Light and Optics

Light is the passage of energy through space in the form of electromagnetic radiation. It is in the form of particles which are called photons. Photons travel at a constant speed, which is entirely unchanged by the motion of any observer. The speed of light is almost exactly 300 megameters per second. Photons have no rest mass, and probably don't even exist at rest, and they have no electrical charge. While in motion, however, light particles (photons) have momentum, and so a tiny mass can be calculated.

Every single photon has a characteristic color which can vary if the observer is moving toward or away from the light source. Moving toward a source makes the light more bluish, and moving away makes it appear more red. This effect (the Doppler effect) is also observed with sound.

The range of colors of light forms a continuous "spectrum", most of which is not directly observable, and was unknown until the last century. At the low-frequency (long wavelength) end of the spectrum we find radio, television, microwave, and infrared radiation. Then we encounter a tiny slice of the spectrum which we can see with our eyes from red to violet (ROYGBIV). Above violet the high-frequency (short wavelength) "colors" are ultraviolet, x-rays, and gamma rays.

Electromagnetic radiation (light) is produced, one photon at a time, when electrons fall into a lower orbit about a nucleus. The negatively-charged electrons are pulled down by the positively-charged nucleus and the energy that results is in the form of a photon. Falling into an outer orbit (3rd, 4th, etc.) produces infrared light. Falling into the innermost orbit produces ultraviolet light.

Visible light is only produced when an electron falls into the second shell of an atom (the Balmer series of spectral lines). Different colors happen because atoms have different sizes, different nuclear charges, and influences on each other when they are close together. Most of our knowledge of atoms comes from studying the light they produce. The element helium, named for the sun, was discovered on the sun by analyzing sunlight, before it was found on the earth. It was later found on the earth mixed in the methane gas in Texas oil wells. (No other nation on earth has appreciable sources of helium.)

Optics is the science of working with light. Light can be produced, reflected, refracted, diffracted, dispersed, focused, diffused, interfered (both constructively and destructively), amplified, polarized, divided, and absorbed using a variety of optical devices. A good example might be a set of ordinary eyeglasses. Suppose your eyeglasses have +3 diopter lenses (you are farsighted). The curvature of the lenses causes the light to strike and/or leave the glass at an angle. Refraction bends the path of the light at those points, causing parallel light rays to converge (focus) at a point 1/3 meter from the lens (3 diopters means the focal length is 1/3 meter). A bonus is the fact that ordinary glass is a good absorber of ultraviolet light, the cause of sunburn, and this helps protect your eyes from damage.

Using the Hubble Telescope to gather the weak light from distant galaxies, separate it into its various colors, amplify it electronically, and broadcast it back to earth by satellite transmission, we can now study the universe in ways that were thought to be impossible a few decades ago.