Electromagnetic Spectrum: An electromagnetic spectrum is a “map” of the total range of waves. All are forms of “LIGHT”! Usually, high frequency (low λ) is on the right. Low frequency (high λ) is on the left.
Notice the wide range of waves. We only perceive visible light, a tiny fraction of the whole spectrum...
Speed of Light: In a vacuum, all electromagnetic waves have a velocity of 300,000,000 m/s! (3x108m/s) That’s 186,000 miles per second!
There are a number of wave phenomenon that are characteristic of all Waves: • #1 – Reflection • #2 – Refraction • #3 – Diffraction • #4 – Interference • #5 – Resonance • They all involve the movement of energy in to form of waves – sometimes from 1 material to another!
Moving from one medium to another Once a wave(incident wave)has reached a change in media, part of the energy is transferred to the medium that is immediately next to it(transmitted wave)and part is reflected backward(reflected wave).The energy transferred depends on the difference between the mediums.If there is a significant difference, almost all the energy will be reflected.
If the mediums are similar, most of the energy will be transferred. However, the reflected waves will be inverted if the medium that comes next is more dense or it won't be inverted if the medium is less dense. • Ex: • sound moving from air to water • light moving into a piece of glass • earthquake waves moving from solid rock to molten rock
#1: Reflection of Waves If we draw a line perpendicular to a surface, this line is thenormalof the surface. When a ray of light hits the surface of an object, part of the light is reflected. If the ray of light is at an angle with the surface, then the angle between the incident ray and the normal [incident angle]will be the same angle between the normal and the reflected ray [reflected angle]. This is called the law of reflection.
Normal Line Incident Ray Reflected Ray Incident Angle Reflected Angle Boundary / Mirror
http://library.advanced.org/10796/ch10/ch10.htm Most surfaces are not completely flat. When millions of rays of light hit the rough surface of an object, they are reflected in all directions. This is how we can see illuminated objects. http://micro.magnet.fsu.edu/primer/java/reflection/reflectionangles/index.html
White light Another discovery by Isaac Newton White Light: White light is not a distinct color. Instead, white light is the combination of all the other colors.
Newton’s Prism Work • Before Newton, it was thought that the prism added color to the light… • Newton tested this idea by breaking the light into the spectrum, then again dividing a single color of the spectrum with another prism to test if it would again “add” color to the light. • Once he discovered it did not, he reached the conclusion that the original white light had contained all of the colors that were seen coming from the prism. http://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/index.html
Selective Reflection: When we perceive an object to be a particular color, we actually are receiving only one particular color of light in our eyes. Ex: A banana looks yellow because it reflects only yellow light. It absorbs all the other colors.
Black: When an object appears black, it means that all colors ( frequencies) of light are being absorbed by that object. None are being reflected. Often, black objects are hotter because they are absorbing more light/energy.
reflected absorbed If a rose were illuminated with a red light, you would see the red rose, but the stem and leaves would look nearly black. Since they are green, that means they reflect only green light ( absorb red).
#2: Refraction of Waves • When a wavetravels from onemedium to a second medium, The original wave is redirected at a different wavelength at a different angle (bend) from the normal to the surface. • The index of refraction determines the amount of change in wavelength and angle. (bending the wave) http://micro.magnet.fsu.edu/primer/java/scienceopticsu/refraction/index.html http://www.physics.uoguelph.ca/applets/Intro_physics/refraction/LightRefract.html
Refraction of Light Beam • Refraction --bending of light wave path as light passes from one material to another material. • Refraction occurs at the boundary and iscaused by a change in the speedof the light wave upon crossing the boundary. • Direction of bending depends upon whether light wave speeds up or slows down at the boundary.
Transmission Across a Boundary • Light wave speed changes • Light wavelength changes - frequency does not change • Only time a wave can be transmitted across a boundary, change its speed, and still not refract is when wave approaches boundary in a direction which is perpendicular to it.
Optical Density • Optical density -- tendency of the atoms of a material to hold on to absorbed energy from a photon in the form of vibrating electrons before reemitting it as a new photon • The more optically dense a material is, the slower a wave will move through the material.
If the new medium is more dense, the light bends because it slows down. How much do you ask? This decrease in speed is given by the formula: v = c / n where v - is the new speed of light and n is…
Index of Refraction • Index of Refraction is a measure of optical density • Represented by n • The higher n is, the more optically dense the material and the slower light travels in the material
Refraction of light When a ray of light passes from one medium to another, itbends. Depending of the new medium the light will travelfaster or slower. If the light travels faster in the second medium, then this medium is called therarer medium(or less dense)
Refraction of light On the other hand, the medium in which the light travels slower, in this case the first one, is called thedenser medium. When a ray of light enters a denser medium,it is bent towards the normal.When a ray of light enters a rarer medium,it is bent away from the normal.
