MINDMAPS FOR UPSCPET 2026 · VISUAL REVISION
Refraction and the convex lens
Connect Snell's law of refraction to how a convex lens forms images
Refraction and lens optics
Cause and direction of refraction
- Refraction is the bending of an oblique light ray when its speed changes across media.
- Entering an optically denser medium (slower speed), the ray bends toward the normal.
- Entering a rarer medium, it bends away from the normal; at i = 0°, it goes straight.
Snell's law and refractive index
- The law of refraction is Snell's law: n₁ sin i = n₂ sin r (angles from the normal).
- For a given pair of media and wavelength, sin i ÷ sin r is a constant (n₂₁).
- Absolute refractive index n = c ÷ v compares light speed in vacuum to that in the medium.
Convex lens as a converging lens
- A usual glass convex lens in air is thicker centrally and converges parallel rays.
- Refracted rays meet at the principal focus F; optical power P = 1 ÷ f (in metres).
- Convex lenses have positive focal length (+f) and positive power in dioptres (D).
Magnifying glass vs real images
- Place the object closer to the lens than its focal length, between the lens and F.
- The eye traces them back to a virtual, erect and magnified image: a magnifying glass.
- Beyond the focal distance, a converging lens can form a real inverted image on a screen.
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