Tutorial Reflection of Light Refraction of Light Index of Refraction Total Internal Reflection Reflection of Light When a ray of light is incident on a boundary of two media, part of the light is reflected and part of the light is refracted. Law of reflection states that angle of incidence is equal to the angle of reflection.
Example Two mirrors intersect at an angle of α = 30o. A light ray is reflected off the horizontal mirror. The path of the reflected beam is shown in the figure below. Show that the angle β = 120o. In general β = 180 - 2α
Before attempting to do this problem, we will redraw the diagram marking all the angles. 
From Snell's law, i = i' and r = r' < BAD = 90 - i -------------------------- (1) < ABD = 90 - r -------------------------- (2) (1) + (2) ==> < BAD + < ABD = (90 - i) + (90 - r) = 180 - (i+r) But < BAD + < ABD = 180 - α Combining these two equations: 180 - α = 180 - (i+r) α = (i+r) -------------------------- (3) In triangle ABC, <C = 180 - (i+i') - (r+r') = 180 - (2i - 2r) since i' = i and r' = r So <C = 180 - 2(i+r) From (3) <C = 180 - 2α But β = <C (opposite angles) β = 180 - 2α β = 180 - 2 (30) = 120o β = 120o |
Refraction of Light We learnt that when a ray of light is incident on a boundary of two media, part of the light is reflected and part of the light is refracted. The angle that the refracted beam makes with the normal is called angle of refraction. If the incident beam is in a medium of lower refractive index, then angle of refraction is smaller than the angle of incidence. If the incident beam is in a medium of higher refractive index, then angle of refraction is larger than the angle of incidence. These two types of refractions are illustrated in the following diagrams. 
Snell’s Law
Assume a ray travels from medium A to medium B. Let the refractive indices of medium A ande medium B be n1 and n2 respectively. Let the angle of incidence and angle of refraction be i and r respectively. According to Snell's Law,
Refractive index of a medium is defined as the ratio of the speed of light in vacuum (c) and the speed of light in the medium (v). Refractive index is a dimensionless quantity and is always larger than one.
Wavelength and Index of Refraction
Wavelength of light depends on the medium it travels. In fact, wavelength is inversely proportional to the index of refraction.
Total Internal Reflection and Critical Angle
Total internal reflection is possible when light travels from a medium of higher refractive index to a medium of lower refractive index.
As the angle of incidence is gradually increased, angle of refraction increases as well. At a certain point, angle of incidence is such that the refracted beam travels along the boundary. i.e. Angle of refraction = 90o. This angle is called the Critical Angle. For angles of incidence larger than the critical angle, total internal reflection takes place.
Using Snell’s law,
n1 sin θc = n2 sin 90 = n2
θc = sin-1(n1/n2) |
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