Title Med-RM_Phy_SP-4_Ch-24_Ray Optics and Optical Instruments.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Introduction Reflection of Light by Spherical Mirrors Refraction Total Internal Reflection Refraction at Spherical Surfaces Refraction by Lenses Refraction through a Prism Dispersion by a Prism Optical Instruments Introduction Electromagnetic radiations belonging to the wavelength range about 400 nm to 750 nm is called light. Human eye (ratina) has the sensitivity to detect electromagnetic wave of this range. Sir Isaac Newton, held the theory that light was made up of tiny particles. In 1678, Dutch Physicist, Christian Huygens, believed that light was made up of waves. This became known as “Huygens’ Principle”. Huygens theory was the successful theory of light wave motion in three dimensions. Newton could not explain the phenomenon of light interference, this forced Newton’s particle theory in favour of the Wave theory. In 1803, Thomas Young studied the interference of light waves by shining light through a screen with two slits equally separated, the light emerging from the two slits, spread out according to Huygen’s principle. In 1900 Max Planck proposed the existence of a light quantum, a finite packet of energy which depends on the frequency. In 1905, Albert Einstein had proposed a solution to the problem of observations made on the behaviour of light having characteristics of both Wave and Particle theory. From work of Planck on emission of light from hot bodies, Einstein suggested that light is composed of tiny particles called photons, and each photon has energy. In this chapter we deal with phenomena of reflection, refraction, dispersion, using ray picture of light. Using the simple law of reflection and refraction, we shall study the image formation by spherical reflecting surfaces, spherical refracting surfaces, plane surfaces, optical instruments, including human eye. Chapter 24 Ray Optics and Optical Instruments
184 Ray Optics and Optical Instruments NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 REFLECTION OF LIGHT BY SPHERICAL MIRRORS Law of Reflection (i) The laws of reflection state that the angle of reflection (angle between reflected ray and the normal to the reflecting surface) equals the angle of incidence (Angle between incident ray and the normal). (ii) The incident ray, reflected ray lie in the same plane with normal to the reflecting surface. These laws are valid at each point on any reflecting surface whether plane or curved.   Incident ray Normal Reflected ray Mirror Pole and optical centre: Geometric centre of a spherical mirror is called its pole while that of a spherical lens is called its optical centre. The line joining the pole and centre of curvature of spherical mirror is known as principal axis. Sign Convention : To derive any formulae for reflection at spherical surfaces we must first adopt a sign convention for measuring distances. According to Cartesian sign convention all distances are measured from pole of the mirror, or optical centre of lens. F M C P Concave Mirror Upward height (+ve) Incident light (+ve) Downward height (–ve) Distance opposite to incident light (–ve) Distance along incident light (+ve) Principal axis P – Pole ; F – Focus ; C – Centre of Curvature PF = f = Focal length of mirror. CP = R = Radius of curvature of mirror. The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative. Heights measured above principal axis are taken as positive and the heights below the principal axis are taken as negative. Focal Length of Spherical Mirrors As we know parallel beam of light ray is incident on a concave or convex mirror as shown in figure, the rays will converge (appear to diverge) at a point F called principal focus. F M C P Concave Mirror P F C Convex Mirror M
NEET Ray Optics and Optical Instruments 185 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 In case of a concave mirror the reflected rays will converge at focus F whereas in case of convex mirror the reflected rays appear to diverge from focus F. Convex Mirror P F C M   2  M Concave Mirror 2 FC P    2 R f  The Mirror Equation The rays emanating from a point actually meet at another point after reflection (refraction) is called as the image. Hence, an image is a point-to-point correspondence of object. In practice we can take any two rays emanating from a point on an object, trace their paths and find the intersection to obtain image. M P B C F B D A f v N u A 111 vuf    This is called as mirror equation.  m = – I O h v h u  (Lateral Magnification) These relations are true for both concave mirror and convex mirror. Multiplying both sides of 111 by , u uvf   We get f u v u 1  or, f fu f u v u  1   v f m u fu     Similarly, multiplying the mirror formula by v, we get f vf m  
186 Ray Optics and Optical Instruments NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Note: (1) Lateral magnification is also called transverse or linear magnification. (2) Areal magnification = 2 2 2 Area of the image Area of the object v m u   It is also called superficial magnification. (3) Newton's formula : For spherical mirrors, let x = distance of the object from the focus y = distance of the image from the focus 2 xy  f Rules for Image Formation (a) A ray passing parallel to principal axis after reflection from the mirror passes or appears to pass through focus. 0 2  R f 0 2  R f C F R f P R F C (b) A ray initially passing through or directed towards focus becomes parallel to the principal axis after reflection from the mirror. F C F C P (c) A ray passing through the centre of curvature retraces its path after reflection. C P C