Light
Light is a form of energy. It helps our vision i.e. objects around us become visible in presence of light.
Reflection of light
Reflection of light is an optical phenomenon where light energy from a source after traveling through vacuum or any homogeneous transparent medium when incidents on an interface, a part of the incident energy returns to the first medium obeying some laws.
Laws of reflection of light
(i) The incident ray, the reflected ray and the normal to the reflecting surface at the point of incidence all lies on the same plane.
(ii) The angle of incidence is equal to the angle of reflection.
Regular reflection
Reflection of light taking place from a plane mirror or any other highly polished surface is regular reflection. Here, a beam of incident parallel rays after-reflection from a plane polished reflector returns also as a beam of parallel rays.
Irregular or Diffuse reflection
This type of reflection occurs when light rays are incident on a rough surface, like, ground glass, walls, rough paper etc. Here, even if the incident rays are parallel, the reflected rays are not parallel, they are scattered in different directions.
At dawn, the eastern sky looks bright reddish yellow although the sun is below horizon. This is due to diffuse reflection of sunlight taking place from floating dust and other particles. So, we receive the scattered light that causes the eastern sky look bright. The scattered light is reddish yellow, because, blue and indigo colors are of smaller wavelengths suffer more scattering than those of longer wavelengths. Also sunrays travel a very long path from the horizon when the scattered blue colors are absorbed in the atmosphere and hence the remaining light looks reddish. White light devoid of blue and indigo looks reddish yellow. So, at dawn, when the Sun is below horizon, the eastern sky looks bright reddish yellow. For the same reason, the western sky looks reddish yellow at dusk
Refraction of light
Refraction of light is an optical phenomenon where rays of light coming through vacuum or any other homogeneous optical medium when transmits into a second homogeneous optical medium, the speed as well as path of the rays change in the second medium.
Laws of refraction
First law: The incident ray, the refracted ray and the normal at the point of incidence on the refracting surface all lie in the same plane.
Second law (also known as Snell's law): For refraction of an obliquely incident ray of light of any given color in a given pair of optical media, the ratio of sine of the angle of incidence to sine of the angle of refraction is constant.
Definition of critical angle
When a ray of light of any given color tends to travel from a denser medium to a rarer medium, then, for a certain angle of incidence the angle of refraction is 90 degree. This angle of incidence is the critical angle of the given pair of media for the given color of light.
Total Internal Reflection
When a ray of light tends to travel from a denser medium to a rarer medium, then if the angle of incidence exceeds the critical angle, the incident light totally reflects back to the first medium.
This is called total internal reflection.
Definition of Lens
Any homogeneous transparent refractive medium bounded by at least one spherical surface is called lens.
Lenses may be of different shapes, but generally two types of lens are considered: (i) Biconvex or simply convex lens, (ii) Biconcave or concave lens.
A convex lens is made by joining two sections cut out from a single or two different spherical glass globes. The two globes may or may not be of same radius. This lens is thick at the middle and thin at the edge.
A concave lens is also constructed from two glass globes. This lens is thin at the middle but thick at the edge.
Spectrum
A ray of white light after refraction through a prism is split up into its seven component colors (VIBGYOR) which occupy different positions on a screen. This band of seven colors on the screen is called a spectrum.
Spectrum is of two types - (a) Pure and (b) Impure spectrum.
(a) Pure spectrum: It is the spectrum in which all the component colors of the incident light ray, after dispersion, occupy distinct separate positions on a screen.
(b) Impure spectrum: It is the spectrum in which some of the component colors, the difference of wavelengths of which are very small, overlap on one another. Thus, after dispersion, the component colors do not occupy separate distinct positions on a screen.