Physics

 

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:

1. Natural sources
2. Artificial sources

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;

• Parallel Beam:- Is a collection of light rays which are parallel to one another. Examples of parallel beams are: Rays from search light and rays from the sun.

• Convergent Beam:- Is the beam in which light rays meet at a point as shown below:

 

A hard lens can be used to produce convergent beam. The lens brings parallel rays of light from the sun to a point.

• Divergent Beam:- is the beam in which all the rays come from a point and diverge (spread) outwards as shown below;

 

   

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

• Light travels in a straight line (Rectili-near propagation of light) an experiment performed in the laboratory to illustrate the rectilinear propagation of light as follows:

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:

• Formation of shadows (a shadow is an area which light cannot reach due to the obstruction of an opaque object). Black shadow is formed at region of total eclipse of the light source (Umbra region) while grey shadow is formed at the region of partial eclipse of the light source (penumbra region). If a point source of light is used instead of a large source only the umbra region is produced (thus black shadow).
• Eclipses: An eclipse is a result of a shadow cast by one heavenly body on another.
• Eclipse of the sun:- this is a situation whereby the moon comes between the sun and the earth so that the shadow of the moon is cast on the earth thus preventing people on earth from seeing the sun. People on the Umbra region of the shadow experience total eclipse of the sun while people in the penumbra region of the shadow experience partial eclipse of the sun. (i.e. to them, the sun is partially covered). Complete eclipse of the sun cannot be achieved in all parts of the earth at the same time because the moon is smaller than the earth and so its umbra region is not large enough to cover the whole earth.

 

 

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:

    

• Eclipse of the moon:- this is a situation whereby the earth comes between the sun and the moon so that the shadow of the earth is cast on the moon, thus preventing people on earth from seeing the moon.

       

        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.

Example:

• An object 50cm tall is placed 1m in front of pin-hole camera with length 20cm.
• What is the height of the image and the magnification produced?
• If the object distance is doubled, what is the size of the image produced? How is the magnification affected?

Solution:      

Now

 

Thus on doubling the object distance, the magnification became halved.

• Reflection of light:- this is the throwing back of light into the former medium on becoming incident to the surface of an opaque object. Opaque objects with smooth or polished surfaces cause regular reflection (i.e. the rays are reflected in definite and parallel directions). On the other hand, opaque objects with rough or non-polished surfaces cause diffuse reflection of light (i.e. the reflected rays are scattered in non-parallel directions.

 

 

 

 

 

 

 

 

 

 

 

There are two laws of reflection and they are stated as follows:

• The angle of incidence is equal to the angle of reflection
• The incidence ray, the reflected ray and the normal at the point of the incidence all lie on the same plane.

 

 

 

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.

 Also

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 size of the image is equal to the size of the object.
• The distance of the image behind the mirror is equal to the distance of the object in front of the mirror.
• The image formed is always erect
• The virtual image is formed behind the mirror
• The image formed by a plane mirror is laterally inverted

Image

Object

Example:

                   

 

 

 

 

 

 

The differences between a real and a virtual image are as follows:

• A real image is formed by the actual intersection of reflected rays while a virtual image is formed by the apparent intersection of the reflected rays when their directions have been produced backward.
• A real image is formed in front of the mirror that is on the same side of the mirror to that of the object while a virtual image is formed behind the mirror that is on a side opposite to that of the object.
• A real image can be formed on a screen while a virtual image cannot be formed on a screen.
• A real image is inverted relative to the object while a virtual image is erect relative to the object

Uses of plane mirrors (i.e. practical applications of plane mirrors)

• For seeing one’s image
• For seeing round corners at cross-roads and in corridors
• It is used in periscopes which are used in seeing beyond obstacles

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.