# Difference between the increase in optical path due to refraction and that due to reflection

If we place two glass plates of refractive index $$\mu$$ and each having thickness $$t ,$$ on the way of a light ray the increase in the optical path becomes $$\left(S_2 P - S_1 P \right) = 2 \left(\mu - 1\right) t$$ due to refraction through them, and the path difference (or extra path traversed by light) due to reflection at the second surface of one glass plate is $$2 \mu t \cos{\left(\alpha\right)} .$$

Without seeing the expression if we look at the phenomenon directly, aren't they basically same, as both are going through a glass media of the same length?

• I think you need to draw a diagram to explain what you mean – John Rennie Apr 8 '14 at 17:09
• The experiment you are describing is very unclear. How many sources are there? How many beams? Which ones are you considering (reflected, transmitted, which surface...)? Please provide some additional information. – Anael Apr 9 '14 at 0:10
• I have said that in first line "a light ray",and also the reflection is at second surface of the plate.. – Diya Apr 9 '14 at 4:03