# Light and the Electromagnetic Spectrum

Most people don't think about the importance of light to living things. The truth is that without light, there would be no plants. Without plants, there would be no animals. This is because plants use light energy from the sun to convert carbon dioxide and water into sugar and oxygen, a process called photosynthesis. When you eat a burger, you are getting energy that was extracted from the sun and converted by a plant into sugar.

The Electromagnetic Spectrum

Light travels in a wave. Some waves must travel through matter. Matter is anything that has mass and takes up space. Examples of matter include your desk, your classroom, and the air in your classroom. Seismic waves and sound waves are both examples of waves that must travel through matter. Light, though, is a type of wave that does not need matter to travel through. Waves that can travel through matter or through empty space are called electromagnetic waves. An electromagnetic wave is a wave that consists of changing electric and magnetic fields. Light is one type of electromagnetic wave.

The electromagnetic spectrum is made of all the different kinds of electromagnetic waves. Visible light, ultraviolet light, and infrared light make up a very small part of the electromagnetic spectrum. All three of these waves, though, are important to organisms.

As you can see in the illustration above, light consists of visible light (the colors you can see) along with ultraviolet light (also known as "black light") and infrared light (also known as heat). Light waves, along with the other waves on the electromagnetic spectrum, have both wavelength and frequency. The relationship between wavelength and frequency is simple: long wavelengths have low frequencies and short wavelengths have high frequencies. How many of the other types of electromagnetic waves shown on the electromagnetic spectrum are you familiar with?

The illustration below also shows the electromagnetic spectrum. Just to confuse you, it is the opposite of the illustration above. You will notice that in the illustration below the longer wavelengths are on the left, whereas in the illustration above, the longer wavelengths are on the right. That's what happens when you rely on the internet for illustrations!

You will notice that there is a huge range of wavelengths on the electromagnetic spectrum. The longest wavelength listed is a radio wave with a wavelength of 106 (1,000,000) meters, also known as a megameter. The shortest is an x-ray with a wavelength of 10-13 (one ten trillionth) meters, also known as 100 femtometers.

Infrared Waves, Visible Light, and Ultraviolet Light

Infrared waves are electromagnetic waves with wavelengths between about 1 millimeter (mm) and 700 nanometers (nm). A nanometer is a very short distance. How short? Well, an atom, which is the smallest unit of matter, is about .1 nm. Infrared waves are important to living things because they warm the Earth. If it weren't for infrared waves coming from the sun, the Earth would be too cold to support life. It's not just the sun that emits infrared waves. All things, including living things, emit infrared waves. Hotter objects emit more infrared waves than cooler objects do. Certain types of cancer produce tumors that are warmer than the tissue surrounding them. These tumors can be detected with equipment that is sensitive to infrared waves.

Visible light represents a very narrow range in the electromagnetic spectrum. Visible light waves have wavelengths that range from about 400 nm (violet light) to about 700 nm (red light). Plants, using a process called photosynthesis, are able to change light energy from the sun into chemical energy that can be used by animals (sugar). The visible light that comes from the sun is white light. White light is the visible light of all wavelengths combined. Most incandescent bulbs, fluorescent bulbs, and LED bulbs also produce white light.

Special cells in our eyes called cones are sensitive to colored light. The cones react differently to different wavelengths of light, and our brain interprets these different reactions as different colors. The colors of light found in the spectrum are red, orange, yellow, green, blue, indigo, and violet. It might be easier to remember these colors by memorizing the name ROY G. BIV.

Ultraviolet light waves have shorter wavelengths than visible light. The wavelengths of UV light range from 60 nm to 400 nm. UV waves are also important to living things. Most of the effects of UV waves are negative. UV waves can cause sunburn. They can also cause skin cancer. Too much UV can cause your skin to develop wrinkles. Finally, UV waves can damage your eyes. Because of the ozone layer that surrounds the Earth, most of the UV waves emitted by the sun do not reach the Earth. There is still enough UV to cause damage, though. That's why you need to protect yourself against UV. You can do this by wearing sunscreen with a high SPF (sun protection factor) rating, wearing sunglasses that block UV, and wearing clothing that covers most of your skin.

On the positive side, UV waves help your cells to make Vitamin D which is important to teeth and bones. More important, though, is the way that UV waves can be used to kill bacteria and make things sterile.

Interactions of Light with Matter

Four of the ways that light interacts with matter are reflection, absorption, transmission, and scattering. These four interactions are described below.

Reflection

Light travels in a straight line so long as the medium through which it is traveling does not change. When light hits an object, it reflects, or bounces off the object. The way that light reflects follows the law of reflection. The law of reflection states that the angle of incidence is equal to the angle of reflection. You can see an explanation of the law of reflection in the illustration below.

There are actually two different ways that light can reflect. The first way, which is shown in the illustration above, is called regular reflection. This is the kind of reflection that occurs with a mirror or with a shiny surface. The second kind of reflection is called diffuse reflection. It occurs when light is reflected off a surface that is rough, like a wall. So the light rays are bouncing off many different surfaces. You can see a good illustration of the difference between regular reflection and diffuse reflection below.

