Properties of Light.

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Properties of Light

ELECTROMAGNETIC WAVES
Light is an example of an electromagnetic wave. It requires no medium through which to travel.

VISIBLE LIGHT

History Isaac Newton knew that light traveled in straight lines and cast shadows. Other scientists discovered that light was made from a stream of tiny particles or traveled as waves. Olaus Roemer estimated the speed of light in the late 1600’s, but his value was low. William Herschel, 1799, discovered that different colors of light have different temperatures. He discovered that red was the hottest color while violet was the coolest.

Ultraviolet was discovered by Wilhelm Ritter in 1801.
In 1972, the National Institute of Standards and Technology discovered that the actual speed of light was 299,792,458 meters per second. Albert Einstein proved that light moves fastest in a vacuum. However, the speed of light varies depending on the medium through which it travels.

TRANSVERSE NATURE

Polarization Polarization results when we separate electromagnetic waves by the plane in which they are vibrating. By taking only those waves which vibrate in one plane, we cut down the amount of energy.

Waves and Rays

Geometrical Optics We assume that light follows straight-line paths (rays) Changes occur when a ray hits a boundary rays may bounce off (reflection) rays may bend into the other medium (refraction) rays may be absorbed (light energy  thermal energy) Diffraction and interference are phenomena best described by light waves (not rays)

Doppler Shift

Doppler Shift

Reflection

Incident vs Reflected Rays
Incident Ray – leaves the object and strikes the mirror Reflected Ray – leaves mirror and strikes your eye The reflected ray is on the line of sight from the image to your eye.

Angles are Measured from the Normal
In geometrical optics, angles are always measured with respect to the normal of the interface normal Incident ray 60o 30o interface

Reflection The law of reflection is always valid! The angle of incidence is always equal to the angle of reflection. This is why “rough” surfaces reflect light in all directions

Specular Reflection

Diffuse Reflection

Refraction

Refraction of Light Beam
Part of the incident ray is reflected.

Refraction Electromagnetic waves travel at 3 x 108 m/s (c) in a vacuum
Refraction occurs when a wave changes speed as it passes from one medium to another with different characteristics. Electromagnetic waves travel at 3 x 108 m/s (c) in a vacuum

Transmission Across a Boundary
The only time a wave can be transmitted across a boundary, change its speed, and still not refract is when the wave approaches boundary in a direction which is perpendicular to the boundary.

Consider a light ray which traverses a thick slab
ray bends towards the normal upon entering the glass ray bends away from the normal when it exits from the glass exiting light ray is at same angle as original ray, but is shifted over to one side

Why refraction happens?
Because speed of light changes. Because light is a wave. Because its wavelength changes in different medium.

Fata Morgana Fata Morgana, also known as Morgan le Fay, was a fairy enchantress skilled in the art of changing shape. In one traditional story she was King Arthur's sister and learned many of her skills from Merlin the Magician. A special type of complex mirage, one that sometimes gives the impression of a castle half in the air and half in the sea, is named after Fata Morgana. She was known to live in a marvelous castle under the sea. Sometimes the enchantress made this castle appear reflected up in the air, causing seamen who mistook it for a safe harbor to be lured to their deaths. The fate morgana mirage is one that can occur only where there are alternating warm and cold layers of air near the ground or water surface. Instead of traveling straight through these layers, light is bent towards the colder, hence denser, air. The result can be a rather complicated light path and a strange image of a distant object. A fate morgana actually is a superposition of several images of one object. Typically one image is upright more or less above two inverted images that may be mingled together. The images may undergo rapid changes as the air layers move slightly up and down relative to the observer. In Alaska the best chance of seeing the relatively rare fate morgana is in winter when temperature inversions develop in the larger valleys. When seeing a complex mountain image out across a valley or bay one can attempt to sort out in the mind the paths that the light rays must have taken. Perhaps it's best just to acknowledge that it is Morgan le Fay beckoning.

Halos

Speed Change A ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal. A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal

REFRACTION

speed of light in vacuum speed of light in medium n =
Index of Refraction Every substance has an optical density called the substance's index of refraction (n). The index of refraction indicates is how well light passes through the substance (vs ) compared to how fast light travels through a vacuum (c). c vs speed of light in vacuum speed of light in medium n =

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

Laws of refraction Index of refraction depends on: The medium
The wavelength (or color of the light)

Indices of Refraction

Refraction is Dispersive
Light of different frequencies is refracted by different amounts

Rainbows are a result of dispersion

Table of Indices

ni sin qi = nr sin qr Snell’s Law
Snell’s Law relates the index of refraction to the angles of incidence and refraction. ni sin qi = nr sin qr ni is the refractive index of the medium the light is leaving, i is the incident angle between the light ray and the normal to the medium to medium interface, nr is the refractive index of the medium the light is entering, r is the refractive angle between the light ray and the normal to the medium to medium interface.

Question 1 To shoot a fish with a gun, should you aim directly at
the image, slightly above, or slightly below? (1) aim directly at the image (2) aim slightly above (3) aim slightly below [CORRECT 5 ANSWER]

Answer 1 (1) aim directly at the image (2) aim slightly above (3) aim slightly below Due to refraction, the image will appear higher than the actual fish, so you have to aim lower to compensate.

Question 2 To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below? (1) aim directly at the image (2) aim slightly above (3) aim slightly below [CORRECT 5 ANSWER]

Answer 2 (1) aim directly at the image (2) aim slightly above
(3) aim slightly below The light from the laser beam will also bend when it hits the air-water interface, so aim directly at the fish. light from fish laser beam

Total Internal Reflection

Internal Reflection All rays reflect internally, but the top three rays reflect only a small percentage internally; most energy leaves the prism. The fourth and fifth rays are reflected 100 % internally

Total Internal Reflection
Total internal reflection is a special case of reflection created when when a light passes from a more optically dense medium to a less dense one at an angle such that there is no refracted ray.

Fiber Optics Total internal reflection is used to send signals through fiber optic cables.

Total internal reflection
There is a critical angle for every boundary When light hits the boundary at this angle it is no longer refracted, but reflected! Remember !! this only works when the incident medium has the higher index of refraction. WHY?

n1sin qc = n2 sin 90o Critical Angle
The critical angle is the angle of incidence which causes the angle of refraction to be 90°, so Snell’s law can be written as n1sin qc = n2 sin 90o The higher the index of refraction the lower qc

Critical Angle When you exceed the critical angle, you have total internal reflection

Diffraction

Diffraction is the change in direction of a wave as it encounters the edge of a barrier.

Wavelength vs Diffraction
Shorter wavelengths bend the least when encountering the edge of a barrier. They are in contact with the edge for a shorter period of time.

Double-Slit Interference
Bright fringes Thomas Young (1802) used double-slit interference to prove the wave nature of light.

Remember that white light contains all the colors of the
The Visible Spectrum Remember that white light contains all the colors of the s p e c t r u m each color in the spectrum has a different wavelength and so bends at a different rate.

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