Light is that part of radiant energy which can be detected by the eye and which is propagated inform of electromagnetic or transverse waves. OR
Light is a form of energy that causes a sensation of vision.
Sources of light
There are two sources of light: They are:
Examples of natural sources – Are the sun, stars, fireflies (glow-worm), some deep sea – fishes. These sources are said to be self-luminous since they produce and emit light themselves.
Examples of Artificial or man-made sources are candle light, gas light, Incandescent lamp, electric lamps etc. These sources are said to be artificial luminous objects.
NON- LUMINOUS OBJECTS: Are those objects that cannot produce their own light but reflects light falling on them. We see these objects because light reflected from them enters our eye. Examples: moon, mirror, paper, wall, wood, bricks, earth etc.
Transmission of light
Light does not need a material medium for its transmission or propagation. Thus it can pass through a vacuum. Some materials allow light to pass through them while some don’t. An object which reflects most of the light that falls on it, absorb very little of it and transmit none of it is said to be opaque. Thus opaque objects do not allow light to pass through them. Example of opaque objects are; the moon, mirror, mercury, stone, human body etc.
An object which transmit most of the light falling on it and also reflects very little of it is said to be transparent. Thus transparent objects are those that allow light to pass through them. Examples of transparent objects are; plain glass, clean water, gases and cellophane.
An object which transmit light poorly such that, the material from which the light is covering cannot be seen through them is said to be translucent. Examples of translucent objects are; tissues paper, cloudy liquid, white paper smeared without (or oil paper), newspaper, frosted glass, waved paper, fluorescent tubes etc.
Rays and beams of light
Every moving body has a path or way through which it travels. Vehicles and Lorries travel on road, aeroplane have airways and ships have sea routes.
The path followed by light as it moves away from its sources is called a ray.
A ray of light is the path or direction along which the light energy passes.
It is represented by a straight line with an arrow. The arrow head shows the direction of the transmission.
When a rays of light is more than one we call it a beam of light. A beam of light is a collection of rays.
There are three types of beams. They are;
A hard lens can be used to produce convergent beam. The lens brings parallel rays of light from the sun to a point.
Rays from lamps, bulbs, torch light are divergent.
Rays of light are produced in the Laboratory by using a ray bore.
Characteristics or properties of light
There Cardboard sheets with holes at the centres are arranged such that their holes are in a straight line, candle light is then placed at one end of the three holes. On looking through the hole at the other extreme. We can see the light from the candle. If either screen is shifted, this light travels in a straight line.
The practical application of rectili-near propagation of light (which also stand on evidence that light travel in a straight line are:
If the moon comes between the sun and the earth but such that the moon is far from the earth so that extreme rays that would have formed the umbra region intersect before getting to the earth, the phenomenon is known as annular eclipse of the sun. During the annular eclipse of the sun, the central part of the sun is covered while a ring of bright light radiates only from the edged of the sun.
This is shown below:
One of the practical applicators of rectilinear propagation of light is the pin-hole camera since this device makes use of fact that light travel in a straight line.
If the object is brought closer to the pin-hole, the size of the image (i.e. image height) increases, while if the distance between the object and the pin-hole (i.e. u) increases the image height (IH) decreases. Thus, U is inversely proportional to IH;
If the size of the pin-hole increases a brighter but lured image is formed but if the size of the pin-hole is reduced, a darker but sharper image is formed.
The image distance from the pin-hole (i.e. v) is equivalent to the length of the camera.
Magnification:- is defined as the ratio of image distance to the object distance
Where object distance, v image distance, Image height, OHobject height.
Thus on doubling the object distance, the magnification became halved.
There are two laws of reflection and they are stated as follows:
This diagram shows how an image of a point object is formed in a plane mirror. From the above diagram, one can establish the relationship between the object distance and the image distance from the mirror.
When an image of an object is formed in a plane mirror, the object and the image are at equal perpendicular distances from the plane mirror. Moreover, the object and the image are at equal distances from any point on the plane mirror.
Moreover is common to and so both triangles are congruent.
thus an object and its image in a plane mirror are at equal perpendicular distance from the plane mirror. Also since It is established that an object and its image are at equal distance from any point on the plane mirror.
It should be noted that when a mirror is rotated through an angle of 20 (i.e. twice the angle of rotation of the mirror and so total angler between the incident ray and the present reflected ray
Characteristics of images formed by a plane mirror
The differences between a real and a virtual image are as follows:
Uses of plane mirrors (i.e. practical applications of plane mirrors)
The simple periscope consists of two plane mirrors fixed facing each other at an angles of 450 to the line joining them. A ray of light from an object on entering the periscope through the upper opening will meet the mirror at an angle of 450 and so will be reflected through a right angle and will pass vertically down the tube. Thus it also meets the second mirror at and is again reflected such that it turns through a right angle and moves horizontally to the eye at Thus the periscope is used. In viewing objects over the heads of a crowd of people or over the top of any obstacle.