(LSEL) The Nature of Lights Lesson

The Nature of Lights

As you look out upon the world for the first time, your eyes start to see images that your brain converts to memory. When you see your mother's face for the first time, the ceiling above the hospital bed or the smiling faces of onlookers as you lay in the maternity ward, you are using light to display the world around you. As light is reflected by objects, it enters your eyes and you perceive that light as an image. That image is stored in your brain and you retain those images more and more as your brain develops. Over time when you see an object again, you can recognize its shape and color so that you can associate it with a previous memory. The energy carried in light amazingly can be transformed into those memories, so that we remember some things for the rest of our lives.  

The light that creates these images has some unique characteristics we will explore in this lesson. Light has a dual nature. It acts as both a particle and a wave as it moves through the universe. This characteristic makes it unique from any other thing science has discovered to date. Light travels in small bundles of energy that act like particles, except they do not move in a straight line. Instead they move with a specific frequency and wavelength, which gives them a property of a wave. These bundles of energy are called photons. Each photon carries a specific amount of energy based on the wavelength of the light. Each individual wavelength corresponds to the color of light that is perceived by the human eye. There are other wavelengths of light invisible to the human eye, but they still carry energy. These forms of electromagnetic energy can either have a wavelength above or below the visible spectrum. The visible spectrum is the narrow range of wavelengths our eyes can see. We typically break this down into the colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. Just remember ROY-G-BIV and this will help you remember the colors of the visible spectrum.

There are other forms of light outside of our range of vision. If the light falls below the red end of the spectrum, it is called infrared. If the light is above the violet end of the spectrum, it is called ultraviolet. Even radio waves, microwaves, television waves and x-rays are part of the electromagnetic spectrum. Some of these are high frequency waves carry a lot of energy. Others have low frequency and can travel long distances. We use the low frequency waves to transmit signals and high frequency waves to view inside of the human body.

As you can see there is a wide range of wavelengths that are considered part of the electromagnetic spectrum. Each wavelength has its own specific characteristics. We will examine the entire electromagnetic spectrum in the next lesson. The visible spectrum ranges from 300nm to 790nm. This is the only range of electromagnetic radiation we can see. These wavelengths possess wavelengths we can view with the naked eye and make up the white light we see all around us. Light as we know it has many properties we need to investigate in order to understand how we perceive light and color.

Light is composed of billions of tiny particles called photons. Photons are little bundles of energy that travel from one place to another as both a wave and a particle. This concept is called the dual nature of light. In other words, photons act like a rock being thrown at an object.   A photon moves up and down in a wavelike pattern as it travels. This was puzzling to scientists and it took a long time to explain this phenomenon. The physics behind light is very complex, but we can look at the dual nature of light in a way that is easy to understand.

Think of a photon as a tiny object that is moving through space. When a photon of light hits a small object like the electron of an atom, the electron is moved out of position. This demonstrates that light has a particle nature. We can see this when we use a solar panel. When light of a certain frequency hits certain metal atoms, an electron is ejected. This can be sent through an electric circuit creating electricity. This can only be possible if light acts as a particle. This phenomenon is called the photoelectric effect.

Light can also act as a wave. When you have light of one color go through two slits, the light from the two paths interfere and cause bands of light and dark on a screen behind the slits. This interference is only possible if light travels as a wave. So you can see we have to look at light as both a particle and a wave at the same time. This concept is hard to understand because we are used to classifying things as one particular thing. A person is a person, a car is a car or a dog is a dog. With light we have to think of things in a different way. An easy way to explain this is to look at the toy robots called transformers. Transformers are both robots and vehicles at the same time. Depending on the situation, the toy can be a robot or a vehicle. The same thing is true with light; depending on how you look at light it can behave as either a particle or a wave. In this lesson we will mainly focus on the wave nature of light since most of the properties of light we describe are based on wave characteristics.

Frequency and Wavelength

As we have learned, one of the characteristics of light is it behaves like a wave. Light is unique in that all light moves at the same speed (3.00×10⁸ m/s). Since all light travels at a standard speed, a light wave can be defined by its wavelength and frequency. The frequency is how fast the wave vibrates or moves up and down. The wavelength is the distance between two peaks of wave.

Frequency(ƒ) is measured in Hertz (Hz) and wavelength(λ) is measured in nanometers (nm). Frequency and wavelength are inversely related, meaning that a low frequency wave has a long wavelength and vice versa. This relationship can be expressed mathematically by the formula below.

c = ƒ x λ

c = Speed of light (3.00 x 10⁸ m/s)

ƒ = frequency

λ = Wavelength

We know that the speed of light is always constant so the variable c always equals 3.00 x 10⁸ m/s. With that information we can determine the wavelength or frequency of a wave if we know one or the other. For example, if we know the frequency of light we can determine the wavelength by dividing the speed of light by the frequency that we know. You can see this relationship in the example below.

If a light from a star has a frequency of 6.10 x 10¹⁴ s^-1 what is the wavelength of the light emitted by the star?

Start with our formula

c = ƒ x λ

3.00 x 10⁸ m/s = 6.10 x 10¹⁴s^-1 x λ

λ = 4.92 x 10^-7 m

Reflection

Reflection is the change in direction of a wave after it impacts another object. A good example is the mirror in your bathroom. The light from your face hits the mirror and then changes direction and comes back to the eye. This allows you to see your image as well as your surroundings. As a wave hits an object, it is reflected back at an equal but opposite angle; this is called the law of reflection. The incoming wave comes in at an angle called the angle of incidence, and the light leaves at an angle called the angle of reflection. If a wave enters from the left at 45° it will leave the reflective surface at 45° to the right. This always holds true in a vacuum, and this principle can be used to direct light and lasers in modern devices.

Refraction

In many transparent materials, like glass, plastic, or water, light can partially pass through the material. For example, I can see my face when light is reflected from the surface of a lake. This means water had to reflect light back to my eyes. I can also see the bottom of the lake and fish swimming underneath, which means light has to be reflected from the fish and the bottom of the lake as well. This light has to be transmitted through the water, through the air and then back to my eye. An interesting thing occurs when we see that reflected light. The features under the water are distorted and appear larger, broken or elongated. This phenomenon is called refraction. Refraction is due to the difference in speed at which the light travels in each material. As stated earlier, light travels at a set speed of 3.00 x 10⁸ m/s but this only holds true in a vacuum, such as outer space. When light travels through materials, they can alter the speed of light making it reach the eye at different times. This is why things look different when you view them under water. Light travels at different speeds through the air and water.   This causes the two reflections to reach the eye at different times. Therefore, you perceive each reflection at a slightly different time making objects appear larger or broken. Many things we see every day can be explained with the concepts of refraction and reflection. Some of these are: fish appearing larger in the water, a spoon appearing bent or broken in a glass of water, and mirages forming on a long highway or in a desert. All of these things are illusions the brain perceives as real, but they are actually just products of the properties of light.

Diffraction

Diffraction is the apparent bending of waves around small obstacles and the spreading out of waves past small openings. We can see this when we are in a dark room as light seems to bend around the corner from a source that is not visible. We can also see diffraction when light spreads out as it moves through a small opening. If you were to place a flashlight in front of a small hole, light will move through the opening and spread out on the other side.

Play around with this simulation from PHET to learn more.

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