CBSE Class 6 Science Light Shadows and Reflections Exam Notes

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Light, Shadows and Reflections

LIGHT — A FORM OF ENERGY
Light is a form of energy which excites the sensation of vision. Imagine how the world would be if there was no light around us.

SOURCES OF LIGHT
An object which emits light is called a source of light. The various sources of light can be grouped under two main heads, viz:
1. Natural sources of light
2. Man-made or artificial sources of light

Natural Sources of Light
Some of the natural sources of light are mentioned below:

a) The Sun: The most important natural source of light for us is the Sun. It is a very bright source. Even though it is 150 million kilometres away from the Earth, it gives sufficient amount of light to make the day bright and warm for us on the Earth.

b) The Moon: The Moon has no light of its own but reflects the light of the Sun. The light from the Moon is cool and pleasant.

c) The Stars: The stars are other natural sources of light. Some of them are even brighter than the Sun. But they are so far away from us that they are seen at right only, as twinkling points in the sky.

Artificial Sources of Light
a) Candles, oil lamps, earthen lamps, lanterns and gas lamps are man-made sources of light.
b) Electric bulbs and fluorescent tubes: At right, the main source of light is either the electric bulb or the fluorescent tube. Sodium lamps and mercury lamps are used increasingly nowadays at important crossings at the roads.

Point Source and Extended Source of Light
a) Point Source of Light: A source of light which is of the size of the pin head of a common pin is called extended source of light.
b) Extended source of light: Any source of light which is bigger than the point source of light is called extended source of light. A burning candle, a glowing electric bulb or a fluorescent tube etc., are the examples of extended source of light.

LUMINOUS AND NON-LUMINOUS BODIES
You know that the Sun, the stars, a candle, an oil lamp, etc. give us light. Such bodies, which give out light of their own are called luminous bodies.

How can Non-Luminous Bodies be Made Luminous?
Any non-luminous body can be made luminous by heating it. Take an iron wire and heat it on a gas flame. After sometime the iron wire will become red hot and start emitting light. At this point the temperature of iron is between 600°C to 800°C.

Why are the Moon and the Planets Considered as Non-luminous Bodies?
All of you must have seen the Moon at night giving out cool milky white light. Similarly, you must have seen the planet Venus, which appears like a bright star during early evening. Why do we call them non-luminous bodies, when actually light is coming from them? Well, the simple reason is that they do not produce light of their own. Instead, they reflect the light from the Sun falling on them. Hence, they are non-luminous bodies.

Transparent, Translucent and Opaque Objects
Substances which allow light to pass through them easily, and through which we can see clearly are called transparent substances. Glass, water, air and cellophane paper are examples of transparent substances. 

Substances through which light can pass partially but we cannot see through them clearly are called translucent substances. Frosted glass, greased paper, butter paper and wax paper are examples of such substances.

Substances which do not allow light to pass through them at all are called opaque substances. Wood, metals, bricks, and stones are examples of opaque substances. 

RECTILINEAR PROPAGATION OF LIGHT

Can you see your fried on the other side of a wall? Of course not, because light can’t bend. Also it can’t travel through the wall.

While seeing a film in a cinema hall you may have noticed that the light from the projector appears to go in a straight line towards the screen. In a cinema hall, what you see is not light itself, but innumerable dust particles in the path of light, which become visible when light falls on them.

The above examples show that light travels in straight lines. This phenomenon is called the rectilinear propagation of light.

PINHOLE CAMERA

A pinhole camera is based upon the principle of rectilinear propagation of light. It consists of a rectangular light proof box with a very small hole in the middle of one face and a shutter arrangement to open and close the hole. A photographic plate is usually placed on the opposite side of the hole. When rays of light coming from an object pass through the fine hole, they form an inverted, small and real image on the screen. The sharpness and size of the image can be altered by adjusting the distance of the object from the hole.

Using the property of similar triangles we can write —

Height of the image (I) / Height of the object (O) = Distance of the image from the pinhole (V) / Distance of the object from the pinhole (U) 

or

I/O = V/U

the image remains sharp when the distance between the screen and the pinhole is increased. If the pinhole camera is moved nearer the object the image on the screen will become large.

On advantage that the pinhole camera has over the more usual lens camera is that it does not need to be focused. Objects at all distances are able to produce a sharp image. Moving the photographic plate does not affect the clarity of the image; i t only changes the size of the image produced.

SHADOWS

You must have seen shadows of plants, animals and other objects. If an opaque body is placed in the path of light, the rays cannot pass through it. Thus, the rays of light are blocked from going to the other side of the opaque body and the space behind the object does not receive any light from that source.

The space behind the opaque object, where the light is wholly, or partially cut off by it is called shadow.

Eclipses are examples of the formation of shadows in nature.

Conditions for the Formation of a Shadow

1. There must be a source of light.

2.. There must be an opaque body to obstruct the path of light.

3. There must be an opaque screen to receive the shadow as it cannot be formed in air.

The nature and the size of the shadows depend on

i) The size of the source of light and the object.

ii) The distance between the source of light and the object.

