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Concept Labby Aryaman

Light — Reflection and Refraction

Mirrors, lenses, ray diagrams, mirror & lens formula, refractive index and power.

Chapter Notes

Based on NCERT Class 10 Science, Chapter 9

9.1 Reflection of light

  • A highly polished surface such as a mirror reflects most of the light falling on it.
  • Laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
  • A plane mirror always forms a virtual, erect image of the same size, as far behind the mirror as the object is in front, laterally inverted.
∠i = ∠r
Board-style practice questions

Q1.State the laws of reflection of light.

2 marks

9.2 Spherical mirrors

  • A concave mirror curves inwards (reflecting surface on the inside); a convex mirror bulges outwards.
  • Pole (P), centre of curvature (C), radius of curvature (R), principal axis and principal focus (F) describe the mirror.
  • For a spherical mirror the focus lies midway between the pole and the centre of curvature.
  • Concave mirrors are used in torches, headlights, shaving mirrors, dentists' mirrors and solar cookers. Convex mirrors are used as rear-view mirrors because they give an erect, diminished image of a wide field.
f = R / 2

Watch it happen — Concave Mirror Image Table

Controls

Live result

Image position
Between F and C
Image nature
Diminished, real, inverted
Typical use
Shaving/torch design
Ray diagram — spherical mirrorImage position: Between F and C
imagerays converge at the focus

Concave mirrors converge light; convex mirrors always give a diminished virtual image.

9.2.4 Mirror formula and magnification

  • Distances are measured from the pole; distances measured against the incident light are negative (New Cartesian sign convention).
  • As the object moves from infinity to the pole of a concave mirror, the image moves from F outwards and changes from real-inverted-tiny to virtual-erect-enlarged (object between P and F).
1/v + 1/u = 1/f m = h′/h = −v/u

Watch it happen — Mirror Formula Simulator

Controls

15 cm
30 cm

Live result

Image distance v
-30cm
Magnification m
-1
Nature
Real & inverted
Size
Same size
Ray diagram — spherical mirrorImage distance v: -30 cm
imagerays converge at the focus
Click to start animation

Sign convention: distances measured against the incident light are negative.

Board-style practice questions

Q1.An object 5 cm tall is placed 20 cm from a concave mirror of focal length 15 cm. Find the position, size and nature of the image.

3 marks

9.3 Refraction of light

  • Light bends when it passes from one transparent medium into another because its speed changes.
  • Going from a rarer to a denser medium the ray bends towards the normal; going from denser to rarer it bends away from the normal.
  • Through a rectangular glass slab the emergent ray is parallel to the incident ray but shifted sideways (lateral displacement).
Snell's law: sin i / sin r = n₂₁ (constant)

Watch it happen — Snell's Law Bench

Controls

1
1.5
30 °

Live result

Angle of refraction
19.47°
Critical angle
—°
Speed in medium 2
2×10⁸ m/s
Ray diagram — lensAngle of refraction: 19.47 °
imagerays converge at the focus
Click to start animation

Light bends towards the normal when entering a denser medium (n₂ > n₁).

9.3.2 The refractive index

  • The refractive index compares the speed of light in two media. Light travels at c = 3 × 10⁸ m/s in vacuum.
  • Absolute refractive index of water is 1.33, of crown glass 1.52, of diamond 2.42 — a higher value means an optically denser medium in which light travels slower.
n = speed of light in vacuum / speed in medium = c / v

Watch it happen — Snell's Law Bench

Controls

1
1.5
30 °

Live result

Angle of refraction
19.47°
Critical angle
—°
Speed in medium 2
2×10⁸ m/s
Ray diagram — lensAngle of refraction: 19.47 °
imagerays converge at the focus
Click to start animation

Light bends towards the normal when entering a denser medium (n₂ > n₁).

Board-style practice questions

Q1.The refractive index of glass is 1.5 and speed of light in vacuum is 3 × 10⁸ m/s. Find the speed of light in glass.

2 marks

9.3.3–9.3.5 Spherical lenses and ray diagrams

  • A convex (converging) lens is thicker in the middle; a concave (diverging) lens is thinner in the middle.
  • Standard rays: a ray parallel to the principal axis passes through F after refraction; a ray through the optical centre goes straight; a ray through F emerges parallel to the axis.
  • A convex lens gives real inverted images except when the object is inside the focus, when it acts as a magnifying glass. A concave lens always gives a virtual, erect, diminished image.

Watch it happen — Lens Formula & Power

Controls

20 cm
30 cm

Live result

Image distance v
60cm
Magnification
-2
Power P = 1/f(m)
5D
Image
Real & inverted
Ray diagram — lensImage distance v: 60 cm
imagerays converge at the focus
Click to start animation

Power is positive for convex lenses and negative for concave lenses.

9.3.7–9.3.8 Lens formula, magnification and power

  • The lens formula relates object distance, image distance and focal length for thin lenses.
  • The power of a lens tells how strongly it converges or diverges light; its SI unit is the dioptre (D), with f in metres. Convex lens power is positive, concave lens power negative.
  • For lenses placed in contact the powers simply add up.
1/v − 1/u = 1/f m = h′/h = v/u P = 1/f (m) P = P₁ + P₂ + …

Watch it happen — Combining Lens Powers

Controls

4 D
-2 D

Live result

Net power
2D
Net focal length
50cm
Behaves like
Convex lens
Ray diagram — lensNet power: 2 D
imagerays converge at the focus
Click to start animation

P = P₁ + P₂ for lenses in contact; unit of power is the dioptre (D).

Board-style practice questions

Q1.A convex lens of focal length 25 cm is used as a magnifying glass. Find its power. What is the power of a concave lens of focal length 50 cm?

3 marks