Significant information about celestial bodies and the universe comes from an analysis of their visible light. A basic property of an isolated light is that it travels in a straight line. This property can be demonstrated when one turns on a flashlight and observes a ray of light emitted by the flashlight. However, light can change direction under certain conditions, for example, when it is reflected from a surface. When light is not completely absorbed by an object, the light will be reflected. Placing objects, for example, a book in front of a mirror will show a mirror image of the book. A mirror therefore reflects light and forms images.
When light moves from one medium to another, the direction of light changes. The path of the light will be bent, or refracted. This concept may be demonstrated by passing a beam of light into a glass filled with water. The direction a ray of light travels changes when it passes from one medium such as air into another such as glass. This bending of light as it passes from one medium to another is called refraction.
Light is a type of electromagnetic wave. It is made up of the colors of the visible spectrum. The color with the longest wavelength is red and the color with the shortest wavelength is violet. Some electromagnetic waves have shorter and longer wavelengths than the visible spectrum. Light with a wavelength that is slightly longer than red light is called infrared light. Light with a wavelength that is slightly shorter than violet light is called ultraviolet light. Infrared and ultraviolet light are not part of the visible spectrum and cannot be seen by the human eye. Visible light is only a small part of a larger electromagnetic spectrum. The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays and gamma rays.
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radio waves have the longest wavelength and the lowest frequency. They are used for radio, television and radar.
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infrared rays can be felt as heat.
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ultraviolet rays are present in sunlight. They can cause sunburn and also be used to kill bacteria.
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x-rays have a very short wavelength an high frequency. X-rays have a great deal of energy and are used to make images of internal organs.
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gamma rays have a shorter wavelength and high frequency than x-rays. They are given off during nuclear reactions.
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When light is not completely absorbed by an object, the light will be reflected. Reflection occurs when a wave of light bounces back after striking a barrier. When a beam of light strikes a smooth surface, the light is reflected at the same angle. For example, the light reflected from the surface of a still pond will form clear images of the objects around the pond.
There are two types of reflection: a) regular reflection b) diffuse reflection. Regular reflections are formed by a mirror which reflects light and forms images. A beam of light striking a mirror at a certain angle will be reflected from the mirror at the same angle. The mirror forms a clear image because of its smooth surface. In the case of diffuse reflection, the light will be reflected at different angles forming a fuzzy image. The rough surface upon which light strikes will also accounts for an unclear image. For example, the surface of a pond will not be smooth on a windy day. Thus the light reflected on the surface of the pond will form fuzzy images.