# Physical Light

Topics: Optics, Geometrical optics, Lens Pages: 13 (2705 words) Published: June 24, 2013
Name:_______________________ Class: F.___ Class No:_____

1. Light and sight
We see an object when light from that object enters our eyes. Many rays from the object enter the eye at the same time. However, in diagrams, we only show two or some rays only.

A point gives out many rays but we usually draw _two___ rays to mark out the cone of rays from the point of the object reaching the eye (Fig 1a). To indicate that the eye sees the whole object, two cones of rays are drawn from the two _end____ of the object to the eye (Fig 1b). |[pic] |[pic] | |Fig 1a |Fig 1b |

To represent light from a distant object (e.g. the sun), we draw parallel rays. Light rays that are parallel to one another come from the same point on the distant object.

1.1Figure 1c shows that light from a point on a near object reaches our eyes as _divergent _________ rays.

Fig 1c [pic]

1.2Figure 1d shows that if the object moves away from us, the rays from the same point reaching our eyes become __ less __(less/more) divergent.

[pic]
Fig 1d

1.3 The rays reaching our eyes from a point of a distant object become __parallel_____ (Fig 1e).

[pic]
Fig 1e

2. Light rays and beams
Objects that can give out light by themselves are called _luminous____ objects, for example, candles, the sun and light bulbs. Objects do not give out light by themselves are called __non-luminous_______ objects.

Light consists of narrow rays travelling in __straight________ lines.

A light beam is a stream of light. Figure 1f shows three kinds of light beams.

|[pic] | |a __parallel ___ beam |a __divergent__ beam |a __convergent_ beam |

Fig 1f

3. Lens – Convex Lens & Concave Lens
A lens which is thicker at its center than at the edges is called a convex lens. A lens which is thinner at its center is called a concave lens.

3.1 Convex lens
A convex lens is a converging lens. The refracted rays are converging to the principal axis If the rays of light from the ray box are parallel rays, the rays refracted through the lens converge to one point called the principal focus (marked F) The distance from F to the center of the lens is called the focal length of the lens. [pic]

Experiment 1– Seeing images formed by a convex lens
Aims: To observe the images formed by a convex lens.

|Apparatus: | 1 spherical convex lens | | | 1 translucent screen | | | 1 lens holder | | | 1 lamp-house |

Procedure:
1. Setup the apparatus as shown. The letter F on the lamp housing is the object. First start with a larger object distance, say 50cm. Move the translucent screen until a sharp image of F is formed on it. [pic]

The image can be captured on a screen. It is a real image. It is __ diminished __ (magnified / diminished / of the same size as the object) and ___ inverted ____ (erect / inverted)

2. Reduce the distance between the illuminated letter “F” and the lens to 15 cm. View the letter through the lens. The...