Let a ray from the source S strike the interface of the two media at the point O. The normal to the surface at the point O is ON. The incident ray SO makes an angle i with the normal. This angle is called the angle of incidence. It is observed that part of the incident ray along SO is reflected along OA. The reflected ray follows the normal laws of reflection i.e. the angle of reflection ∠ NOA = i. The intensity of the reflected light will depend on the smoothness of the interface and also on the angle of incidence. This is a normal observation; we do see our reflection in a puddle of water, or, on the car, or shop window.
2. Ray tracing and Definition of angles
These blogs are written to help understand the concepts in physics at the Grade 10 level. The student is not exposed to the wave nature of light until grades 11 or 12. The phenomenon of reflection is generally taught in earlier grades; hence we proceed with the assumption that the student already has a good grasp of the reflection phenomena. We focus here to learn the phenomena of refraction of light.
Light rays will be emanating from the source in all directions. Let us select any one ray along the direction shown. Let this ray encounter a change of medium. Let this ray encounter a change of medium.
Let us consider that the ray is going from a rarer medium like vacuum or air to a denser medium like water or glass. Will the ray continue straight? Does light interact with the medium? Yes it does. Light waves are electro-magnetic waves. The electrons in the medium move in response to the electric field of the wave. The effect is to slow down the speed of the wave. In vacuum there are no electrons to interact hence the speed of light is maximum. The refractive index of a medium is defined as,
μ = Speed of light in vacuum / Speed of light in Medium Eq. (1)
As the speed of light is largest in vacuum, the refractive index is always greater than 1. The refractive index of air (μ =1.0003) is very close to that of vacuum (μ =1.0), hence the refractive index value of a medium with respect to air is taken to be that with respect to vacuum.
On entering the denser medium like water or glass from a rarer medium like vacuum or air at O, the ray does not continue along the original direction but bends towards the normal. This phenomenon is called the refraction of light.
A detailed explanation for this will be taught in later years when we will study the electromagnetic nature of light and its interaction with matter. The refracted ray OT makes an angle r with the normal OM to the surface. This angle is called the angle of refraction. The refracted ray is deviated by an angle, δ = ∠ POT from the original direction, this is called as the angle of deviation.