Regions of Shadow

A shadow consists of two regions:

a) Umbra: The region of total darkness is called umbra. Now ray of light enters in this region.

b) Penumbra: The region of partial darkness which surrounds the umbra is called penumbra. Some rays of light always reach this region and partially illuminate it.

MIRRORS AND REFLECTIONS

Suppose a ray of light, while travelling from one optical medium to another (say from air to glass), strikes the surface of separation of two media. If the ray of light is returned by the second optical medium into the first optical medium with a change in angle, the phenomenon is known as reflection of light.

Any smooth, polished surface which can turn back the rays of light into the same medium is known as mirror. A looking glass is the best example of a mirror. A highly polished metal surface, still water or oil are examples of mirrors. It must be noted that reflection from a mirror gives us clear images. The images are different from shadows.

REFLECTION OF LIGHT

The process of bouncing back the light to the same medium after striking the surface of another medium is called reflection.

A surface which reflects the light is called reflector. Silver metal is one of the best reflectors of light. A highly polished surface, such as a mirror, reflects most of the light falling on it.

Types of reflection: There are two types of reflection

Regular Reflection: When a parallel beam of light falls on a smooth and highly polished surface, then the reflected beam is also parallel and directed in a fixed direction. Such type of reflection is called regular reflection. For example, light reflected from search light, automobile head lights, etc.

Diffused Reflection: When a parallel beam of light falls on a rough surface, then the reflected light is not parallel but spreads in all directions, such type of reflection of light is called irregular or diffused reflection. For example, light reflected from the wooden surface.

LAWS OF REFLECTION

The reflection of light from a plane surface or from a spherical surface takes place according to two laws which are:

i) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

ii) The angle of incidence is equal to the angle of reflection

These laws of reflection are applicable to all types of reflecting surfaces such as plane surface, spherical surfaces or any irregular surface.

FORMATION OF IMAGE BY PLANE MIRROR

Consider a point object O placed infront of a plane mirror MM´. The mirror will form an image I of the object O. Here, two reflected rays AB and CO when produced backward, they meet at a point behind the mirror and hence form the virtual image I at that point.

Properties of Images formed by a Plane Mirror

i) The image formed by a plane mirror is virtual and erect.

ii) The distance of the object from the mirror is equal to the distance of the image from the mirror.

iii) The size of the image is equal to the size of the object.

Iv) The linear magnification produced by a plane mirror is unity. i.e., m =v/u  = h'/h=1.

v) The image formed is laterally inverted, i.e., the left side of the image appears to the right side and vice- versa.

Uses of Plane Mirror

i) Plane mirror is used as a looking gla

ii) Plane mirror is used in solar cooker to reflect the sun light.

iii) Plane mirrors are used in periscopes usually used in submarines.

iv) Plane mirros are used in barbar’s shop to see the back portion of the head.

Spherical Mirrors

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. Spherical mirrors are of two types: Concave mirror and Convex mirror. A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror. A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

Important Terms used in Spherical Mirrors

Centre of Curvature: The centre of the hollow sphere of which the spherical mirror forms a part is called centre of curvature. It is denoted by C. The centre of curvature of a concave mirror lies in front of it but the centre of curvature of a convex mirror lies behind the mirror.

Radius of Curvature: The radius of the hollow sphere of which the spherical mirror forms a part is called radius of curvature. It is denoted by R.

Pole: The centre of the reflecting surface of a spherical mirror is called its pole. It is denoted by P.

Principal Axis: The straight line passing through the centre of curvature and the pole of a spherical mirror is called principal axis.

Aperture: The part of the spherical mirror from which the reflection of light actually takes place is called aperture of the mirror. In other words, the diameter of a spherical mirror is called its aperture.

Principal Focus: A point on the principal axis of a spherical mirror where the parallel rays of light closed to the principal axis meet or appear to meet after reflection from the mirror is called principal focus. It is denoted by the letter F.

Focal Length: The distance between the pole and the principal focus of a spherical mirror is called the focal length. It is represented by the letter f.

Note: For spherical mirrors of small aperture, the radius of curvature is equal to twice of its focal length i.e., R = 2f  or  f =R/2

Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

To construct the ray diagrams in order to locate the image of an object, an arbitrarily large number of rays emanating from a point could be considered. However, it is more convenient to consider only two rays while making a ray diagram. These rays are so chosen that it is easy to know their directions after reflection from the mirror. The interaction of at least two reflected rays gives the position of image of an object.

Image Formation by Concave Mirror

Image Formation by Concave Mirror for Different Positions of the Object from the Pole of Mirror

Uses of Concave Mirrors

i) Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.

ii) They are often used as shaving mirrors to see a larger image of the face.

iii) The dentists use concave mirrors to see large images of the teeth of patients.

iv) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace

Image formation by a Convex Mirror

Nature, Position and Relative Size of the Image formed by a Convex Mirror

Uses of Convex Mirrors

i) Convex mirrors are commonly used as rear-view (wing) mirrors in automobiles to see the traffic at the back side.

ii) Convex mirrors are preferred because they always give an erect, though diminished image. Also, they have a wider field of view as they are curved outward Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

Mirror Formulae

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