Thursday, November 28, 2019

[pic] Essays (3148 words) - Physics, Chemistry, Nature, Spectroscopy

[pic] This activity will focus on the visible portion of the electromagnetic spectrum. Background Information: About 300 years ago, Sir Isaac Newton saw a beam of sunlight through a glass prism. He discovered that light is made up of a spectrum of seven distinct visible colors. This spectrum of colors always appears in the same order. You can see this color spectrum (Red, Orange, Yellow, Green, Blue, Indigo, Violet and all the colors in between) when you look through a diffraction grating. There are two color ranges that are not visible to our eyes in this spectrum: below red is infra-red and above violet is ultra- violet. In a rainbow after a rainstorm this same color spectrum appears in the same order. Rainbows are created when sunlight passes through rain drops that act as millions of tiny prisms. The color of a solid object depends on the colors of light that it reflects. A red object looks red because it reflects red light and absorbs all other colors. A blue object looks blue because it reflects blue light and absorbs all other colors. A white object reflects all colors of light equally and appears white. A black object absorbs all colors and reflects no visible light and appears black. Just like when you color with too many colors in one area with crayons or markers, all colors are absorbed, none are reflected and it appears black! Explanation of visible light at the electronic level: What do fireworks, lasers, and neon signs have in common? In each case, we see the brilliant colors because the atoms and molecules are emitting energy in the form of visible light. The chemistry of an element strongly depends on the arrangement of the electrons. Electrons in an atom are normally found in the lowest energy level called the ground state. However, they can be "excited" to a higher energy level if given the right amount of energy, usually in the form of heat or electricity. Once the electron is excited to a higher energy level, it quickly loses the energy and "relaxes" back to a more stable, lower energy level. If the energy released is the same amount as the energy that makes up visible light, the element produces a color. The visible spectrum, showing the wavelengths corresponding to each color, is shown below: [pic] Note: [1 = 0.1 nm] Is light a particle or a wave? Is light composed of waves or of particles? If light is waves, then one can always reduce the amount of light by making the waves weaker, while if light is particles, there is a minimum amount of light you can have - a single ``particle'' of light. In 1905, Einstein found the answer: Light is both! In some situations it behaves like waves, while in others it behaves like particles. This may seem odd. How can light act like both a wave and a particle at the same time? Consider a duck-billed platypus. It has some duck-like properties and some beaver-like properties, but it is neither. Similarly, light has some wavelike properties and some particle like properties, but it is neither a pure wave nor a pure particle. [pic] A wave of light has a wavelength, defined as the distance from one crest of the wave to the next, and written using the symbol [pic]. The wavelengths of visible light are quite small: between 400 mm and 650 nm, where 1 nm = 10-9 m is a ``nanometer'' - one billionth of a meter. Red light has long wavelengths, while blue light has short wavelengths. A particle of light, known as a photon, has an energy E. The energy of a single photon of visible light is tiny, barely enough to disturb one atom; we use units of "electron-volts", abbreviated as eV, to measure the energy of photons. Photons of red light have low energies, while photons of blue light have high energies. The energy E of a photon is proportional to the wave frequency f, E = h f where the constant of proportionality h is the Planck's Constant, h = 6.626 x 10-34 J s. Also, the relationship between frequency and wavelength can be defined as: f = c ? where c is the speed of light (3108 metres per second). So photons still have a wavelength. A famous result of quantum mechanics is that the wavelength relates to the energy of the photon. The longer the wavelength, the smaller the energy. For instance, ultraviolet photons have shorter wavelengths than visible photons, and thus more energy. This is why they can give you sunburn,

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