Light and Geometric Optics

Transcription

1 By the end of this week I will be able to... label a diagram of a wave Solve word problems using the wave equation identify the types of radiation on the EM spectrum explain how light behaves like a wave and a particle Light and Geometric Optics

2 The first real theories about light came from the ancient Greeks. Many of these described light as a ray a straight line moving from one point to another. Pythagoras proposed that vision resulted from light rays emerging from a person's eye Epicurus argued the opposite: Objects produce light rays

3 Arab scholars took these ideas and honed them even further, developing what is now known as geometrical optics applying geometrical methods to the optics of lenses, mirrors and prisms. They identified the optical components of the human eye and correctly described vision.

4 They also invented the pinhole camera, discovered the laws of refraction and studied rainbows and eclipses. By the 17th century,the Dutch mathematicianastronomer Christiaan Huygens described a theory that speculated on the existence of some invisible medium (ether) that light waves or vibrations traveled through.

5 In 1704, Newton proposed a different take one describing light as corpuscles, or particles. The Electromagnetic Spectrum We now know light is a form of energy. This energy travels in waves, which together with visible light are called electromagnetic radiation.

6 Electromagnetic radiation is energy (travelling in waves) that radiates outward in all directions from a source that can travel through empty space at the speed of light, and is absorbed by other matter. A wave is a disturbance that transfers energy from one point to another without transferring matter.

7 The highest point in a wave is called a crest, and the lowest point is called a trough Crest Trough Wavelength is the distance from crest to crest or trough to trough. Symbol is (lambda) Measured in metres/nanometres. Wavelength Wavelength Crest Trough Wavelength

8 Amplitude is the wave height from the rest position to the crest (or the trough). The larger the amplitude, the more energy that is carried. Wavelength Wavelength Crest Amplitude Rest Position Trough Wavelength Frequency is the rate of repetition of a wave. The symbol is f. Measured in hertz (Hz), (cycles per second) It can also be found using the equation: f = # cycles # seconds If wave crests pass a point 10 times in a second, the frequency of the wave is 10 cycles/second.

9 The energy transferred by a wave often depends on the frequency of the wave and its amplitude. The higher the frequency, the more energy the wave passes along. Frequency and wavelength have an inverse relationship, which means that when one value increases, the other decreases. There is a mathematical relationship among the speed (v), the frequency (f), and the wavelength ( ) of the wave:

10 Ex. Red light has a wavelength of 700 nm. If its frequency is x Hz, what is the SPEED OF LIGHT? THIS IS THE SPEED OF LIGHT IN A VACCUUM (3 x 10 8 m/s) Example 2. Knowing that the speed of light is 3.0 x 10 8 m/s and that some X rays have a wavelength of 5.25 nm, what is the frequency of the X rays?

11 Ultraviolet light has a frequency of 5.21 x Hz. What is the wavelength? (Give your answer in scientific notation to two decimal places) The wavelength is 5.76 x 10 5 m. The electromagnetic spectrum represents the range of radiation from low energy, to high energy

12 We can only see a tiny portion of the spectrum called visible light. What we see is a rainbow of colors. The difference between colours of light is: each colour has a different wavelength and frequency Visible light, wavelenght and frequency

13 What is the relationship?

14 Radio: Yes, this is the same kind of energy that radio stations emit into the air. But radio waves are also emitted by other things... such as stars and gases in space. Microwaves: They will cook your popcorn in just a few minutes! Microwaves in space are used by astronomers to learn about the structure of nearby galaxies, and our own Milky Way! Infrared: Our skin emits infrared light, which is why we can be seen in the dark by someone using night vision goggles. In space, IR light maps the dust between stars. Visible: Yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars Ultraviolet: We know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn! Stars and other "hot" objects in space emit UV radiation. X rays: Your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X rays. Gamma rays: Radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma rays.. We can also think about light as a particle: It is made of photons (massless particles that travel in a wave like pattern at the speed of light) Photons contain a specific amount (bundle) of energy How much energy the photons contain tells us where the radiation is on the electromagnetic spectrum