Absorption

Not all light is reflected. Some light is absorbed. For example, if white light shines on a red object, only red light is reflected. All of other colors are absorbed. Particles in air also absorb light. That's one of the reasons that light becomes dimmer the farther it travels.

Scattering

Scattering is an interaction between light and matter that results in light changing direction. Diffuse reflection is an example of scattering. So is the way that light is scattered when it moves through fog or smoke. Scattering is what makes the sky blue. Light with shorter wavelengths is scattered more than light with longer wavelengths. You might wonder why the sky isn't violet, since violet has a shorter wavelength than blue. First, the light from the sun contains more blue light than violet light. The larger factor, though, is that your eyes are far more sensitive to blue light than to violet light.

Transmission

Transmission is light passing through matter. All of the light that you can see is transmitted through air. Certain other types of matter also allow the transmission of light. Glass and water are both examples of matter than allow the transmission of light. Materials that allow the transmission of light are either transparent or translucent. Examples of transparent items include glass and sandwich bags. When you look through something that is transparent, it is easy to see what is on the other side. Materials that are translucent allow light to pass through, but the light is diffused so that you can't really see what is on the other side. Wax paper and frosted glass windows (like the kind you often see in a bathroom) are examples of translucent materials.

Refraction

The fifth way that light interacts with matter is called refraction. When light refracts, it bends. Light will refract anytime the medium through which it is traveling changes. If a light beam traveling through the air strikes glass, water, or any other matter that allows the transmission of light, it will refract.

You have seen this effect many times, though you may not have known that you were seeing an example of refraction. Look at the picture below of a pencil in a glass of water. Look at where the pencil enters the water. Do you notice that the pencil looks like it is bent or broken? That's because of refraction. The light that is reflecting off of the pencil bends when it is traveling through water compared to when it is traveling through air.

When you look at a rainbow, you are seeing another example of refraction. In the case of a rainbow, you are seeing the refraction caused when light traveling through air encounters tiny drops of water. The light bends when it hits the water drop. But what causes the rainbow? Why does the white light get broken up into all of the colors that it is made of?

The answer is easier to understand if you study the illustration of the prism below.

So, what is happening? White light that is traveling through one medium (air) enters another medium (glass). When the medium through which light is traveling changes, the light refracts. When the light comes out the other side of the prism, the medium changes again, this time from glass to air. Not all wavelengths of light refract equally. Short wavelengths bend more than long wavelengths. Remember that white light contains all of the different wavelengths. But as the different wavelengths bend, and since they all bend different amounts, the white light gets "spread" into the full spectrum of colors.

Refraction is how lenses work. A lens is a specially-shaped piece of glass or plastic that bends light waves so that they are focused on to a small spot. Below is a drawing of the major parts of an eye. The light rays go through the iris, which can open to let in more light or close to reduce the amount of light entering the eye. The light then passes through the lens. Muscles change the shape of the lens so that it can focus on objects. You can feel these muscles move if you pay careful attention when you change your focus from something very close to your eye to something that is far away. The lens bends the light so that the rays converge where they hit the retina. The retina is where your rods (sensitive to light) and cones (sensitive to color) are. The rods and cones convert the light to an electrical signal that travels along the optic nerve to the brain. The brain converts the electrical signal to an image and, sometimes, will store the image as a memory.

There are many commonalities between an eye and a camera. That's not surprising, since cameras were designed to mimic the function of the eye. A camera has an aperture which, like the iris in your eye, can be opened and closed to change the amount of light let into the camera. The camera also has a lens which can focus light rays. In a camera, the focus point of the lens is controlled not by tiny muscles, but by your hand or by tiny motors built into the lens. The light is focused onto the digital sensor (or film in old school cameras). The digital sensor is made of a material that is sensitive to light. Just like the retina does, the sensor converts the light to an electrical signal. This signal is then saved to a memory card. You can see the different parts of a camera in the illustration below.

Congratulations! You have reached the end of Chapter 3! Have a look below for a list of the things you need to know in order to earn a top grade on the test.

What You Need to Know for the Test

1. You should be able to define each of the following terms:

• electromagnetic wave
• electromagnetic spectrum
• infrared waves
• ultraviolet waves
• visible light
• wavelength
• regular reflection
• diffuse reflection
• incident beam
• reflected beam
• angle of incidence
• angle of reflection
• absorption
• scattering
• refraction

2. You should be able to provide examples for each of the following terms:

• regular reflection
• diffuse reflection
• absorption
• scattering
• refraction
• visible light

3. You should be able to provide an estimate of the range of wavelengths found in the electromagnetic spectrum.

4. You should know the relationship between wavelength and frequency.

5. You should know which color of visible light has the longest wavelength and which has the shortest wavelength.

6. You should be able to explain what happens when white light strikes an object that your eyes interpret as blue.

7. You should be able to explain the law of reflection.

8. You should be able to explain why the sky is blue.

9. You should be able to explain how a prism breaks white light up into a spectrum.

10. You should be able to explain what happens when the medium through which light is traveling changes.

11. You should be able to make a drawing that shows the law of reflection and be able to label the normal, the incident beam, the reflected beam, the angle of incidence, and the angle of reflection.

12. You should be able to explain how our eyes are like a camera.

13. You should be able to explain why light is important to living things.

14. You should be able to use the information in this video: