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CLASS - 10 SCIENCE IMPORTANT QUESTIONS OF MARK 3   FOR BOARD EXAMINATION

QUESTION NO. 40

Law of reflection of light. Define absolute refractive index and Express it mathematically.

ANSWER NO. 40

LAW OF REFLECTION 

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident Ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

REFRACTIVE INDEX

Refractive index is a number which measures the bending of light when passing from one medium to another. It is the ratio of velocity of light in vacuum to the velocity of light in the medium.

Refractive index = velocity of light in vacuum
                                     velocity of light in medium

QUESTION NO. 41

Define the following and show in the diagram.
  1. Centre of curvature
  2. Pole
  3. Radius of curvature of the mirror
  4. Principal axis

ANSWER NO. 41

  1.  CENTRE OF CURVATURE  --  The reflecting surface of a spherical mirror form a part of sphere. This point is called centre of curvature.
  2. POLE --  The centre of the reflecting surface of a spherical mirror is a point called pole.                        
  3.  RADIUS OF CURVATURE OF THE MIRROR   - The radius of the sphere of which reflecting surface of a spherical mirror form a part, is called the radius of curvature.
  4.  PRINCIPLE AXIS - It is a straight line passing through the pole and the centre of curvature of spherical mirror.    
       

QUESTION NO. 42

What is law of reflection of light ?

ANSWER NO. 42

LAW OF REFRACTION OF LIGHT

  1. The incident ray, the reflected ray and the normal to the interface of two transparent media at the point of incidence, all lie in same plane.
  2. The ratio of sign of angle of incident to the Sine of angle of reflection is a constant, for the light of a given colour and for the given pair of media. This law is also known as snell's law of reflection.
 If (i) is the angle of incident (r) is the angle of reflection then,
          Sine i   = Constant
           Sine r
  This constant value is called the reflective index of the second medium with respect to the first.

QUESTION NO. 43

define the following
  1. Centre of curvature
  2. Principle Axis
  3. Optical centre
  4. Aperture

ANSWER NO. 43

  1. CENTRE OF CURVATURE

     A lens, either a convex lens or a concave lens, has two spherical surface. Each of these surface forms a part of sphere. The centre of this sphere are called centre of curvature of the lens. Represented by letter C.

2. PRINCIPLE  AXIS

  An imaginary straight line passing through two two centres of curvature of a lens is called its principle axis.

3. OPTICAL CENTRE

 The point of a lens is its optical centre. It is usually represented by the letter O.

4. APERTURE 

  The effective diameter of the circular outline of a spherical lens s called its aperture.

QUESTION NO. 44

Why is the colour of the sky  is blue.

ANSWER NO. 44

the molecules of the air and other fine particles in the atmosphere have size smaller than the wave length of the visible light. These are more effective in scattering light of shorter wavelength at the red end.
     The red light has a wavelength about 1.8 times greater than blue light. Thus, When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour more strongly than red. The scattered blue light enters in our eyes. 

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