Index Matching Explanation

When a lens is immersed in a liquid with the same refractive index:

• The lens becomes invisible because light doesn't bend at the interfaces
• The lens formula shows 1/f = 0, meaning f → ∞ (no focusing power)
• The lens acts like a flat sheet of glass with parallel surfaces

This principle is used in microscopy (index matching oils) and magic tricks (as in this example). Water (n=1.33) won't make a glass lens (n≈1.5) disappear, but glycerine (n=1.47) can for specially made lenses.