Light - Reflection and Refraction — Class 10 Science

Light is a form of electromagnetic radiation that travels as both waves and particles (called photons). This dual nature explains all optical phenomena.

Key Points:

• Speed of light in vacuum: c = 3 × 10⁸ m/s
• Refractive index: n = c/v (v = speed in medium)
• Light travels in straight lines in a uniform medium (rectilinear propagation)
• Light can be reflected, refracted, and absorbed

Reflection is the bouncing back of light when it strikes a smooth surface. It follows two important laws:

Important Points:

• Angle of incidence (i): Angle between incident ray and normal
• Angle of reflection (r): Angle between reflected ray and normal
• Both angles are measured from the normal, NOT from the mirror
• Applies to all types of reflecting surfaces (plane, curved, rough, smooth)
• For rough surfaces, light reflects in all directions (diffuse reflection)

A plane mirror is a flat reflecting surface that forms a virtual, erect, and magnified image of an object.

Magnification: m = h'/h = 1
where h' = height of image, h = height of object

Properties of Image in Plane Mirror:

• Virtual: Image cannot be projected on screen (formed behind mirror)
• Erect: Image is upright (not inverted)
• Same size: Magnification m = 1 (image same size as object)
• Symmetrical: Distance of object from mirror = distance of image from mirror
• Laterally inverted: Left and right are interchanged

Curved mirrors are either concave (converging) or convex (diverging). They form different types of images depending on object position.

Mirror Formula: 1/f = 1/v + 1/u
Magnification: m = -v/u
where f = focal length, u = object distance, v = image distance

Sign Convention (New Cartesian):

• Object distance (u): Always positive (measured from mirror)
• Focal length & radius (f, R): Positive for concave, negative for convex
• Image distance (v): Positive if image on same side as reflected light (real), negative if opposite (virtual)
• R = 2f (radius = 2 × focal length)

Image formed by Concave Mirror at different positions:

Refraction is the bending of light when it passes from one medium to another. The degree of bending depends on the refractive indices of the two media.

Snell's Law: n₁ sin i = n₂ sin r
or sin i / sin r = n₂ / n₁
where n = refractive index of medium

Important Points:

• Refractive index: n = c/v (c = speed in vacuum, v = speed in medium)
• Greater the difference in refractive indices, greater is the bending
• Light bends TOWARDS normal when entering denser medium (sin i > sin r)
• Light bends AWAY from normal when entering rarer medium (sin i < sin r)
• Refractive index of air ≈ 1, water ≈ 1.33, glass ≈ 1.5

Critical Angle and Total Internal Reflection:

When light travels from denser to rarer medium, at a specific angle (called critical angle θc), the refracted ray grazes the interface. Beyond this angle, total internal reflection occurs.

sin θc = n₂/n₁ (for light going from medium 1 to medium 2)
sin θc = 1/n (for light going from medium with refractive index n to vacuum)

Lenses are transparent optical devices made of glass or plastic that refract light to form images. There are two main types: convex (converging) and concave (diverging).

Lens Formula: 1/f = 1/v + 1/u
Magnification: m = v/u = h'/h
where f = focal length, u = object distance, v = image distance

Sign Convention (Cartesian):

• Focal length (f): Positive for convex lens, negative for concave lens
• Object distance (u): Always positive (object on left of lens)
• Image distance (v): Positive if image on right (real), negative if on left (virtual)
• Magnification (m): Positive for virtual/erect, negative for real/inverted

Image positions by Convex Lens:

Power of lens is the ability of a lens to converge or diverge light. It is the reciprocal of focal length in meters.

Power of Lens: P = 1/f (in metres)
SI unit: Dioptre (D)
1 Dioptre = 1 m⁻¹

Important Points:

• Power is positive for convex lens (converging)
• Power is negative for concave lens (diverging)
• Higher the power, shorter the focal length (stronger lens)
• Example: A lens with f = 20 cm = 0.2 m has P = 1/0.2 = 5 D

Lens Combinations:

When two or more lenses are placed in contact, their powers add up:

Combined Power: P = P₁ + P₂ + P₃ + ...
Combined Focal Length: 1/f = 1/f₁ + 1/f₂ + 1/f₃ + ...

Examples:

• Two convex lenses (+3D and +2D): Combined P = +5D (stronger converging)
• One convex (+3D) and one concave (-2D): Combined P = +1D (net converging)
• Two equal convex lenses: Combined magnifying power is much greater

Optical instruments like microscopes, telescopes, periscopes, and kaleidoscopes use combinations of mirrors and lenses to magnify or redirect light.

Key Characteristics:

• Microscope: Uses objective of very short focal length and long focal length eyepiece. Magnification = magnification of objective × magnification of eyepiece
• Telescope: Uses objective of long focal length and short focal length eyepiece. Magnifying power M = fo/fe
• Periscope: Uses two plane mirrors at 45° angles to see over obstacles. Image at same height as object
• Kaleidoscope: Uses three plane mirrors forming an equilateral triangle. Creates symmetrical patterns by multiple reflections