7th Class Science Lights and Sound Light

Light

Category : 7th Class

*        Light

 

Light is the important part of our life. Some objects produce light and are called luminous object, while some do not produce light and therefore are called non luminous object. The big source of light is the Sun. The other sources of light are, burning candles, torches, electric bulbs, etc. Light travels through vacuum and also through some mediums, for example, through air, glass and other transparent objects. The best example of traveling of light through vacuum is traveling of sun light from the Sun to the Earth.

The objects that allow the passing of light through them are called transparent object, i.e. glass, water, air, etc. The objects that allow passing only of some portion of light through them, are called translucent objects.

 

Look at the following picture of translucent object

 

Opaque objects do not allow passing of light through them. A piece of wood does not allow passing of light through it, therefore is called opaque object. The shadow of an object is formed due to the interruption of light. The light from light producing body is radiated towards different direction and a shadow of an object, which interrupt light, is formed at the opposite direction from where light is coming. For example, if the light is coming from east direction, the shadow of the object, which interrupt the light, forms in west direction. Shadow is not possible of a the transparent object. Because it allows the passing of light through

 

Look at the following picture of shadow formation

In the above picture, light from the candle is passing through the hole on the globe and shadow is formed on the opposite side of the light producing body.There are two parts of a shadow, umbra and penumbra. Umbra is the part of shadow, which is in complete darkness and does not receive light from the source. Penumbra is the part of shadow, which receives some part of light from the source.

 

Look at the following picture of path of the light

 

In the picture above, light from candle is passing through the hole of the three cardboards. A thin streak of light is appeared at the far end of the cardboard. This proves that the light travels in straight line. This property of light is called rectilinear propagation of light.

 

Look at the following picture of the solar eclipse

 

Solar eclipse is the formation of shadow of Moon on the surface of the Earth.

The solar eclipse is possible due to the passing of Moon between the Earth and the Sun. Light from the Sun is reflected by the surface of the Moon and shadow is formed on the surface of the Earth.

 

Look at the following picture of the lunar eclipse

 

Lunar eclipse is the formation of shadow of the Earth on the surface of the Moon. It is possible when the Earth is between the Sun and the Moon. Light form the Sun is reflected by the Earth and forms a shadow on the surface of the Moon. The Moon is completely covered by the umbra part of the shadow as shown in the picture above.

The colour of light is generally white or light yellow. The actual colour of light can be seen during the rain, in the form of rainbow. Light is the composition of seven colours, i.e., red, violet, yellow, orange, green, blue and indigo. The application of light is everywhere in nature. One major application of light is in the photosynthesis process.

 

*            Source of Light

There are two sources of light, natural and artificial. Natural sources of light is the Sun. Some other celestial bodies also produce light, for example, stars. Some lights are also produced by the atmospheric reaction like, thunder and volcano etc. The Sun is the biggest source of energy for us. It is mainly made up of hydrogen. Hydrogen of the Sun is converted into helium through nuclear fusion, thereby producing light. The core of the Sun is highly heated.

There are various kinds of artificial sources of light around us. For example, different kinds of electric bulbs, electric as well as oil burning lamps, solar lantern, etc. The intensity of light mainly depends on the burningobjects.For example, the intensity of light of burning of wood is less than that of an electric bulb.

 

 

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       A small object is placed behind the larger object. Which one of the following parts of shadow covers the entire part of the smaller object?

(a) Penumbra

(b) Umbra

(c) Shadow of smaller object

(d) All of these

(e) None of these

 

Answer: (b)

 

 

       Which one of the following allows light to pass through it partially?

(a) Translucent

(b) Opaque

(c) Some transparent objects

(d) Ail of these

(e) None of these

 

Answer: (a)

 

 

*          Reflection of Light

Every object absorbs and reflects light. In the picture below a plane mirror is reflecting light. The beam of light (AO) from the source is falling on the surface (O) of the plane mirror. It is called incident ray. The incident ray is reflected by the mirror from the same point (point of incidence at 0) and called reflected ray (OC). A perpendicular line (OB) from the point of incidence is called normal. The angle between the incident ray (AO) and the normal (OB) is called angle of incidence \[(\angle AOB)\] and denoted by \[\angle \]i. The angle between the reflected ray (OC) and equals (OB) is called the angle of reflection \[(\angle COB)\] and denoted by \[\angle \,r\].

 

Look at the following picture of reflection of a beam of light               

 

 

*            Laws of Reflections

Laws of reflection states that:

The angle of incidence is always equal to the angle of reflection.

The incident ray, the normal (perpendicular line on the plane mirror) and the reflected ray lie on the same plane.

 

According to the laws of reflection, the angle of incidence (ZPOA = ZAOR) is equal to the angle of reflection. Incident ray, reflected ray and the normal lie on the same plane. The point, at which the smallest possible cross sectional area of the light is fallen on the surface of the plane mirror, is called point of incidence. In the figure above point 0 is the point of incidence.

The reflection of light mainly depends on the medium from which light is reflected. Reflection is classified into two types; regular reflection (specular reflection) and irregular reflection (diffuse reflection). When incident parallel rays (two or more than two parallel incident rays) are reflected parallel from the smooth surface, the phenomenon is known as specular or regular reflection. If parallel incident rays are reflected by the rough surface then the reflected rays are diverted and called irregular reflection or diffuse reflection.

 

Look at the following pictures of regular and irregular reflection of light

 

                           

        Fig (1) Regular reflection Fig                                          Fig (2) Irregular reflection  

In the figure (1), three rays of light has fallen on a plane mirror. The angle of incidence and angle of reflection are equal in all cases. But In the figure (2), angle of incidence is not equal to the angle of reflection.

 

*           Periscope

Periscope is the optical instrument used to see the object from the long distance. Periscope is widely used in marine to see the far end of the surface of water in the sea. A simple periscope is made up of two glasses at the end of the long tube. The angle between the walls of the tube and each glass is fixed at 45°. The incident ray in the periscope is reflected by 90°.

 

Look at the following picture of the periscope

     

 

 

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        The angle of incidence and angle of reflection are measured by taking a perpendicular line on the point where incident ray touch the surface. Name that perpendicular line.

(a) Incident ray

(b) Reflected ray

(c) The normal

(d) All of these

(e) None of these

 

Answer: (c)

 

 

        John performs an experiment in the physics lab for the verification of the laws of reflection. Which one of the following parameters should he measure for the same?

(a) Refractive index of the medium

(b) Angle of incidence and angle of reflection

(c) Cross sectional area of the medium

(d) All of these

(e) None of these

 

Answer: (b)

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