Eduqo
General Science›Ch 1: Physics›Light: Refraction, Lenses and Eye
Master refraction, total internal reflection, lens power, eye defects and optical phenomena for competitive exams.
Light bends when it travels from one medium to another due to a change in speed. Convex lenses converge light and correct hypermetropia, while concave lenses diverge light and correct myopia.
Memory Hooks
| Defect | Also Called | Corrective Lens |
|---|---|---|
| Myopia | Near-sightedness | Concave |
| Hypermetropia | Far-sightedness | Convex |
| Presbyopia | Old age defect | Bifocal |
TIR
(Total Internal Reflection)
Total Internal Reflection
Light reflects completely inside a denser medium at an angle greater than critical angle.
Dispersion
(Splitting of White Light)
Splitting of White Light
Prism splits white light into its seven constituent colours.
Scattering
(Rayleigh Scattering)
Rayleigh Scattering
Shorter blue wavelengths scatter much more than longer red wavelengths.
Power (P)
(Dioptre Formula)
Dioptre Formula
Power equals reciprocal of focal length in metres (P = 1/f).
Concave lens has positive focal lengthConcave lens has negative focal length
Convex has positive focal length; concave has negative focal length.
Sky appears blue due to refractionSky appears blue due to scattering
Rayleigh scattering of shorter blue wavelengths causes the blue sky.
Focal length in cm is used directly in P = 1/fFocal length must be in metres for P = 1/f
Always convert cm to metres (P = 100/f) before calculating dioptres.
Q1.Which lens is used to correct Myopia (near-sightedness)?
Q2.What is the power of a lens with a focal length of +50 cm?