Light
Light Fundamentals Light is a transverse wave Light exhibits all of the wave behaviors: reflection, refraction, diffraction, etc. Light does not require matter for a medium Light can travel through a vacuum (no matter) Can light travel through matter?
The electromagnetic spectrum includes Visible light is a small range of wavelengths in the electromagnetic spectrum Visible light is just the part of this spectrum we can see with our eyes The electromagnetic spectrum includes radio waves microwaves infrared visible light ultraviolet X rays Gamma rays
All electromagnetic (EM) waves travel at the same speed in a vacuum This speed is called the speed of light and is represented by the small letter “c” c = 3.00 108 m/s c = f is the wave equation for all EM waves Example: What is the frequency of yellow light, = 556 nm? Ans: f = c/ = (3.00 108 m/s)/(556 109 m) = 5.40 1014 1/s = 5.40 1014 Hz
The EM Spectrum
Light Behavior When light strikes matter, three things can happen to the light energy It is absorbed and becomes heat It is transmitted (i.e., travels through the matter) It is reflected Reflection of Light Light can be reflected in two different ways: Regular reflection (a mirror) Diffuse reflection (a white piece of paper)
The Law of Reflection: The angle of reflection is equal to the angle of incidence, or r = i reflected ray incident ray normal line mirror i r
Using mirrors Two examples: 2) A car headlight 1) A periscope
Color
Color What we see as color are different frequencies of visible light The spectrum (or rainbow) is all of the colors in order of frequency (ROYGBIV) Added together, the colors of the spectrum make white light An object appears green because it reflects green light and absorbs the others
Additive Colors Our eyes have cone cells that detect three colors: Red Green Blue These are called the additive primary colors The additive primary colors can be combined to form the other colors
Cyan, Yellow, and Magenta are often called the secondary colors Two colors are complimentary when they add to make white red + cyan = white green + magenta = white blue + yellow = white
Here is a glass prism: A ray of white light arrives
What happens to the ray of light? It is (mainly) reflected or refracted ? But different colours are refracted by different angles.
cted refra In fact, the ray is : (bent through an angle), and dispersed (split up into separate colours) like this.
Red is deviated most / least ? screen Red is deviated most / least ? Violet is deviated most / least ?
There is a spectrum on the screen: V screen There is a spectrum on the screen: Red Orange Yellow Green Blue Indigo Violet ROY G BIV
What is the real difference between red and violet light ? 380-450 nm wavelength violet red 620-750 nm
Clear vs. Diffuse Reflection Smooth, shiny surfaces have a clear reflection: Rough, dull surfaces have a diffuse reflection. Diffuse reflection is when light is scattered in different directions
Colour White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow. We can demonstrate this by splitting white light with a prism: This is how rainbows are formed: sunlight is “split up” by raindrops.
Only red light is reflected Seeing color The apparent color of an object depends on the colors of light it reflects. For example, a red book only reflects red light: Homework White light Only red light is reflected
A white hat would reflect all seven colours: A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colours: White light
Using coloured light If we look at a coloured object in coloured light we see something different. For example, consider a football kit: Shirt looks red White light Shorts look blue
In different colours of light this kit would look different: Red light Shirt looks red Shorts look black Shirt looks black Blue light Shorts look blue
Using filters Red Filter Filters can be used to “block” out different colours of light: Red Filter Magenta Filter