SCIENCE || CLASS 10TH || CHAPTER 10 || BASIC CONCEPTS || CBSE AND MP BOARD

 "AKC SCIENCE CLASSES"

CLASS 10 TH (CBSE AND MP BOARD)

CHAPTER 10

LIGHT-REFLECTION AND REFRACTION

EXERCISE QUESTIONS AND ANSWERS

1) Light :- It is a form of electromagnetic radiation (radiant energy) which produces in us the sensation of vision.

2) Electromagnetic waves :- An electromagnetic wave is a wave constituted by oscillating electric and magnetic fields which oscillate in mutually perpendicular directions and also perpendicular to the direction of propagation of the wave.

3) Important properties of electromagnetic waves :- 

• They are transverse in nature.
• They do not require any medium for their propagation.
• They travel through vacuum with an enormous speed of 3 ✖ 10⁸ m/s.
• The speed of an electromagnetic wave depends on the nature of the medium.

4) Electromagnetic nature of light waves :- According to Maxwell, light travels through space in the form of electromagnetic waves. The wave-length range of visible light is 4 ✖ 10⁻⁷ m to 8 ✖ 10⁻⁷ m.

5) Ray of light :- The direction or path along which light energy travels in a medium is called a ray of light.

6) Beam of light :- A group of rays is called a beam of light; which can be parallel, convergent or divergent.

7) Reflection of light :- When light travelling from one medium falls on the surface of another medium, a portion of the incident light is turned back into the first medium. This is called 'reflection of light'.
8) Law of reflection of light :-

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

                         ㄥi = ㄥr.

9) Optical image :- When a beam of rays starting from a point source of light suffers a change in direction due to reflection or refraction and the reflected or refracted rays actually converge or appear to diverge from another point, then the second point is called the image of the first point. An optical image of the first point. An optical image may be real or inverted.

10) Real image :- If a beam of rays starting from a point source of light, after reflection or refraction, actually converges to a point, then the second point is called the real image of the first. A real image is inverted and can be obtained on a screen. 

11) Virtual Image :- If a beam of rays starting from a point source of light, after reflection or refraction, actually converges to a point, then the second point is called the real image of the first. A virtual image is erect and cannot be obtained on a screen.

12) Mirror :- A highly polished surface which is smooth enough to reflect a good fraction of light incident on it is called a mirror.

13) Properties of image  formed by a plane mirror :-

• It is virtual and erect.
• It is of same size as the object.
• It is formed as far behind the mirror as the object is in front of the mirror.
• It is laterally inverted with respect to the object.

14) Lateral inversion :- The sideways reversal of the image formed by a plane mirror is known as lateral inversion.

15) Definitions in connection with spherical mirrors :-

1. Spherical mirror :- It is a mirror whose reflecting surface forms a part of a hollow sphere. 
2. Concave mirror :- It is the spherical mirror in which the reflection of light takes place from the inner hollow surface.
3. Convex mirror :- It is the spherical mirror in which the reflection of light takes place from the outer bulged surface.
4. Pole :- It is the middle point of the spherical mirror.
5. Centre of curvature :- It is the centre of the sphere of which the mirror forms a part.
6. Radius of curvature :- It is the radius of the sphere of which the mirror forms a part.
7. Principle axis :- The line passing through the pole and the centre of curvature of the mirror is called its 'principal axis'.
8. Linear aperture :- It is the diameter of the circular boundary of the mirror.
9. Angular aperture :- The angle subtended by the circular boundary of the mirror is called its 'angular aperture'.
10. Principal focus :- It is point on the principal axis where a beam of light parallel to the principal axis either actually converges into or appears to diverge from, after reflection from the mirror.
11. Focal length :- It is the distance between the pole of the mirror.
12. Focal plane :- The vertical plane passing through the principal focus and perpendicular to the principal axis is called focal plane.

16) Relation between f and R of a spherical mirror :- The focal length f of a spherical mirror is half of its radius of curvature R.

                                        f = R/2

The focal length or radius of curvature of a plane mirror is infinity.

17) New cartesian sign convention for spherical mirrors :-

• All the ray diagrams are drawn with the incident light travelling from left to right.
• All the distances parallel to the principal axis are measured from the pole of the mirror.
• All distances measured in the direction of incident light are taken as positive.
• All distances measured in the opposite direction of incident light are taken as negative.
• Heights measured upward and perpendicular to the principal axis are taken positive.
• Heights measured downwards and perpendicular to the principal axis are taken negative. Fora concave mirror, f and R are negative and for a convex mirror these are positive.

