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Light of intensity 50 Wm2 is incident on a glass windowpane

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Light of intensity 50 W/m2 is incident on a glass windowpane (n = 1.5), with rays approximately perpendicular to the surface.
 
Optics - Light incident on a glass windowpane Optics - Light of intensity 50 W/m2 is incident on a glass windowpane - Physics Problems
Light of intensity 50 W/m2 is incident on a glass windowpane (n = 1.5), with rays approximately perpendicular to the surface, as show in figure below. Find the intensity I1, I2, and I3.
 
Light incident on a glass windowpane

At point A and B, part of the incident light is transmitted and part is reflected. We defined the reflectance R as the ratio of the intensity of the reflected light Ir to the intensity of incident light Ii.
Equation (1) - reflectance
Equation (1)


Transmittance is similarly defined as the ratio of transmitted intensity It to incident intensity Ii.
Equation (2) - Transmittance
Equation (2)


From conservers ion of energy
conservers ion of energy
Equation (3)


Using above three equations
Reflectance - Transmittance

R + T = 1     Equation (4)

In the special case of a ray incident normal to the surface, R is found to be a simple function of the indices n and n’.
ray incident normal to the surface
Equation (5)


Air to glass
air to glass reflectance

R = 0.04

From equation 4
T = 0.96

Apply equation 1 at point A
equation (1)

Ir = 2 W/m2
Intensity of the reflected light at point A is 2 W/m2

I1 = 2 W/m2

Using equation 3
equation (3)

It = 48 W/m2

Intensity of the transmitted light at point A is 48 W/m2

Apply equation 2 at point B
equation (2)

It = 46.08 W/m2

Intensity of the transmitted light at point B is 46.08 W/m2

I2 = 46.08 W/m2

Apply equation 1 at point B
equation (1)

Ir = 1.92 W/m2

Intensity of the reflected light at point B is 1.92 W/m2

Apply equation 2 at point C
equation (2)

It = 1.84 W/m2

Intensity of the transmitted light at point C is 1.84 W/m2
I3 = 1.84 W/m2
 
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