Light | Class 9 Physics Formula

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Relation between the radius of curvature (r) and the focal length (f) of a spherical mirror for paraxial rays:

r = 2f or, f = r/2

Images formed by a concave mirror :

Position of the object Position of the image  Size of the image Nature of the image
At infinity At the focus Highly diminished Real and Inverted
Beyond C Between F and C Diminished Real and Inverted
At C At C Same size Real and Inverted
Between C and F Beyond C Magnified Real and Inverted
At F At infinity Highly Magnified Real and Inverted
Between P and F Behind the mirror Magnified Virtual and erect

Images formed by a convex mirror :

Position of the object Position of the image Size of the image Name of the image
At infinity At the focus, F Highly diminished Virtual and erect
Anywhere between infinity and pole Between F and pole Diminished Virtual and erect

Snell’s law:

When a light ray enters into the second medium from the first medium, then the refractive index (R.I) of the second medium with respect to the first medium is given by

1μ2 = sin\ i\over sin\ r [\katex]

Wave theory of light :

Absolute R.I of a medium,

μ = Velocity\ of\ light\ in\ vacuum\ (or\ air)\over Velocity\ of\ light\ in\ the\ medium[\katex]

Principle of reversibility of light :

When a ray after refraction, retraces its path, then 1µ2 =  1/ 2µ1

(where a is the rarer and b is the denser medium)

Real and apparent depth: 

When an object is situated in an optically denser medium 2, then when viewed normally from the rarer medium 1

  1. 1µ2 = Real\ depth\over Apparen\ depth[\katex]
  2. Shift = Real depth (1 - 1/1µ2)

Refraction through optical slabs :

  1. The incident ray is parallel to the emergent ray
  2. The angle of incidence is equal to the angle of emergence

Refraction through prism :

  1. δ = δ1 + δ2
  2. δ = i1 + i2 - (r1 + r2)
  3. r1 + r2 = A
  4. δ = i1 + i2 - A

Notation meaning

  • δ = Angle of deviation
  • i1 = Angle of incidence
  • i2 = Angle of emergence
  • r1 = Angle of refraction on the first surface
  • r2 = Angle of refraction on the first surface
  • A = Angle of prism

Images formed by the convex lens :

Position of the object Position of the image  Size of the image Nature of the image
At infinity At the focus Highly diminished Real and Inverted
Beyond 2F1 Between F2 and 2F2 Diminished Real and Inverted
At 2F1 At 2F2 Same size Real and Inverted
Between 2F1 and F1 Beyond 2F2 Magnified Real and Inverted
At F1 At infinity Highly Magnified Real and Inverted
Between P and F1 On the same side of the lens as the object Magnified Virtual and erect

Images formed by a concave lens :

Position of the object Position of the image Size of the image Name of the image
At infinity At the focus, F2 Highly diminished Virtual and erect
Anywhere in front of lens Between F2 and O Diminished Virtual and erect

Linear magnification :

  1. Linear magnification (m) = Linear\ size\ of\ the \ image\ (I)\over Linear\ size\ of\ the \ object\ (O)[\katex]
  2. Linear magnification (m) = Distance\ of\ the\ image\ from\ the\ lens\ (v)\over Distance\ of\ the\ object\ from\ the\ lens\ (v) [\katex]

Power of lens:

Power = 1\over focal\ length\ (in\ metre) [\katex]

Relation among speed (c), wavelength (λ) and frequency (v) of light wave:

c = v.λ

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