# What is Electromagnetic Spectrum?

Electromagnetic radiation is made from two sorts of energy— electrical and magnetic. As the radiation moves outward from its source, its energy constantly changes from strong to weak and back again. The changing energy sets up a pattern that scientists call waves.

### What is wavelength?

Scientists can measure the distance between two points where the magnetic energy or the electrical energy is strongest. This distance is called the wavelength of the electromagnetic radiation and is usually measured in meters or fractions of meters.

### What is frequency?

Count how often a ball bounces against a wall and back in one minute. You are measuring how often, or how frequently, the ball bounces during a certain time. Scientists measure electromagnetic radiation by counting the number of times magnetic energy and electric energy change from strong to weak. The number of complete changes in one second is called the frequency. We measure frequency in units called hertz, or Hz.

Scientists draw pictures like this to help us understand electromagnetic radiation, such as radio waves. The red waves show how the strength of the magnetic field changes as a radio wave travels.
The frequency of the red waves is lower than the frequency of the blue waves. This is because the wavelength of the red waves is longer.

### The electromagnetic spectrum

Electromagnetic radiation travels out in every direction from the point, or source, where it is generated. This radiation travels out at an enormous speed—186,282 miles (299,792 kilometers) per second in a vacuum. In other words, electromagnetic radiation would take more than one second to reach the moon, which is about 239,000 miles (384,473 kilometers) away from the earth.
The frequency of electromagnetic radiation can be as low as a few hertz. Or it may be as high as thousands of billions of hertz. Scientists call the whole range of different frequencies the electromagnetic spectrum.
At one end of the electromagnetic spectrum are gamma rays, which have the highest frequencies. At the other end of the spectrum are radio waves, which have the lowest frequencies. In between are X rays, ultraviolet waves, visible light, infrared radiation and microwaves.
Scientists have developed appliances that use electromagnetic radiation to help us in our daily life. These appliances include X-ray machines, cameras, microwave ovens, television and radios.
X rays have very high energy and so pass easily through many different materials. X rays are used to make X-ray photographs of the bones inside our bodies.