define basic of light

Jul 25, 2022, 16:45 IST

Important Notations about Light 

Focal length f

Magnification m

Distance of the object from the mirror or lens u

Distance of the image from the mirror or lens v

Size of the object (say, height)   or ho

Size of the image   or hi

Least distance of distinct vision D

Pole of mirror P

Optical centre of a lens O

What is Light and what are the characteristic of light 

(i)It is an invisible energy, which causes the sensation of sight. 

(ii) It is the form of energy, which gives in us a sensation of sight. It, itself is not visible but helps us in seeing objects.

Laws of Reflection:

(i)Angle of incidence is equal to the angle of reflection. ( i =  r)

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

Total internal reflection:

This is a phenomenon when the surface of a refracting medium behaves as a reflecting medium (for certain angles of incidence).

Laws of refraction:

(i)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, i.e.,   = constant = . This is also called Refractive Index and also denoted by n.

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

Refraction through prism: When light passes through a prism 

(i)It always bends towards the base of the prism.

(ii)A prism splits the light passing through it into its corresponding wavelengths. This process is called dispersion of light. When sunlight passes through prism it disperses into seven colours, i.e., seven wavelengths (VIBGYOR).

Sign convention for spherical mirrors and lenses:

According to the new Cartesian sign convention:

(i) The pole ‘P’ of the mirror is taken as the origin and the principal axis of the mirror is taken as the x-axis of the coordinate system.

(ii) The object is always placed to the left of the mirror i.e. the light (incident rays) from the object falls on the mirror from the left hand side.

(iii) All the distances parallel to the principal axis of the spherical mirrors are measured from the pole ‘P’ of the mirror.

(iv) All the distances measured to the right of the origin (along +ve x-axis) are taken as positive.

(v) All the distances measured to the left of the origin (along –ve x-axis) are taken as negative.

(vi) The distances (heights) measured upwards (i.e. above the x-axis) and perpendicular to the principal axis of the mirror are taken as positive.

(vii)  The distances (heights) measured downwards (i.e. below the x-axis) and perpendicular to the principal axis of the mirror are taken as negative.

The following illustrates all the points of the new Cartesian sign convention stated above.Origin of Cartesian plane is similar to pole of mirror and optical centre of lenses.

 According to the sign convention for mirrors, the focal length of a concave mirror is negative and that of a convex mirror is positive. The focal length of convex lens is positive. While that of a concave lens is negative.