Fundamentals of light

illumination Light is a form of radiant electromagnetic radiation.

It exhibits both particle-like properties (photons) and wave-like properties and travels in a straight line. Light is unique in that it doesn’t require a medium to travel through. It can travel through a vacuum (unlike sound). Given that light travels in straight lines, scientists developed the ray model of light. The direction of the travel of light rays can only be changed by placing an obstruction in the path. The study of light is called ray optics or geometric optics. Rays of light come from light sources. The major source of light for us is the Sun. The Sun is approx. 1.5 x 108 km from Earth and the light from the Sun takes 8 minutes to reach us (The speed of light is 2.9979 x108 m/s. We round this to 3.00 x 108 m/s for simplicity. We denote the speed of light as c). We are surrounded by many, many sources of artificial light – TVs, LEDs, Incandescent lights, or lasers.

What is the difference between the sunlight and moonlight? Sunlight is much brighter than moonlight because it a luminous source and the moon is considered an illuminated source. A luminous source is an object that emits light (like the overhead lights). An illuminated source is an object that becomes visible when light reflects off of it. We see objects because of different wavelengths of light reflected back to our eyes or different wavelengths of light being emitted (traffic lights are an example). We see the traffic light colors because of the transparent covers that are red, green, or amber. We see reflected light because the materials are opaque. The third type of medium involved with light are translucent materials. Translucent materials allow some light through but don’t allow objects to be seen clearly.

Calculating the speed of light using marshmallows and a microwave To do this at home, you will require a microwave, a microwave safe plate, and a bag of marshmallows. Firstly, place the marshmallows on the plate. I used regular marshmallows

Marshmallow experiment con’t. Important !! You must remove the turntable and glass plate from your microwave. The plate must remain stationary. I placed my plate into the microwave and set the time for 30 seconds at full power.

I then looked at the frequency of the microwave. It was 2450 MHz After the 30 seconds had finished up, I removed the plate and measured the distance between 2 un-melted marshmallows. (I picked 2 marshmallows close to each other and measured the distance from the center of each marshmallow). The distance between the 2 un-touched marshmallows was 12.2 cm. It should be noted that these ‘zones’ of melted and un-melted marshmallows was quite regular. I then looked at the frequency of the microwave. It was 2450 MHz v = f  microwave Lab assistant

% error = (2.99 x 108 / 2.99 x 108) x 100 = 100 % accuracy The frequency was important because I used the formula: v = f  where v = speed of light f = frequency (2450 MHz) and wavelength was 12 cm. V = 2450 MHz x 0.122 m V (c) = 2.99 x 108 m/s % error = (2.99 x 108 / 2.99 x 108) x 100 = 100 % accuracy

The wave nature of light White light is composed of different wavelengths of color. We remember this composition by using ROY G BIV (red, orange, yellow, green, blue, indigo, and violet). The composition can be seen by looking at a shadow on a white surface. The edges of the shadow are not sharp and will show a slight rainbow effect. This is because the light ‘bends’ around the object. This phenomenon is called diffraction.

The single/double slit experiment was the first experiment to prove light has wave like properties.

A medium where the speed of the wave depends on the wave frequency is called a dispersive medium. An example would be a quartz prism that disperses light into a rainbow because higher frequency light (blue) refracts differently than lower frequency light (red). Higher frequency waves, like blue light, don’t ’bend’ or refract as much as the lower frequency waves. This difference gives us the ‘spread’ of light seen with prisms. (We will discuss prisms and refraction later in the section). Polarization of light. Light travels in waves and these waves travel in all directions (except lasers – laser light is ‘coherent’ meaning the light is all the same frequency and travels in the same direction kind of like a really really bright firehose.)

Polarizing filters allow only a single direction of light waves to pass through, all other directions of travel are blocked. Polarization of light occurs when it is reflected to form a single plane of oscillation. You can check this out if you are lucky enough to have a pair of polarized sunglasses. Rotate the sunglasses when you are looking at a reflecting surface like a roadway. Rotating the glasses will show the road appearing to get dark and then light again. This ‘banding’ shows that light waves travelling in the same orientation as the filter will get through to the viewer. All other waves will be blocked.