2.4: Convergence
- Last updated
- Mar 5, 2022
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Figure II.5 shows a lens made of glass of refractive index 1.50. To the left of the lens is air (refractive index 1.00). To the right of the lens is water (refractive index 1.33). A converging beam of light is incident upon the lens directed toward a virtual object O that is 60 cm from the lens. After refraction through the lens, the light converges to a real image I that is 20 cm from the lens.
I am not at this stage going to ask you to calculate the radii of curvature of the lens. (You can’t – you need one more item of information.) I just want to use this diagram to define what I mean by convergence.
The convergence of the light at the moment when it is incident upon the lens is called the initial convergence
The convergence of the light at the moment when it leaves the lens is called the final convergence
Sign convention
- Converging light has positive convergence;
- Diverging light has negative convergence.
Example
Initial convergence =
Final convergence =
Notice that, before the light enters the lens, it is in a medium of refractive index 1.00. Thus the relevant refractive index is 1.00, even though the virtual object is in the water.
