The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:
The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:
Introduction
Human beings can see electromagnetic radiation in the wavelength range from 400 nm to 750 nm, which is called visible light. Light travels at the speed of 3 x 108 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.
Spherical Surfaces
For any curved spherical surface n2/v –n1/u = (n2 – n1)/R which gives a relation between object and image distance in terms of the refractive index of both the mediums (n1 and n2) and the radius of curvature (R).
Lens
The thin lens formula is 1/v – 1/u = 1/f and the magnification is m = h’/h = v/u
where m is the linear magnification of the lens, h’ is the image height and h is the object height.
Power of a lens
Power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it. The power P of a lens is defined as the tangent of the angle by which it converges or diverges a beam of light falling at a unit distance from the optical centre.
P = 1/f,
where f is the focal length.
The SI unit of power of a lens is dioptre (D) 1D = 1m-1.
When a narrow beam of white light is incident on a glass prism, the emergent light consists of several colours. The sequence of these colours is violet, indigo, blue, green, yellow, orange and red (acronym VIBGYOR).
The phenomenon of splitting of light into its component colours is known as dispersion. The pattern of colour components of light is called the spectrum of light.
The rainbow is an example of dispersion of sunlight by water drops in the atmosphere. The phenomenon is a combined effect of dispersion, reflection and refraction.
Using the reflection and refraction properties of mirrors, lenses and prisms, several optical devices and instruments are made such as telescopes, microscopes, binoculars, periscope etc. The human eye is the best example created by nature which is used to create these other devices.
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.
