# Lesson 3 - Light

Light sources

You probably know that light isn’t always around. If you wake up in the dark you have to search for a light switch or a flashlight so you can see what is going on around you. These things weren’t always around and people made lighted candles to see during the night. During the day the light that we see is radiated from the sun. If something emits light we call it a light source.

Seeing things

In everyday life we see a lot of things around us. In order to see light must reach our eyes and then our brains can processes this light in order to make a picture from it. However not all of the things we see emit light and still we can see them. You might wonder how is this possible. The answer is very simple: light is reflected from surfaces of objects and after that it travels to your eye. We now know there are two ways for light to reach your eye:

Light source → eye

Light source → surface reflection → eye

It is due to these reflections that people are able to see different colours. Light from the sun contains every possible colour. When light is reflected not all colours are reflected and therefore some objects appear blue and others appear red. An example of an object that doesn’t emit light but only reflects light is the moon.

What is light?

What is light? This question kept a lot of physicist busy during the 17th century. One argued that light would be some sort of particle. The other argued that light would be some kind of wave. Now why would they say such a thing. The statement of the particle is supported by observations that show light being bended due to the effect of gravitation. This must mean that light has some kind of mass and therefore must be a particle. However if we shine a beam of light through a double slit we see an interference pattern as shown in figure 13. This interference pattern is the same as would arise when two bricks are thrown into the water. These bricks would create wave circles as can be seen in figure 14. In some points the waves amplify each other, while at other points they cancel each other. This phenomenon can’t be explained by seeing light as particles.

So what is light? Is it a particle or is it a wave or is it even something else? The answer is: we don’t know exactly. Physicist have agreed to see light in the way that is easiest for their calculations. So if we for example want to do calculations with gravitation we see light as particles and if we want to know the interference pattern we do calculations as if it is a wave.

Properties of light:

It takes a certain time for light from to sun to reach the earth. Therefore light must have a finite velocity. This velocity is determined to be:

Light can be described as a wave and therefore light must have a wavelength and a frequency . The relation between these properties is:

You can heat things by shining light on them. For example when a sportsman wants to relax his muscles he uses an lamp that shines infrared light on them. This helps the muscles heat up and relax. Therefore light must poses a certain energy . This energy can be related to the frequency as:

h-bar is called ‘Planck’s constant’ and is equal to 6,63·10-34 Js.

A way to see the energy of light is the photoelectric effect. The photoelectric effect occurs when light shines on a surface. In some materials (but not all material) electrons can escape from the material due to energy they get from the light. This effect is called the photoelectric effect. As mentioned before this doesn’t happen with every material. Every material has its own binding energy which is the minimum energy an electron needs to absorb in order to escape from the material. If the electron absorbs more energy than the binding energy, this energy will give the electron a certain velocity, and thus kinetic energy. To understand the photoelectric effect the following questions need to be answered with the uses of the applet that can be found at: http://phet.colorado.edu/en/simulation/photoelectric. This applet looks like figure 15. In the applet photoelectric created electrons are used to close a current loop. The current can be viewed in the display. The lamp that is used to free electrons can be controlled by setting the intensity and wavelength of the light. You will see electrons leave the material as blue dots. In the control panel on the left a material can be selected, some graphs can be shown and you can choose to only view the electrons with the highest energy. Also a battery is present which creates a voltage difference between two parts of current circuit.