stretch the waves out even further, you would get radio waves. Have a longer wavelength and lower frequency. Microwaves have the shortest wavelengths and highest frequency of all radio waves. Strongly absorbed by water particles. Causes them to vibrate and become hot. Only food that contain water particles can be heated using microwaves.

13 Microwaves are also used in radar. The waves that are reflected tell the operator the location and speed of the object. Radio waves with longer wavelengths are used to broadcast radio and television programs. It is not known for certain if long-term exposure to radio waves could be harmful. Ultraviolet Radiation At the violet end of the spectrum, wavelengths of 200 nm are known as ultraviolet (UV) radiation. Extremely energetic, causing tanning, which is a way of trying to protect itself against the UV waves. Major cause of skin cancer.

14 Can also damage the cornea in the eye causing loss of vision. The ozone layer absorbs much of the UV radiation, but due to depletion of this layer in the atmosphere, more UV reaches us. Ozone depletion caused by greenhouse gases and the use of CFC s (chlorofluorocarbons), which used to be used in refrigerators, Styrofoam and aerosol sprays. X rays Even shorter wavelengths and higher frequencies are X rays. Penetrating and extremely energetic. Pass through skin and muscle but absorbed by bone.

15 Gamma Rays Shortest waves and highest frequency. Produced from nuclear reactions and can kill cells. Used to kill cancer cells and is called radiation therapy. Can only be blocked by several feet of concrete or several inches of lead.