# Light

## Laws of Reflection

There are two laws of reflection: 1) The angle of incidence equals the angle of reflection; 2) The incident ray, the reflected ray and the normal all lie in the same plane.

Reflection occurs when light waves are returned (bounced off) after hitting a surface. When energy, such as light or sound, traveling from one medium encounters a different medium, part of the energy usually passes on while part is reflected.

Diffraction is a property of wave motion, in which waves spread and bend as they pass through small openings or around barriers. Diffraction is a property of the motion of all waves. For example, if a radio is turned on in one room, the sound from the radio can be heard in an adjacent room even from around a doorway. Light can bend a round corners in certain circumstances. Diffraction occurs when light passes the edge of an object but it is not easy to detect. This suggests that it has a very small wavelength.

Refraction is the bending of light when it passes from one kind of material into another. Because light travels at a different speed in different materials, it must change speeds at the boundary between two materials. If a beam of light hits this boundary at an angle, then light on the side of the beam that hits first will be forced to slow down or speed up before light on the other side hits the new material. This makes the beam bend, or refract, at the boundary. Light bouncing off an object underwater, for instance, travels first through the water and then through the air to reach an observer’s eye. From certain angles an object that is partially submerged appears bent where it enters the water because light from the part underwater is being refracted.

An object is opaque when light does not pass through it. Black construction paper is a good example of an opaque material. If a pencil is held up behind the paper, we cannot see its image through the paper, and we know the paper is opaque.

An object is translucent if some light can pass through the object. A Tupperware bowl is one example of a translucent object. If you held a pencil behind one wall of the Tupperware, you could see it, but you would not be able to clearly discern all of its features.

Transparent things allow most light to pass through. Clean windows are transparent. It is easy to see objects through a window because light passes through them uninterrupted.

These are two examples of spectroscopes. Scientists use spectroscopes to identify gases.

## White light

By passing a beam of sunlight through a prism, Isaac Newton observed that the beam spread out into a colored band of light, called a spectrum. Newton showed that the differences in color were caused by differing degrees of a property he called refrangibility. Refrangibility is the ability of light rays to be refracted, or bent by a substance. For example, when a ray of violet light passes through a refracting medium such as glass, it bends more than does a ray of red light. Newton concluded through experimentation that sunlight is a combination of all the colors of the spectrum and that the sunlight separates when passed through the prism because its component colors are of differing refrangibility. This property that Newton discovered actually depends directly on the wavelengths of the different components of sunlight.

## Light waves

Although we cannot see how light travels, we know it displays the properties of transverse waves. Light waves, optical tools (eyeglasses, lenses, flashlight, camera, kaleidoscope, binoculars, microscope, light boxes, telescope, prism, spectroscope, mirrors)

* historical contributions in understanding light.