Light

  • Light enables us to see things around us.
  • This is a form of energy that we can detect with our eyes.
  • It falls on objects, bounces off from their surfaces and reaches our eyes.
  • It is the light that causes the sensation of vision in us.

Light || Definition, Nature, Reflection & Properties

  • For example, the sun gives out light, therefore we can see the sun.
  • It takes about 8.3 minutes to reach the earth from the sun.
  • Thomas Alva Edison was the first person who invented electric light bulb.

Definition of Light

  • Light is a form of energy which enables us to see objects which emit or reflect light.
  • We get it from various sources, e.g. sun, electric lamp, candle, and oil lamp, etc.

Characteristics of Light

  • It is an electromagnetic wave.
  • It is a transverse wave, and does not need any medium to travel.
  • They can travel through vacuum.
  • Its speed through vacuum is 3 x 108 m/s.
  • There velocity changes when it travels from one medium to another. While frequency remain same in all medium.
  • It can pass through transparent materials, such as glass or air.
  • They travel in a straight line.
  • It gets reflected back from polished surfaces, such as mirrors, polished metal surfaces, etc.
  • It undergoes refraction when it travels from one transparent medium to another.

Rectilinear Propagation of Light

  • There are a number of everyday phenomena which suggest that light travels in straight lines:-
  • Formation of day and night suggests.
  • Formation of shadows suggests.
  • When a beam of sunlight enters a dark room through a ventilator, we can see the light.
  • When the head light of a car is switched on, there rays appear to travel in straight lines.
  • The burning candle flame, it appears as if it is giving out a few beams of light.
  • Beams of light coming from the projection room in the cinema hall.
  • The ray coming from small laser torches used as pointers.
  • Above all the given examples are suggest that light travels in a straight line.
  • Rectilinear propagation of light refers to the property of light travelling in a straight line.
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Nature of Light

  • There are two theories about the nature of light:-

1. Wave theory 

2. Particle theory 

1. Wave theory

  • According to wave theory, light consists of electromagnetic waves which do not require a material medium for their propagation.
  • The wavelength of visible light waves is very small i.e only about ( 4× 10-7  to 8 × 10-7  )m.
  • The speed of light wave is very high.
  • And it tells about the phenomena of diffraction ( bending of light around the corners of tiny objects), interference and polarization of light.

2. Particle theory

  • According to the particle theory, light is composed of particles which travel in a straight line at very high speed.
  • It tells about the phenomena of reflection and refraction of light, and casting of shadows of objects. 

Conclusion

  • In the past hundred years of physics experiment have demonstrated that light has a dual nature, it exhibits the properties of both waves and particles.
  • The modern theory of light called “Quantum Theory of Light”. It combines both the wave and particle models of light.

Ray and Beam of Light

  • A straight line drawn in the direction of propagation of light is known a ray of light.
  • A ray is described by a straight line with an arrow- head pointing in the direction of propagation of light.
  • A bundle of the adjacent light rays is called a beam of light.
  • A beam of light can be 
  • Parallel beam 
  • Convergent beam
  • Divergent beam of light.
  • The beam of light in which all the rays are parallel to each other is called parallel beam of light.

Divergent_Beam_of_Light

  • The beam of light in which the rays of light starting from a point source move out in different directions is called a divergent beam of light.

Convergent_Beam_of_Light

  • The beam of light in which the rays of light starting from a large source of light get closer to each other is called convergent ray of light.

Reflection of Light

  • When a ray of light falls on a polished smooth surface, such as a mirror, it return back into the same medium.
  • Hence, the bouncing back of light after striking the surface of an object is called reflection of light.
  • Different materials reflect light to different extent.
  • Silver is one of the best reflectors of light.
  • We see our images in a mirror due to reflection of light.

Light || Definition, Nature, Reflection & Properties

  • The ray of light which falls on a cleaned surface is called the incident ray of light.
  • The ray of light which gets reflected from a polished surface is called the reflected ray of light.
  • The normal is a line at right angle to the reflecting surface, such as, a plane mirror, at the point of incidence.
  • The angle made by the incident ray with the normal is known the angle of incidence( <i ).
  • The angle made by the reflected ray with the normal is called the angle of reflection( <r ).

Kinds of reflection

  • Depending on the nature of the reflecting surface, there could be two kinds of reflections:-

1. Regular reflection

2. Irregular reflection

1. Regular reflection

Regular_Reflection

  • The reflection of light from a mirror- like smooth reflecting surface so that the reflected rays are parallel to each other is called regular reflection.
  • It is also known as specular reflection.

2. Irregular reflection

Irregular_Reflection

  • The reflection of light from a rough, irregular surface randomly in various directions is called irregular reflection.
  • It is not parallel to each other.
  • It is also called diffused reflection.

