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Class 9–10 · Physics
Refraction of light
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Refraction of light
Lessons in this chapter
Lessons
1Refraction and optical density
Light changes speed at a boundary. At oblique incidence it bends towards the normal when entering higher refractive index and away when entering lower index. Normal incidence changes speed without changing direction. Optical and mass density are different ideas.
Key relationship
n = c/v
Worked example
c = 3×10⁸, water v ≈ 2.25×10⁸ m/s ⇒ n ≈ 1.33
Next lesson →2Snell's law
Snell's law relates the sines of angles measured from the normal. Frequency is unchanged across a stationary boundary; speed and wavelength change together. The two rays and normal lie in one plane.
Key relationship
n₁ sin i = n₂ sin r; v = fλ
Worked example
n₁ = 1, n₂ = 1.5, i = 30° ⇒ sin r = 1/3 ⇒ r ≈ 19.5°
Next lesson →3Apparent depth, slabs and prisms
Refraction makes submerged objects appear shallower. The simple depth ratio applies approximately near normal viewing. A parallel-sided slab produces lateral displacement with parallel emergence; a prism changes the overall direction.
Key relationship
viewed from air near normal: n ≈ real depth/apparent depth
Worked example
Real depth 1.2 m, n = 4/3 ⇒ apparent depth 0.9 m
Next lesson →4Critical angle and total internal reflection
From higher to lower refractive index, the critical angle produces a refracted ray at 90°. Above it, total internal reflection occurs. Both the index direction and angle condition are necessary. Optical fibres use a higher-index core and lower-index cladding.
Key relationship
sin critical angle = n₂/n₁, n₁ > n₂
Worked example
Glass n = 1.5 to air ⇒ critical angle ≈ 41.8°; i = 50° gives total reflection
Next lesson →5Lenses and principal rays
In air, ordinary glass convex lenses converge and concave lenses diverge. Parallel rays pass through the far focus of a convex lens or appear from the near focus of a concave lens. A ray through a thin lens's optical centre is approximately undeviated.
Key relationship
convex → convergence; concave → divergence
Worked example
A distant object's parallel rays focus near a convex lens's focal plane.
Next lesson →6Lens images
For a convex lens, beyond 2F gives a small inverted real image; at 2F, equal size; between F and 2F, enlarged and inverted. Inside F it is upright and virtual. A concave lens gives a real object a smaller upright virtual image.
Key relationship
real image: rays meet; virtual image: backward extensions meet
Worked example
Magnifier: object inside focal distance of convex lens
Next lesson →7Lens equation and power
With Cartesian signs, a real object has negative u; convex f is positive and concave f negative. Power is reciprocal focal length in metres. Powers approximately add for thin lenses in contact.
Key relationship
1/f = 1/v−1/u; m = v/u; P = 1/f(m), unit D
Worked example
f = +10 cm, u = −30 cm ⇒ v = +15 cm, m = −0.5; P = +10 D
Next lesson →8The eye and accommodation
The cornea and lens form an inverted real retinal image. The iris controls light through the pupil. Ciliary muscles change lens curvature for near and distant focus. A typical textbook near point is about 25 cm for a young normal eye, with individual variation.
Key relationship
near focus usually requires greater lens power
Worked example
Looking from a distant object to a book changes lens shape rather than moving the retina.
Next lesson →9Vision defects
Myopia focuses distant objects before the retina and is corrected with a diverging lens. Hyperopia can require a converging lens for near focus. Presbyopia is declining accommodation with age. A professional eye examination determines correction.
Key relationship
myopia: negative correction; hyperopia: positive correction
Worked example
Correcting lens f = −0.5 m ⇒ P = −2 D
Next lesson →10Spectrum and dispersion
White light contains many frequencies. A prism separates them because refractive index depends on wavelength; ordinary glass bends violet more than red. Rainbows combine refraction, internal reflection and dispersion in droplets.
Key relationship
ordinary glass: nviolet > nred
Worked example
White beam → prism → continuous coloured spectrum
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Air to glass at oblique incidence: bending direction?
Towards the normal: n increases
2. Does light frequency change across the boundary?
No; speed and wavelength change
3. Lens focal length +0.25 m: power?
P = 1/0.25 = +4 D
4. Usual correcting lens for myopia?
Diverging/concave; negative power
5. Two conditions for total internal reflection?
Higher n to lower n; incidence angle above critical angle
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.