Light

What surrounds you and bombards you constantly? Most of it is invisible but you can’t imagine living without it. It is electromagnetic radiation, a type of energy commonly known as light. This energy is produced by the vibration of charged particles. As charged particles move back and forth, the electric field around it vibrates creating a vibrating magnetic field. The two vibrating fields, which are at right angles to each other, produce electromagnetic waves. These waves can travel through materials as well as a vacuum. All electromagnetic waves travel at the incredible speed of about 300,000 km/s in a vacuum, often called the speed of light. This speed is equal to the wavelength of light times its frequency and is represented by the equation c = wavelength x frequency.

Most of us think of light as a wave. Waves easily explain interactions such as reflection. However, early in the 20th century, some scientists noticed that light hitting a metal surface can sometimes eject electrons. How can light waves do this? Albert Einstein showed this can only happen if light is made up of tiny particles called photons. Einstein revolutionized physics by describing light as photons. Scientists now believe light exhibits both wave and particle properties.

Although every electromagnetic wave travels at the same speed, each can have a different wavelength and frequency. The electromagnetic spectrum organizes the types of light by decreasing wavelength and increasing frequency, from left to right. It includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. The energy of the electromagnetic wave is also related to wavelength and frequency. Energy is directly proportional to frequency and inversely proportional to wavelength. Higher frequency, shorter wavelength waves have higher energy. The wave energy increases from left to right across the spectrum. On the spectrum, radio waves have the lowest energy while gamma rays have the highest energy.

Radio waves and microwaves have the longest wavelength but the lowest energy on the electromagnetic spectrum. Radio waves can range in wavelength from thousands of meters to about 30 cm. Heinrich Hertz discovered radio waves in 1888. Today, in addition to using radio waves to broadcast radio and TV signals, astronomers use radio signals from distant parts of the universe, to study the composition of stars and planets. Microwaves have wavelengths from about 30 cm to 1 mm. In addition to cooking our food, microwaves are used by cell phones and GPS devices.

Infrared light has slightly longer wavelengths then visible light in the range of 700 nm and 1mm. Your TV remote uses infrared light to send a signal to change the channel. You can’t see infrared light, but you can sometimes sense it as heat. Infrared cameras and night vision goggles allow you to see infrared light. Warm objects will appear as bright colors. A limited amount of infrared light from the Sun penetrates Earth’s atmosphere and warms the Earth. Carbon dioxide traps this infrared light and causes warmer than normal temperatures creating the greenhouse effect.

Your eyes are tuned to see a very small portion of the electromagnetic spectrum which scientists call visible light. Visible light waves have wavelengths in the range of 400 nm and 700 nm. These wavelengths are seen by humans as different colors. The longest wavelength, 700 nm, is seen as red light, while the shortest wavelength of 400 nm is seen as violet light. This range of colors which humans can see is called the visible spectrum. Some of the energy from the Sun that reaches Earth is the visible light part of the spectrum known as white light. White light is a combination of all of the visible light wavelengths or colors. These colors can be observed by passing light through a triangular prism which separates the light into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion.

Ultraviolet light has a slightly shorter wavelength than visible light. The wavelength of ultraviolet light ranges between 60 nm and 400 nm. A limited amount of ultraviolet light from the Sun reaches Earth. While too much ultraviolet light can cause sunburn and even potentially skin cancer, our skin cells need ultraviolet light to produce vitamin D.

X-rays have wavelengths ranging from 0.001 nm to 60 nm and can penetrate many types of materials, including your body. Gamma rays have wavelengths shorter than 0.1 nm but the highest frequency of all electromagnetic waves. Gamma rays can easily pass through most materials. Fortunately, the Earth’s atmosphere blocks out most of this type of electromagnetic radiation coming from outer space. Doctors use both of these electromagnetic waves in small amounts as either an x ray to look inside the human body, or as gamma rays to kill cancer cells. Astronomers use satellites in outer space to collect x ray and gamma ray information to study the universe.