Class 10 Ch 9 Light – Reflection & Refraction

• Light travels in straight lines.
• Laws of Reflection:
  (i) Angle of incidence $\angle i$ = Angle of reflection $\angle r$.
  (ii) The incident ray, the normal, and the reflected ray all lie in the same plane.

• Concave (अवतल): Reflecting surface curved inwards. (Converging).
• Convex (उत्तल): Reflecting surface curved outwards. (Diverging).
• Important Terms: Pole (P), Centre of Curvature (C), Radius (R), Principal Focus (F).
• $R = 2f$ (Radius is twice focal length).

अवतल दर्पण द्वारा प्रतिबिंब का बनना:

• Concave: Torches, Shaving mirrors, Dentist mirrors, Solar furnaces.
• Convex: Rear-view mirrors in vehicles (gives erect, diminished image & wider field of view).

• All distances measured from Pole (P).
• Distances in direction of light $\to$ (+).
• Distances against direction of light $\to$ (-).
• Object distance ($u$) is Always Negative.

• $v$ = Image distance | $u$ = Object distance | $f$ = Focal length.
• Magnification (m): Ratio of height of image ($h’$) to height of object ($h$).
• $m = \frac{h’}{h} = -\frac{v}{u}$
• If $m$ is (-), image is Real. If $m$ is (+), image is Virtual.

• Bending of light when it passes from one transparent medium to another.
• Laws of Refraction:
  (i) Incident, Refracted, Normal all lie in same plane.
  (ii) Snell’s Law: The ratio of sine of $i$ to sine of $r$ is constant.
  $\frac{\sin i}{\sin r} = \text{constant}$ ($n$).

• $n = \frac{\text{Speed of light in vacuum (c)}}{\text{Speed of light in medium (v)}}$
• Absolute Refractive Index: Vacuum to Medium.
  – Water: 1.33 | Glass: 1.50 | Diamond: 2.42 (Highest).

• Convex Lens: Converging lens. Thicker at middle.
• Concave Lens: Diverging lens. Thinner at middle.

• Power (P): Reciprocal of focal length in metres.
• $P = \frac{1}{f(m)}$
• Unit: Dioptre (D).
• Convex Lens Power is (+). Concave Lens Power is (-).