The Light Ray Model of Light

1. The Light Ray Model of Light

2. The Light Ray Model is one of the simplest and most successful models ever developed. • The Light Ray Model is based on a single property of light (linear motion). • This model has played a role in the development of many optical instruments such as telescopes and cameras.

3. Because light travels in straight lines, the geometric principles related to straight lines can be used to show what happens to light when it strikes various objects. • So many effects produced by light can be explained using this model, which makes up the branch of study called Geometric Optics.

4. To use the Light Ray Model, two terms need to be defined. • The path taken by a fine beam of light through a medium is called a light ray. • The light ray is represented in a diagram by a single, straight line. • The direction of the ray of light is shown by an arrow pointing in the direction of travel. • A group of parallel light rays is called a light beam.

5. A ray box is a device used to produce beams and rays of light for experiments.

6. Rays of light Shadows are places where light is “blocked” Shadows Shadow

7. A shadow is a dark region that forms behind an object. The object is being illuminated brightly on the other side of the shadow. • Shadows are formed when some or all of the light falling on an object is absorbed or reflected. • Transparent and colourless substances do not produce shadows. • Translucent objects may transmit some light, but they also absorb or reflect the striking light. • Opaque objects do not transmit any light so all of the light is absorbed or reflected. Very dark shadows form behind opaque objects.

8. This figure is of a ray box and two screens. It shows a group of light rays (light beam) travelling from the box to the screens. Although a light beam is actually made up of a large number of rays, only a few are drawn to represent the beam.

9. This diagram is the same as the previous diagram showing only a few light rays. This is an even more simplified version showing only the outer limits of the beam.

10. Umbra and Penumbra • A very dark shadow, where there is no light is called an umbra. • Umbra have sharply defined edges. • There is a lighter shadow with edges that are not as sharp as an umbra, called a penumbra.

11. Light Ray Model Explains the Formation of Shadows • Consider the shadow produced by a point light source. The light rays are given off in all directions from the light source. • Some of the light rays strike an opaque object and are absorbed, forming an umbra behind the object. • The other light rays continue to travel in straight lines from the light source as they were not stopped by an opaque object.

12. Notice that to define a shadow, it is only necessary to draw the light rays travelling from the point source to the outside edges of the opaque object . The shape of the umbra is the same as the opaque object.

13. The size of the umbra depends on the distance between the light source and the opaque object. If the object is far from the light, a small shadow will be produced. When the object is closer to the point source, the shadow will be large.

14. A different kind of shadow is formed if a large light source is used. The light is no longer coming from one single point. • The larger light sources are called extended light sources.

15. When an extended light source is used, the resulting shadow does not have sharply defined edges like the shadow of a point source. • Instead, a lighter area of a shadow is formed (penumbra) because light from the extended source overlaps at the outside edges of the opaque object

16. Penumbra Umbra Penumbra