##### Chapter 9

## Ray Optics and Optical Instruments

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

#### Introduction

^{8}m/s and in a straight line. The path taken by it is called a ray of light, and a bunch of rays is called a beam of light. Through the chapter of

**Ray optics and optical instruments,**we will study various concepts like reflection,

**refraction,**dispersion,

**total internal reflection,**etc.

We have studied the laws of reflection and know that the angle of incidence equals the angle of reflection and the incident ray, reflected ray and normal to the reflecting surface lie in the same plane. *In spherical surfaces, the normal is the line joining the point of incidence and the centre of curvature of the mirror.*

*Pole is the geometric center of a spherical mirror, and for a spherical lens, it is called optical center.*

*The principal axis of a spherical mirror is the line joining the center of curvature and the pole whereas, in a lens, it is the line joining the principal focus and the optical center. *

*Sign Convention:* According to the Cartesian sign convention, all the distances are measured from the optical center of the lens or the pole of the mirror. The distances measured in the same direction as the incident light are positive and the distances measured in the opposite direction are negative. The heights measured upwards with respect to the x-axis or normal to the principal axis of the mirror or lens are positive, and those measured downwards are negative.

*Focal Length of Spherical Mirrors**: *When a parallel ray of light is reflected on a concave mirror and a convex mirror, then the reflected rays from them either converge at a point and diverge from a point in the mirrors, respectively. This point is the principal focus. *The **focal length of the spherical mirror** f* is the *distance between the focus F and the pole P. *And f = R/2, where R is the radius of curvature of the mirror.

*Mirror Equation: 1/v* + 1/u = 1/f where *v* is the image distance, *u* is the object distance, and *f* is the focal length.

And, *m *= h’/h = -v/u, where *m* is the linear magnification of the mirror, *h’* is the image height, and *h* is the object height.

*When an incident ray of light enters another medium and the direction of propagation changes at the interface of the two media, this phenomenon is called **refraction** of light. *Snell’s laws of **refraction** are:

– The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

– The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. sin i/sin r = *n21*, where *n21* is the refractive index of the second medium with respect to the first medium.

These laws will help to understand **refraction through a prism.**

**Total Internal Reflection**

*When light travels from an optically denser medium to a rarer medium, it is partly reflected in the same medium and partly refracted to the second medium, at the interface of the two media. The reflection taking place is called internal reflection.* *If the angle of incidence is larger than the critical angle, then refraction is not possible, and it gets totally reflected. This is called **total internal reflection**.*

These concepts of **ray optics** are also used to correct myopia and hypermetropia in human beings by using appropriate lenses and their power. This allows us to see our surroundings correctly. Visit www.MVSgo.com to know more.

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