Optics
Easy Overview
Have you ever tried to grab a coin from a bucket of water, only to find it's not where it looks like it is? That's refraction bending light. This chapter covers how light behaves when it hits mirrors, passes through lenses, and travels through different materials. Plus how your eyes, microscopes, and telescopes use these principles to see the tiny and the far away.
Reflection — Bouncing Light
Law of reflection: angle of incidence = angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane. That's it. Works for flat mirrors (plane mirrors give virtual, erect, same-size images) and curved mirrors. Concave mirrors converge light (used in torches, telescopes). Convex mirrors diverge light (used in rearview mirrors — gives a wider field of view). Mirror formula: 1/f = 1/u + 1/v.
Refraction — Light Bends
When light goes from one medium to another, it bends. That's refraction. Snell's law: n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index. Light bends toward the normal when going to a denser medium (air → water), away from normal when going to a rarer medium (water → air). Refractive index n = c/v, where c is speed of light in vacuum and v in the medium. It's also n = real depth / apparent depth.
Lenses — The Power to Focus
Convex lenses converge light (f positive), concave lenses diverge light (f negative). Lens formula: 1/f = 1/v − 1/u. Power of a lens P = 1/f (in diopters, when f is in meters). Lens maker's formula: 1/f = (n − 1)(1/R₁ − 1/R₂). Magnification m = v/u. A real image can be projected on a screen; a virtual image can't. Your eye's lens changes shape to focus — that's accommodation.
Optical Instruments — Seeing More
Simple microscope: a single convex lens magnifies objects (maximum ~10×). Compound microscope: two lenses — objective (small f) and eyepiece — for much higher magnification. Telescope: objective collects light from distant objects (large f), eyepiece magnifies the image. Astronomical telescopes use two convex lenses. Terrestrial telescopes need an extra lens to make the image upright. Magnification power M = fₒ/fₑ for a telescope in normal adjustment.
Dispersion — Rainbows and Prisms
White light is made of all colors. A prism splits it into its component colors — that's dispersion. Red bends the least, violet bends the most (because refractive index is slightly different for each wavelength). That's why you get VIBGYOR. Rainbows are just dispersion by water droplets in the atmosphere. Angular dispersion = δ_v − δ_r, and dispersive power ω = (δ_v − δ_r)/δ_y.
Key Points
- •Reflection: ∠i = ∠r (laws of reflection)
- •Mirror formula: 1/f = 1/u + 1/v
- •Snell's law: n₁ sin θ₁ = n₂ sin θ₂
- •Refractive index n = c/v = real depth / apparent depth
- •Lens formula: 1/f = 1/v − 1/u
- •Power of lens P = 1/f (diopters)
- •Total internal reflection happens when light goes from denser to rarer medium at θ > critical angle
- •Dispersion: white light splits into VIBGYOR through a prism
Practice Questions
- An object is placed 30 cm from a concave mirror of focal length 20 cm. Find image position and nature.
- A convex lens has focal length 15 cm. Where should an object be placed to get a real image 3 times magnified?
- Explain total internal reflection with a neat diagram. Give two applications.
- Derive the lens maker's formula.