NCERT Class 10 the most Important points of chapter 10 (Light—Reflection and Refraction) part-1

The most Important points of light reflection and Refraction part-1

1. LIGHT is a form of electromagnetic radiation (radian 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: 

(i) They are transverse in nature.

(ii) They do not require any medium for their propagation.

(iii) They travel through vacuum with an enormous speed of 3 × 10⁸ m/s.

4. Electromagnetic nature of light waves: According to Maxwell, light travels through space in the form of electromagnetic waves.

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. Laws of reflection of light:

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

(ii) The angle of incidence is equal to the angle of reflection. Thus, 

                   くi  = くr.

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 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, appears to diverge from another point, then the second point is called the virtual image of the first. A virtual image is errect and cannot be obtained on a screen. 

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

13. Properties of image formed y a plain mirror:

(i) It is virtual and erect.

(ii) It is of same size as the object.

(iii) It is formed as far behind the mirror as the object is in front of the mirror.

(iv) 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 sphereical mirrors:

(i) sherical mirror: It is a mirror whose reflecting surface forms a part of a hollow sphere.

(ii) Concave mirror: It is the spherical mirror in which the reflection of light takes place from the inner hollow surface.

(iii) convex mirror: It is the spherical mirror in which the reflection of light takes place from the outer bulged surface.

(iv) pole: It is the middle point of the spherical mirror.

(v) Centre of curvature: It is the centre of the sphere of which the mirror forms a part.

(vi) Radius of curvature: It is the radius of the sphere of which the mirror forms a part.

(vii) Principal axis: The line passing through the pole and the centre of curvature of the mirror is called it's ‘principal axis’.

(viii) linear aperture: It is the diameter of the circular boundary of the mirror.

(ix) Angular aperture: The angle subtended by the circular boundary of the mirror is called it's ‘angular aperture’.

(x) 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.

(xi) Focal length: It is the distance between the focus and the pole of the mirror.

(xii) 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:

(i) All the ray diagrams are drawn with the incident light travelling from left to right.

(ii) All the distances parallel to the principal axis are measured from the pole of the mirror.

(iii) All distances measured in the direction of incident light are taken as positive.

(iv) All distances measured in the opposite direction of incident light are taken negative.

(v) Heights measured upward and perpendicular to the principal axis are taken positive.

(vi) Heights measured downwards and perpendicular to the principal axis are taken negative.

For a concave mirror, f and R are negative and for a convex mirror these are positive.

18. Mirror formula: This formula gives the relationship between object 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 mirror. 

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

In case of concave mirror, for a real, inverted image magnification is negative and for a virtual, errect 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.