18) Mirror Formula :- This formula gives the relationship between objects distance (u), image distance (v) and focal length (f) of a spherical mirror.

      

                 1/u + 1/v = 1/f

This formula  is applicable to both concave and convex mirrors.

19) Linear magnification (m) :- It is the ratio of the size of the image (h') to the size of the object (h).

    m = h'/h = -v/u = f/(f-u) = (f-v)/f 

In the case of concave mirror, for a real, inverted image magnification is negative and for a virtual, erect image it is positive. In case of convex mirror, the magnification is always positive, as the image formed is always virtual and erect.

In case of a spherical mirror, remember for magnification :-  

• Virtual positive 
• real negative.

20) Refraction of Light :- When a ray of light passes from one transparent medium to another, it bends from its path at the surface of separation. This phenomenon is known as " refraction of light ".

21) Law of refraction of light :-

• The incident rays, the refracted rays and the normal at the normal at the point of incident all lie in the same plane.
• For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This constant is called refractive index (n) of the second medium with respect to the first medium.

      

$sinisinr=constant$

  

The second law is called Snell's law of refraction.

22) Refractive index :- The refractive index n of a transparent medium is the ratio of the speed of light in vacuum (c) to the speed of light in that medium (v).

        speed of light in vacuum           c

n  = ---------------------------------------   =  -------

        speed of light in medium        v

23) Relative refractive index :- The relative refractive index of medium 2 with respect to medium 1 is the ratio of speed of light (v₁) in medium 1 to the speed of light (v₂) in medium 2.

  

                     v₁

            n₂₁ =  -------

                     v₂

24) Refraction through a rectangular glass slab :- For refraction through a glass slab, the emergent ray is parallel to the incident ray but it is laterally displaced from the direction of incident ray.

25) Relation between real depth and apparent depth :- Due to refraction of light, the apparent depth of an object placed in a denser medium is less than the real depth.

                    Real depth

          n = -------------------------------

                        Apparent depth

26) Definitions in connection with spherical lenses :-              

1. Lens :- It is a portion of a transparent medium bounded by two surfaces, at least one of which is curved surface.
2. Convex lens :- It is thicker at the centre than at the edges. It converges a parallel beam of light on refraction through it. It has a real focus.
3. Concave lens :- It is thinner at the centre than at the edges. It diverges a parallel beam of light on refraction through it. It has a virtual focus.
4. Centre of curvature :- The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part. Because a lens has two surfaces, so it has two centres of curvature.
5. Radius of curvature :- The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part.
6. Principal axis :- It is the line passing through the two centers of curvature of the lens.
7. Principal focus :- A beam of light parallel to the principal axis either converges to a point or appears to diverge from a point on the principal axis after refraction through the lens. This point is called 'principal focus'. A lens has two principal focii
8. Optical centre :- It is a point situated within the lens through which a ray of light passes undeviated.
9. Focal length :- It is the distance between the principal focus and the optical centre of the lens.
10. Aperture :- It is the diameter of the circular boundary of the lens.  

27) New cartesian sign convention for spherical lenses :-

• All distances are measured from the optical centre of the lens.
• The distances measured in the direction of incident light are taken positive. 
• The distances measured in the opposite direction of incident light are taken negative.
• Heights measured upwards and perpendicular to the principal axis are taken positive.
• Heights measured downwards and perpendicular to the principal axis are taken negative.

For a convex lens, the focal length f is positive and for a concave lens f is negative.

28) Lens formula :- This formula gives relationship between object distance (u), image distance (v) and focal length (f) of a spherical lens.

          

               1/v - 1/u = 1/f

This formula is applicable to both convex and concave lenses. 

29) Linear magnification of a spherical lens (m) :- It is the ratio of the size of the image formed by the lens (h') to the size of the object (h).

                

               h'         v

        m = ------  =  ------

               h         u

In case of convex lens, the magnification m is positive when image is virtual and it is negative when image is real. In case of concave lens, m is always positive as the image formed is always positive as the image formed is always virtual.

For a spherical lens, remember for the linear magnification (m) :-

• Virtual positive,
• real negative.

30) Power of a lens :- It is defined as the reciprocal of the force length expressed in metres.

           1              100

P = ----------------- =  ---------------

        f(in m)       f( in cm)

S.I. unit of power is m⁻¹, also called dioptre (D). One dioptre is the power of a lens whose focal length is 1 metre. The power of a convex lens is positive and that of a concave lens is negative.

31) Power of a lens combination :- If a number of lenses are placed in contact with each other, then the net power of the combination is equal to the algebraic sum of their individual powers.

P = P₁ + P₂ + P₃ + ...............