LIGHT AND OPTICAL INSTRUMENT

Objectives

- To describe the process of image making.

- To mention the law of light reflection and provide examples.

- To explain how to diffuse white lights to become a spectrum.

Now, can you see any objects around you?

Observe the nature around you. The sky is blue, the color of vegetations in a rice-field is green, and flowers have a variety of colors. You can see an object if light which is either reflected or emitted from the object enters your eyes. Light is the range of frequencies of electromagnetic waves that stimulate to the retina of the eyes. Light waves have wave lengths from about 400 nm to 700 nm. The shortest wavelength is seen as violet light. As the wavelength increases, the color changes to blue, green, yellow, orange, and finally, red.

1. Phenomena of Light

1.1. Light travels in a straight line

How does light travel?

Light spreads outwards from its sources to all directions. If an object absorbs the entire light incident upon the object and reflects none to the eyes, it appears black. Similarly, none of the objects in a completely dark room can be seen because no light energy comes from the objects to the eyes. Everything, therefore, appears black. Black is thus ‘seen’ when there is no light energy coming from an object.

Direction of Light Travel

Light travels in a straight line in a vacuum or uniform medium. When your body blocks the sunlight, you will see a sharp shadow. This happens like when the sunlight streams through trees, and light from a cinema projector travels on its way to the screen. All of those phenomena suggest that light travels in straight lines. The beams of light are visible because dust particles in the air reflect light into our eyes. We locate objects by assuming that light travels from the objects to our eyes in straight lines.

The straight-line path of light has led to the ray model of light. A ray is a straight line representing the path of a very narrow beam of light. The study of the travel of light using a ray diagram is called ray optics. Even though ray optics ignore the wave nature of light, it is very useful to describe how light is reflected and refracted.

1.2. Shadows

Shadows of an object are formed for two reasons. First, because some objects, which are said to be opaque, do not allow light to pass through them. Second, light travels in straight lines. The sharpness of the shadow depends on the size of the lightsource. A very small source of light, called a point source, gives a sharp shadow which is equally dark all over. A large source or extended source (Figure 5.39A) gives larger and has a central dark region. This region is calledthe umbra. The umbra is surrounded by a ring of partial shadow, which is called the penumbra.

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