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Refractive index from real and apparent depth – travelling microscope lab

PhysicsOpticsAges 14–15Virtual lab

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Use with my class

A virtual practical to measure refractive index with a travelling microscope: focus on a mark with nothing on it, through a transparent slab or a liquid layer, then on powder sprinkled on the top surface. Read R₁, R₂ and R₃ on the vernier scale (least count 0.001 cm) and work out n = real depth / apparent depth = (R₃ − R₁)/(R₃ − R₂); a graph of real against apparent depth has slope n. The Concave mirror screen finds the no-parallax position of a pin and its image (h = R), then again with a little liquid in the mirror (h = R′), giving n ≈ R/R′.

Lesson: Refraction of light: real and apparent depth

What it shows

A mark under a glass slab or a layer of water looks closer than it really is. Light from the mark bends away from the normal as it leaves the denser medium, so seen from above the rays seem to come from a point raised by t − t/n. The apparent depth is t/n, so n = real depth / apparent depth. A travelling microscope measures both depths from three readings. A concave mirror gives another method: a pin coincides with its own image at the centre of curvature, and a thin liquid layer moves that point down to about R/n. The model uses rays close to the normal or axis.

How to use

On Travelling microscope, choose Sample on the mark and Real thickness t. Drag or tap the side view, then use Focusing knob (reading R) and its − / + buttons until the eyepiece view is sharpest; press Take reading for each of the three steps. On Concave mirror, leave Move the eye on and change Pin height h until the tips stay together, then press Take reading before and after the liquid is poured.

Parameters you can change

  • Screen Travelling microscope, Concave mirror
  • Sample on the mark Crown glass slab, Acrylic slab, Slab X (unknown), Water, Glycerol, Liquid X (unknown)
  • Real thickness t of the slab or liquid layer 0.5–5 cm
  • Show rays
  • Show reading
  • Mirror radius R 10–30 cm
  • Liquid in the mirror Water, Glycerol, Liquid X (unknown)

Questions to explore

  1. Why does a coin at the bottom of a pool look closer to the surface than it really is?
  2. How does the apparent depth change when the real depth of the same liquid is doubled?
  3. Why does a little liquid in a concave mirror move the no-parallax point closer to the mirror?

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