Class 10 Science Chapter 9 Light Reflection Refraction Short Notes

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A quick revision summary for right before the exam: every definition, formula and named fact from the chapter, all in one place. The full notes are available separately for detailed reading and ray diagrams.

1 Reflection and Spherical Mirrors

Laws of reflection: (i) angle of incidence = angle of reflection (ii) incident ray, normal, reflected ray are coplanar
Plane mirror image: virtual, erect, same size, equidistant behind, laterally inverted
Concave mirror: curves inward, C in front; Convex mirror: curves outward, C behind
Pole (P) · Centre of curvature (C) · Radius of curvature (R = PC) · Principal axis · Principal focus (F) · Focal length (f = PF) · Aperture (diameter of mirror)
$R = 2f$ : focal length is half the radius of curvature

Object (concave mirror)	Image position	Size	Nature
At infinity	At F	Point-sized	Real, inverted
Beyond C	Between F & C	Diminished	Real, inverted
At C	At C	Same size	Real, inverted
Between C & F	Beyond C	Enlarged	Real, inverted
At F	At infinity	No image
Between P & F	Behind mirror	Enlarged	Virtual, erect

Convex mirror: at infinity → at F behind, point-sized, virtual erect; between infinity & P → between P & F behind, diminished, virtual erect. Always virtual, erect, diminished.

Four ray rules (mirrors)
(i) ∥ axis → through F (concave) / appears from F (convex) (ii) through/towards F → emerges ∥ axis (iii) through/towards C → reflects straight back (iv) at pole P → reflects obliquely, equal angles

Uses: Concave: torches, headlights, search-lights, shaving/dentist’s mirror, solar furnace. Convex: rear-view/wing mirrors (erect image + wide field of view).

New Cartesian Sign Convention: pole = origin, principal axis = x-axis, object always to the left (u negative); right/above = positive, left/below = negative.

Mirror formula & magnification

\frac{1}{v}+\frac{1}{u}=\frac{1}{f} \qquad m=\frac{h'}{h}=-\frac{v}{u}

Negative

m

= real, inverted. Positive

m

= virtual, erect.

2 Refraction of Light

Refraction: change in direction of light passing obliquely between transparent media, due to a change in speed
Rarer → denser: bends towards normal, slows down. Denser → rarer: bends away from normal, speeds up
Glass slab: bends at both faces, equal and opposite; emergent ray ∥ incident ray, but laterally shifted

Laws of refraction
(i) incident ray, refracted ray, normal are coplanar (ii) Snell’s law:

\dfrac{\sin i}{\sin r}

= constant = refractive index

Refractive index

n_{21}=\dfrac{v_1}{v_2}

· absolute index

n_m=\dfrac{c}{v}

c=3\times10^8\ \mathrm{m/s}

. Higher

n

= optically denser (NOT the same as mass density; kerosene,

n=1.44

, is optically denser than water,

n=1.33

, though kerosene’s mass density is lower).

Medium	n	Medium	n
Air	1.0003	Canada Balsam	1.53
Ice	1.31	Rock salt	1.54
Water	1.33	Carbon disulphide	1.63
Alcohol	1.36	Dense flint glass	1.65
Kerosene	1.44	Ruby	1.71
Fused quartz	1.46	Sapphire	1.77
Turpentine oil	1.47	Diamond	2.42
Benzene	1.50	Crown glass	1.52

3 Spherical Lenses

Convex lens: thicker at middle, converging. Concave lens: thicker at edges, diverging
Optical centre (O) · Centres of curvature ( $C_1$ , $C_2$ ) · Principal foci ( $F_1$ , $F_2$ ) · ray through O passes undeviated

Object (convex lens)	Image position	Size	Nature
At infinity	At $F_2$	Point-sized	Real, inverted
Beyond $2F_1$	Between $F_2$ & $2F_2$	Diminished	Real, inverted
At $2F_1$	At $2F_2$	Same size	Real, inverted
Between $F_1$ & $2F_1$	Beyond $2F_2$	Enlarged	Real, inverted
At $F_1$	At infinity	No image
Between $F_1$ & O	Same side as object	Enlarged	Virtual, erect

Concave lens: at infinity → at $F_1$ , point-sized, virtual erect; between infinity & O → between $F_1$ & O, diminished, virtual erect. Always virtual, erect, diminished.

Three ray rules (lenses)
(i) ∥ axis → through far focus (convex) / appears from near focus (concave) (ii) through/towards focus → emerges ∥ axis (iii) through optical centre → no deviation

Sign convention: measured from optical centre O; convex lens f positive, concave lens f negative.

Lens formula & magnification

\frac{1}{v}-\frac{1}{u}=\frac{1}{f} \qquad m=\frac{h'}{h}=\frac{v}{u}

Note: mirror is plus and has a minus in magnification; lens is minus and has no minus in magnification.

4 Power of a Lens

Power

P=\frac{1}{f}\ (f\text{ in metres})

SI unit: dioptre (D),

1\ \mathrm{D}=1\ \mathrm{m^{-1}}

. Convex lens:

P

positive. Concave lens:

P

negative. Combined power in contact:

P=P_1+P_2+P_3+\dots

(e.g.

+2.0\ \mathrm{D}

and

+0.25\ \mathrm{D}

together =

+2.25\ \mathrm{D}

Quick Revision

Mirror: $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$ , $m=-\frac{v}{u}$ · Lens: $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ , $m=\frac{v}{u}$
$R=2f$ for mirrors · $c=3\times10^8\ \mathrm{m/s}$ · $n_m=c/v$ · $P=1/f$ , unit dioptre
Concave lens and convex mirror both ALWAYS give virtual, erect, diminished images
Rarer→denser bends towards normal; denser→rarer bends away

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