Law of Refraction: Snell’s Law • There is a formula to predict how much a wave will “bend” as it travels into a new medium: n1sinq1= n2sinq2
n1sin 1 = n2 sin 2 (1.00) (0.643) = (x) (0.53) X = 1.21 Angle of Incidence 1 = 40 AIR - n1 = 1.00 ? - n2 = Angle of Refraction 2 = 32 “New” Angle of Incidence “New” Angle of Refraction
At the boundary of 2 media: Some of the light wave is always reflected. However, when a ray of light goes from a denser medium to a rarer medium, all the light will be reflected if the angle of incidence is greater than thecritical angle. The critical angle is the angle of incidence for which the refracted ray is at 90 degrees with the normal. http://micro.magnet.fsu.edu/primer/java/refraction/criticalangle/index.html
http://micro.magnet.fsu.edu/primer/java/diffraction/basicdiffraction/index.htmlhttp://micro.magnet.fsu.edu/primer/java/diffraction/basicdiffraction/index.html #3: Diffraction When a wave travels through a small hole in a barrier, it bends around the edges. This is called diffraction. • The bending of a wave around an obstacle
#4: InterferenceofWaves The Addition of waves can add (Constructive) or subtract(Destructive). Standing waves are a result of waves combining in phase http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/index.html
Collision of waves When two waves traveling in opposite directions through the same medium collide, the amplitude of the resulting wave will be the sum of the two initial waves. Remember: the energy of the 2 waves is influencing the motion of the media - the energy “pulls” on the media.The resulting phenomenon is called interference and there are two types:
Collision of waves Constructive interferenceis when the amplitudes of the initial waves are in the same direction. The resulting wave will belargerthan the original waves. The highest point of a constructive interference is called anantinode. Constructive!
Destructive interferenceis when the amplitudes of the initial waves areopposite. The amplitude of the resulting wave will be zero. The point in the middle of a destructive interference is called anodeand it never moves(in light - it would be a dark spot)
When waves “add” and “subtract The Principle of Superposition
Summary • Waves are a pulses of energy through a medium • Waves properties: Speed, amplitude, wavelength, and frequency • Waves change through mediums by • Reflection • Refraction • Diffraction • Interference
ORIGIN OF LIGHT Where does light come from? How is it produced? The least complex answer is that light comes from the atom itself - the motion of the electrons around the nucleus. When an electron drops an energy level - a packet of light (photon) is produced. This is the phenomenon that you studied in the online activity.
VISIBLE LIGHT Of all the electromagnetic waves, visible light is the only portion of electromagnetic waves that can be detected by the human eye. It is a very small section of the spectrum and visible wavelengths run from 7.5 x 10-7 m (red) to 3.5 x 10 -7 m (purple).
Luminous and illuminated bodies Light is produced by a luminous body. A light bulb is a luminous body that emits light in almost every direction. Light travels in straight lines at 299,792,458 m/sec in a vacuum. OR 3.0 X 108 m/s
Luminous and illuminated bodies When light hits an object OR another medium, it is reflected or refracted . An illuminated body reflects light. When a ray of light reaches our eyes, the receptors in our eyes will produce a different color sensation depending on the wavelength of the light wave.
COLORS Red, green and blue are known as primary colors, because when they are added together white light is formed. By mixing primary colors in pairs we obtain secondary colors. Blue and red produce magenta, and blue and green produce cyan.
Why the Sky is Blue: Our sky, atmosphere, is made up of various particles ( mostly N2 and O2 ) that vibrate at various frequencies. When hit by light of various frequencies, some react, some don’t.
The violet/blue light is reflected/scattered the most, so we see those colors. Our eye cones aren’t as sensitive to violet, so we see a predominantly blue sky.
LIGHT ORIGIN REVIEW QUIZ 1. WHERE DOES LIGHT COME FROM? - EXPLAIN THE MECHANISM. 2. WHY DO WE SEE COLORS? - HOW IS IT WE SEE COLORS? 3. WHAT ARE THE COLORS OF THE VISIBLE SPECTRUM? 4. NAME AT LEAST 4 OTHER FORMS OF ELECTROMAGNETIC RADIATION.