Refractive Index, Speed, Frequency and Wavelength
For a monochromatic wave in a transparent nondispersive approximation, n = c/v and lambda_medium = lambda_0/n; frequency is continuous across the interface.
Why this shows up in the exam
Material identification · Optical-path calculations · Wavelength selection in media
Learn the idea
Refractive index compares light speed in vacuum with its phase speed in a medium. Light slows and its wavelength shortens in a higher-index medium, while its frequency remains fixed at a stationary boundary.
🧠 Memory hook: Boundary keeps frequency; speed and wavelength change together.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- n = c/v — absolute refractive index
- lambda2/lambda1 = v2/v1 = n1/n2 — frequency unchanged at the boundary
How to approach it
- 1Write n = c/v for each medium
- 2Keep frequency common
- 3Scale wavelength with speed
Common slip-ups that cost marks
- •Changing frequency on refraction
- •Inverting the n-speed relation
- •Using vacuum wavelength inside a medium
🌟 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.
More from Optics
Interference of light
Examine the principle of superposition, Young's double slit experiment, fringe width, intensity distribution, and the conditions for constructive and destructive interference.
Diffraction of light
Understand the bending of light around obstacles, single slit diffraction patterns, their width, and the effect of wavelength on diffraction.
Lenses and mirrors
Explore the image formation, ray diagrams, lens and mirror formulas, and the behavior of light with concave/convex lenses and mirrors, including combinations and virtual objects.
Optical instruments
Understand the working principles, magnification, resolving power, and design of devices like microscopes and telescopes, including their adjustments and measurement techniques.
Polarization of light
Learn about the polarization of light, Brewster's law, Malus' law, and the use and function of polaroids.
Dispersion and rainbow formation
Study how light splits into its constituent colors through dispersion in prisms and natural phenomena like rainbows, including minimum deviation and dispersive power.