NATURE OF LIGHT

CORPUSCULAR or PARTICLE THEORY Sir Isaac Newton Light is made up of very fine particles, or corpuscles, that are emitted from sources at high temperatures. These particles travel in straight lines from the source to the observer at an enormous velocity.

WAVE THEORY OF LIGHT CHRISTIAN HUYGENS Light is a form of wave motion which starts from a vibrating body and is transmitted through space at high speed.

THOMAS YOUNG Two light waves may meet in such a way that they may completely cancel each other. Instead of producing a brighter region, the two waves result in complete darkness.

ELECTROMAGNETIC THEORY of LIGHT A changing electric field produces a magnetic field.

VELOCITY of LIGHT (c) Velocity = frequency x wavelength c = f λ = 3.0 x 10 8 m/s

SOURCES of LIGHT 1. Natural sources

2. Artificial sources Heating bodies to a very high temperature Passing an electrical discharge through a gas at a low pressure Combustion of chemical reaction

RECTILINEAR PROPAGATION OF LIGHT If the light source is very tiny and concentrated in one place (a point source) only a sharp dark shadow (umbra) is formed.

If the source is broader light from the top of the source causes a lower shadow than that from the top. You therefore get partial shadow or penumbra (light shadow) as well as umbra.

REFLECTION When a ray of light strikes any surface, a portion penetrates the surface and is either absorbed by the second medium or is transmitted if the second medium is transparent. The rest of the ray is reflected.

LAWS of REFLECTION The angle of incidence is equal to the angle of reflection The incident ray, the normal ray and the reflected ray all lie in the same plane known as the plane of incidence.

TYPES of REFLECTION SPECULAR REFLECTION – occurs when the reflecting surface is smooth and highly polished resulting to very sharp reflected rays DIFFUSED REFLECTION – occurs when the reflecting surface is more or less irregular resulting to scattered or diffused rays.

For PLANE MIRRORS Follow the Law of Reflection. The image will be the same size as the object. The distance of the image will be the same as the distance of the object.

1. Sketch a ray diagram that shows how light will travel from the object to the eye by reflecting from the mirror in the given diagram. Show the position of the image.

2. Measure how far the object is in front of the mirror 2. Measure how far the object is in front of the mirror. Draw a quick sketch of the image behind the mirror at the same distance

3. Draw a light ray (a line) from the observer’s eye to an important part of the image (like the top). The light ray should be dashed when it is behind the mirror to show that the light ray isn’t really there

4. At the point where that light ray hit the mirror, bounce it back at the same angle (law of reflection) so that it hits the same spot on the original object.

5. Draw another separate light ray that shows the path the light takes to get from the bottom of the object to the eye. You should get something that looks like this: Because the image isn’t really a "thing" behind the mirror (meaning that there are no true light rays that are back there), we refer to it as a “virtual image”.