Laws of Reflection of Light

  • The reflection of light from a plane surface or from a spherical surface takes place according to the two laws, which are known as the laws of reflection.

1. First law of reflection

2. Second law of reflection 

Light || Definition, Nature, Reflection & Properties

1. First law of reflection

  • The angle made by the incident ray with the normal at the point is equal to the angle made by the reflected ray with the normal, i.e.

Angle of incidence = Angle of reflection

<i = <r

2. Second law of reflection

  • The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Images formed by reflection

  • The images formed by the reflection, it may be real or virtual.

1. Real image

  • When the rays of light starting from a well illuminated object are reflected from a smooth polished surface in such a way that they meet at some other point or on a screen, then image so formed is called real image.
  • A concave mirror gives a real image if the object is placed at or beyond focus.

Following are the characteristics of a real image:-

  • It can always be taken on screen.
  • It is always inverted as compared to the object.
  • It may be of the same size/bigger/smaller than the object.

2. Virtual image

  • When the rays of light starting from a well illuminated object are reflected from a smooth polished surface in such a way that they appear to meet at some other point, but cannot meet on a screen, then the image so formed is called virtual image.
  • The image formed in a looking glass is always virtual.

Following are the characteristics of a virtual image:-

  • It cannot be taken on a screen.
  • It is always erect as compared to the object.
  • It may be of the same size/bigger/smaller than the object.

Refraction of Light

Refraction_of_Light

  • We know that light travels in straight line through any medium of uniform density.
  • However, when a ray of light travels from one transparent medium to another, it suffers a change in its direction.
  • For e.g, air to glass, etc.
  • The change in direction of a ray of light when it passes from one medium to another, is called refraction of light.
  • Its speed through glass is 2 x 108 m/s.

or

  • The bending of a ray of light when it passes from one medium to another, is called refraction of light.
  • The direction in which the ray of light bends when it travels from one medium to another depends upon the optical density of the two media.

Bending_of_Ray_Due_to_Refraction

  • When a ray of light goes from an optically rarer medium to an optically denser medium, it bends towards the normal. i.e.

Angle of refraction( <r ) < Angle of incidence ( <i )

  • When a ray of light goes from an optically denser medium to an optically rarer medium, it bends away from the normal. i.e.

Angle of refraction ( <r ) > Angle of incidence (<i )

  • Air is optically less denser than water or glass. Therefore
  • A ray of light travelling from air into glass bends towards the normal.
  • A ray of light travelling from glass into air bends away from the normal.

Laws of Refraction of Light

  • There are two laws of refraction:-

1. First law of refraction

2. Second law of refraction

Laws_of_Refraction

1. First law of refraction

  • It tells about the ratio of sine of the angle of incidence to the sine of the angle of refraction for a particular pair of media is constant.
  • Hence, if the angle of incidence is i, and that of refraction is r, then

sine of the angle of incidence/ sine of the angle of refraction = sin i/ sin r = constant

  • The value of this constant depends upon temperature and the wavelength of light used in the measurement.
  • It is also known as Snell’s law.

2. Second law of refraction

  • It tells that the ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.

Cause of Refraction of light

  • The speed of light depends on the nature of the medium in which it travels.
  • When it travels from a rarer medium to a denser medium, its speed decreases.
  • Whereas speed of light increases when it travels from a denser to a rarer medium.
  • It is due to this change in the speed of light that the ray of light bends as it goes from one medium to another.
  • Thus, the change of speed of light as it travels from one medium to another is the cause of refraction.

Effects of Refraction of Light

  • A stick held obliquely and partly immersed in water, it appears to be bent at the water surface.
  • An object submerged under water appears to be raised.
  • A pool of water is appeared to be less deep than it actually is.
  • When a thick glass slab is placed over some printed matter, the letters seem raised when viewed from the top.
  • A lemon is kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
  • The stars appear to sparkle on a clear night.

Let’s known more

1. Define luminous objects.

Ans:- The objects which emit light themselves are called luminous objects.

  • Luminous objects may be natural or man-made.

2. Define non-luminous objects.

Ans:- Those objects which do not emit light themselves but only reflect ( or scatter ) the light which falls on them, are called non-luminous objects.

  • Even the moon is a non-luminous objects. We can see the moon because it reflects the sunlight falling on its surface towards us.

3. What is transparent medium?

Ans:- A medium in which ray can travel freely over large distances is called a transparent medium. For e.g. water, glycerine etc.

4. What is opaque?

Ans:- A medium in which ray cannot travel is called opaque. For e.g. wood, metals, bricks, etc.

5. Define medium.

Ans:- The material through which ray travels is called medium.

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