Class 10 Science Chapter 9: Light – Reflection and Refraction:
📘 Chapter 9: Light – Reflection and Refraction (Class 10 Science)
🔆 What is Light?
- Light is a form of energy that enables us to see objects.
- It travels in a straight line in a homogeneous medium (Rectilinear Propagation).
- Speed of light in vacuum: 3 × 10⁸ m/s.
🔁 Reflection of Light
- Reflection: Bouncing back of light rays from a smooth surface.
🔹 Important Terms:
- Incident Ray: The incoming ray on the surface.
- Reflected Ray: The ray that bounces back.
- Normal: A perpendicular line drawn at the point of incidence.
- Angle of Incidence (i): Between incident ray and normal.
- Angle of Reflection (r): Between reflected ray and normal.
🔹 Laws of Reflection:
- ∠i = ∠r.
- Incident ray, reflected ray, and normal all lie in the same plane.
🪞 Plane Mirror
- Image is virtual, erect, laterally inverted, and same size.
- Distance of object = Distance of image from mirror.
🔮 Spherical Mirrors
Types:
- Concave Mirror: Reflecting surface is inward (converging).
- Convex Mirror: Reflecting surface is outward (diverging).
Mirror Terminology:
- Pole (P): Center of mirror surface.
- Centre of Curvature (C): Center of sphere.
- Radius of Curvature (R): Distance between P and C.
- Principal Axis: Line joining P and C.
- Focus (F): Point where rays converge/diverge.
- Focal Length (f): Distance between P and F. → f = R/2
🧪 Image Formation by Concave Mirror
| Position of Object | Position of Image | Nature |
|---|---|---|
| At infinity | At focus (F) | Real, inverted, point image |
| Beyond C | Between F and C | Real, inverted, diminished |
| At C | At C | Real, inverted, same size |
| Between C and F | Beyond C | Real, inverted, enlarged |
| At F | At infinity | Real, inverted, highly enlarged |
| Between F and P | Behind mirror | Virtual, erect, enlarged |
🪞 Image Formation by Convex Mirror
| Position of Object | Position of Image | Nature |
|---|---|---|
| At infinity | At focus (F) | Virtual, erect, diminished |
| Anywhere else | Between F and P | Virtual, erect, diminished |
➡️ Used in vehicles as rear-view mirrors for wider field of view.
🧮 Mirror Formula:
1f=1v+1u\frac{1}{f} = \frac{1}{v} + \frac{1}{u}
- f = focal length, v = image distance, u = object distance
- All distances are measured from pole (P).
- Sign Convention:
- Distances towards left of mirror = negative
- Distances towards right = positive
🔍 Magnification (Mirror):
m=h′h=−vum = \frac{h’}{h} = -\frac{v}{u}
- h’ = height of image
- h = height of object
- m > 1 → enlarged, m < 1 → diminished, m = 1 → same size
🌈 Refraction of Light
- Refraction: Bending of light when it travels from one medium to another.
- Caused by change in speed of light.
Laws of Refraction (Snell’s Law):
- Incident ray, refracted ray, and normal lie in the same plane.
- sinisinr=constant=n\frac{\sin i}{\sin r} = \text{constant} = n
🔢 Refractive Index:
n=cvn = \frac{c}{v}
- c = speed of light in vacuum
- v = speed in medium
- Denser medium → lower speed → bends towards normal
- Rarer medium → higher speed → bends away from normal
🔲 Refraction through Glass Slab
- Incident ray and emergent ray are parallel but displaced laterally.
🔍 Lenses
- A lens is a transparent material with two refracting surfaces.
Types:
- Convex Lens: Converging (thicker in middle)
- Concave Lens: Diverging (thinner in middle)
🧪 Image Formation by Convex Lens
| Object Position | Image Position | Nature |
|---|---|---|
| At infinity | At focus F2 | Real, inverted, point image |
| Beyond 2F1 | Between F2 and 2F2 | Real, inverted, diminished |
| At 2F1 | At 2F2 | Real, inverted, same size |
| Between F1 and 2F1 | Beyond 2F2 | Real, inverted, enlarged |
| At F1 | At infinity | Real, inverted, highly enlarged |
| Between F1 and O | Same side as object | Virtual, erect, enlarged |
🧪 Image Formation by Concave Lens
- Always forms virtual, erect, and diminished image between F1 and O.
🧮 Lens Formula:
1f=1v−1u\frac{1}{f} = \frac{1}{v} – \frac{1}{u}
- Same sign conventions as mirrors.
🔍 Magnification (Lens):
m=h′h=vum = \frac{h’}{h} = \frac{v}{u}
- m > 1 → Enlarged
- m < 1 → Diminished
⚡ Power of a Lens
P=100f(in cm)P = \frac{100}{f (\text{in cm})}
- Unit: Dioptre (D)
- Convex lens → positive power
- Concave lens → negative power
🔧 Applications of Mirrors and Lenses:
- Concave mirrors: Solar cookers, torches, shaving mirrors, headlights.
- Convex mirrors: Rear-view mirrors.
- Convex lenses: Magnifying glass, projector, camera.
- Concave lenses: Spectacles for myopia (short-sightedness).