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How Light Behaves. Optics Study of light . Illumination. Luminous – when a body gives of light Incandescent – when a body that gives of light through when heated A body is illuminated when you can see it because it reflects light towards your eyes.

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how light behaves
How Light




Study of light

  • Luminous – when a body gives of light
  • Incandescent – when a body that gives of light through when heated

A body is illuminated when you can see it because it reflects light towards your eyes.


Radiant flux – the amount of energy radiated in a unit of time by an electromagnetic wave source. Part of this radiation that enables us to see is called the luminous flux, F. Measured in lumen (lm).

  • Illuminance, E, is measured in lm/m2 or lux.
  • If you don’t have constant\ illumination in all directions, you can specify the luminous intensity of a source in a particular directio, I, using the unit candela (cd)
  • 1 cd = 1lm/Ω
  • The steradian is the unit of solid angle given by A/R2 .

A – Area of sphere

R – Radius of the sphere

  • When a ray of light reaches between two types of materials, several things can happen. Some may reflect from the surface, while some may pass through the surface.
  • Refract – The light that enters from the second material.

Some light may be absorbed by molecules on the surface or within the

second material.

  • Transparent – lets light rays pass through it without mixing them up.
  • Translucent – allows rays to pass through it but mixes them up so that you cannot see them clearly through the material.

Opaque – when it blocks all light.

Imagine a line perpendicular to te reflecting surface. Such line is called normal . The angle between the path of an incoming ray and the normal is called the angle of incidence. The reflected ray makes the same angle to the normal as the incoming ray, but the other side of the normal. Reflection works this way even when it involves rough surfaces. Whenever a ray reflects from a surface, it has an equal angle to the normal at that spot it had before the reflection.


When a light reflects from a smooth surface, all of its rays reflect in the same directions. When light reflects from a rough surface, the rays reflect in many directions because the normals at all spots on the surface points many ways.

  • Waves behave according to the Law of Reflection.
  • If you have a nice, clean, fat, shiny mirror, the light will still be recognizable as the beam of the flashlight. It is called specular reflection. But if you shine a flashlight at foil that has been crumpled and then straightened out, the reflecting surface is no longer flat and shiny; the reflected light goes in many different directions and is much more spread out. This is diffuse reflection.
  • Waves that do not reflect back from a boundary but travel into the new medium instead are said to refract.
  • In one direction, the wave just moves straight into the new material, adjusting only its speed. In two dimensions, the angle formed by the refracted ray and the boundary will change from the incident angle, because the wave moves at a different speed in the new medium.
snell s law
Snell’s Law
  • Gives a relationship between indices of refraction, the angles of incidence and refraction and speeds in this problem:

We have a relation

sin i/sin r = v1/v2

To observe refraction, place a pencil in a glass of water and then look at the pencil from the top and one side. The pencil appears bent at the water surface. The light from the top part of the pencil comes directly to you eyes. The rays from the bottom part pass through the surface between the water and air. There the rays refract, and so they seem to have come from a pencil bottom bent from the pencil top.

index of refraction
Index of Refraction
  • The ratio between the speed of light c and it speed v in a particular medium is called the index of refraction of the medium.The greater the index of refraction, the greater the extent to which a light beam is deflected on entering or leaving the medium. The symbol for the index of refraction is n, so that

n = c/v

n1 sin i=n2 sin r

  • Opaque materials absorb certain colors of light. Transparent materials also absorb certain colors if they contain dyes or pigments.
  • Scattering describes what happens when light rays stike atoms, molecules, or other individual, tiny particles. These particles send the rays of light off in new directions – that is, they cause the rays to scatter. Most of a clear sky appears blye because air molecules scatter more blue rays toward us than they do te other colors in sunlight. When the sun is near the horizon, it looks orange or red because the light reaching us has lost so muc of its other colors by scattering.
  • When two lights waves cross through the same spot, they interfere with each other – that is, they add to or subtract from each other.
  • Constructive Interference, gives brighter light than either wave would have separately.
  • Suppose instead that wheneve a crest of one wave crosses through the spot, a trough of the other wave also does. The trough reduces the height of the crest, leaving the spot dim or even dark. This process is called destructive interference.

The wavelength of the light can be calculated from the distance between the center and the first bright band of the screen.

  • In the thickness of various thin films, different colors of light are able to constructively interfere when path difference between light reflected from the top surface and from the bottom surface matches with the light’s wavelength, causing the changing stripes of color we see in soap bubbles. As you view the reflection t larger angles, the path difference becomes larger and a longer wavelength of light will be able to constructively interfere as well, and more bands of color will appear.

Hologams, are specialized interference patterns recorded on a thin film emulsion on glass or plastic, enabling you eyes to see exactly the pattern of light waves that was reflected by a three-dimensional object.

  • In young’s experiment, the light passing through each slit spread. It results from the fact that light behaves as a wave.
  • However, diffraction serves a purpose when a device called a diffraction grating is used to study the colors in a light beam. A grating used with a telescope can separate the colors in the light from a star, enabling the scientists to learn what materials make up the star.
  • Is the spreading of light into its colors.
  • One way of to disperse a light beam is to send it though a prism. Red light is refracted the least, and purple light is refracted the most. This is because the speed of the various wavelengths in glass is different, slowest for violet light and fastest for red light.
  • It involves the oscillations (regular variations in strength) of the electric fields that make up a light wave. These directions may be represented by arrows.
  • In most of lights we see, the arrows point in many directions perpendicular to the ray’s path. Such light is unpolarized.
  • If these arrows all point in one direction or just opposite it, the light is polarized.