Dispersion and Angular Dispersion
For a thin prism, angular dispersion between wavelengths 1 and 2 is (n1-n2)A; dispersive power is angular dispersion divided by mean deviation.
Why this shows up in the exam
Spectrometers · Wavelength separation · Material dispersion comparison
Learn the idea
A prism separates colours because refractive index depends on wavelength. Shorter visible wavelengths usually bend more in normal dispersion, spreading white light into a spectrum.
🧠 Memory hook: Colour spread comes from index spread.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Delta delta = (n_v-n_r)A — violet-red angular dispersion for a thin prism
- omega = (n_v-n_r)/(n_y-1) — dispersive power using a mean yellow index
How to approach it
- 1Identify wavelength-dependent indices
- 2Compute each deviation or their difference
- 3Separate mean deviation from angular dispersion
Common slip-ups that cost marks
- •Calling dispersion diffraction
- •Assuming frequency changes inside the prism
- •Using one refractive index for all colours
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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