Radio waves have the longest wavelengths in the electromagnetic spectrum. Some radio waves have wavelengths longer than a large building. Others are shorter but still much longer than visible light. These waves are used for communication, broadcasting, and radar. They can travel long distances and often pass through walls or around obstacles.Microwaves follow radio waves on the spectrum. Their wavelengths are shorter, often measured in centimeters or millimeters. Microwaves are used for wireless communication, satellite links, radar, and cooking. In a microwave oven, these waves interact with water molecules in food, causing them to vibrate and heat up.Beyond microwaves lies the infrared region. Infrared light has wavelengths longer than red visible light but shorter than microwaves. Many objects emit infrared radiation according to their temperature. Warm objects such as human bodies glow in infrared. Night vision cameras and thermal cameras detect this radiation, turning temperature patterns into images.Next comes the small band of visible light. This is the only portion that typical human eyes can sense. Within visible light, different wavelengths correspond to different colors. Red has the longest wavelength, followed by orange, yellow, green, blue, and violet. Violet has the shortest wavelength of visible light and the highest frequency in that band.Just beyond visible violet lies ultraviolet radiation. Ultraviolet has shorter wavelengths and higher frequencies than visible light. Sunlight contains ultraviolet, which causes sunburn and can damage DNA in skin cells. Some insects can see ultraviolet patterns on flowers that humans cannot. Ultraviolet light is used for sterilization because it can kill bacteria and viruses.Still shorter wavelengths bring us to X rays. X rays are very energetic and can pass through many materials that block visible light. Medical X ray machines use them to create images of bones and internal structures. Dense materials such as bone absorb X rays more strongly, casting shadows on detectors placed behind the patient.At the extreme short wavelength end lie gamma rays. These have the highest frequencies and energies in the electromagnetic spectrum. Gamma rays are produced in nuclear reactions, radioactive decay, and extreme cosmic events. They can penetrate deeply into materials and are used in cancer treatments and industrial inspections, but they also pose serious health risks if uncontrolled.Across this vast spectrum, all electromagnetic waves share the same basic nature. They all travel at the same speed in empty space, the speed of light. They all consist of oscillating electric and magnetic fields. They all obey wave principles such as reflection, refraction, diffraction, and interference.Light behaves differently when it encounters different materials or boundaries. One fundamental behavior is reflection. Reflection occurs when light hits a surface and bounces back into the original medium. A smooth, polished surface reflects light in a very orderly way, producing a clear image. A rough surface scatters the reflected light in many directions, giving a dull appearance.The law of reflection is simple but powerful. It states that the angle of incidence equals the angle of reflection. The angle of incidence is measured between the incoming ray and a line perpendicular to the surface. The angle of reflection is measured between the outgoing ray and that same perpendicular line. Mirrors obey this law precisely, which is why they reflect images accurately.Reflection is not limited to mirrors. Your vision of almost any ordinary object relies on reflected light. Light from a source such as the sun or a lamp strikes the object. The object reflects some wavelengths toward your eyes and absorbs or transmits others. Your brain interprets the pattern of reflected light as color, shape, and brightness.Refraction is another essential behavior of light. Refraction occurs when light passes from one medium into another and changes direction. This bending happens because the speed of light is different in different materials. When light slows down or speeds up at a boundary, its path bends according to a rule called Snell law.A familiar example of refraction appears when you place a straw in a glass of water. The straw looks bent or shifted at the water surface. Light rays coming from the submerged part of the straw bend as they pass from water into air. Your brain assumes they traveled in a straight line, so the submerged part appears displaced.Lenses use refraction to focus or spread light. A convex lens, thicker in the middle than at the edges, bends incoming parallel rays toward a focal point. This focusing allows lenses in cameras, projectors, and your eyes to form sharp images. A concave lens, thinner in the middle, spreads incoming rays apart. These lenses help correct certain types of vision problems by adjusting how light focuses on the retina.The amount of bending in refraction depends on the refractive index of each medium. The refractive index is a measure of how much the medium slows light compared with empty space. Air has a refractive index slightly above one. Water has a higher value, and typical glass is higher still. When light enters a medium with higher refractive index, it slows and bends toward the perpendicular line.Prisms reveal another interesting result of refraction. White light entering a glass prism is bent differently for different wavelengths. Shorter wavelength violet light refracts more strongly than longer wavelength red light. As a result, the prism separates white light into a spectrum of colors. This dispersion creates rainbows and colorful patterns.Diffraction is the bending and spreading of waves as they pass around obstacles or through openings. When the opening size is comparable to the wavelength, diffraction becomes especially noticeable. This behavior is a fundamental property of all waves, including water waves, sound waves, and light waves.Imagine water waves moving toward a narrow gap in a sea wall. As the waves pass through the gap, they spread outward in an expanding arc. The waves do not simply travel in a straight line after passing through. This arching and spreading is diffraction in action.Sound diffracts noticeably around corners and through doorways. That is why you can hear someone speaking from another room even when you cannot see them. The wavelengths of typical sounds are similar to the size of everyday openings and objects. This size match makes diffraction effects strong for sound.Light also diffracts, but its wavelengths are extremely short compared with most everyday objects. A sheet of paper or a doorway is huge compared with visible wavelengths. So diffraction of light in daily life is usually subtle. However, when light passes through tiny slits or edges on the scale of micrometers, diffraction becomes clear.
