📘Class 10 Science – Chapter: Light – Reflection and Refraction Notes
🔦 Introduction to Light
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Light is a form of energy that enables us to see objects.
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It travels in a straight line and at a speed of 3 × 10⁸ m/s in vacuum.
🔁 Reflection of Light
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Reflection is the bouncing back of light from a smooth surface.
Laws of Reflection:
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Angle of incidence = Angle of reflection.
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Incident ray, reflected ray, and normal lie on the same plane.
Types of Reflection:
| Type | Description | Example |
|---|---|---|
| Regular Reflection | From smooth surface | Plane mirror |
| Diffused Reflection | From rough surface | Wall, paper |
Image Formed by Plane Mirror:
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Virtual and erect
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Same size as object
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Laterally inverted
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Same distance behind the mirror
Example: Image of a candle in a plane mirror appears same-sized and upright.
🔄 Spherical Mirrors
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Part of a hollow sphere with a reflecting surface.
Types:
| Mirror Type | Reflecting Surface |
|---|---|
| Concave | Inward curved surface |
| Convex | Outward curved surface |
Important Terms:
| Term | Description |
|---|---|
| Pole (P) | Centre of mirror’s surface |
| Center of Curvature (C) | Centre of the sphere |
| Focus (F) | Point where rays converge/diverge after reflection |
| Radius of Curvature (R) | Distance between P and C |
| Principal Axis | Line joining P and C |
Rules for Ray Diagram (Concave Mirror):
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Ray parallel to axis → passes through F.
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Ray through F → reflects parallel to axis.
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Ray through C → reflects back along the same path.
Image Formation by Concave Mirror:
| Position of Object | Nature of Image | Size | Example |
|---|---|---|---|
| At infinity | Real, inverted, at focus | Highly diminished | Solar cooker |
| Beyond C | Real, inverted, between F & C | Diminished | Torch, headlights |
| Between F and P | Virtual, erect, magnified | Enlarged | Makeup mirror |
🔄 Refraction of Light
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Refraction is the bending of light as it passes from one medium to another.
Laws of Refraction:
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Incident ray, refracted ray, and normal lie in the same plane.
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Ratio of sine of angle of incidence to sine of angle of refraction is constant. (Snell’s Law):
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sin i / sin r = constant = refractive index (n)
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Refractive Index (n):
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n = speed of light in vacuum / speed of light in medium
Example: Light bends towards the normal when it enters glass from air.
🔍 Lenses
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Transparent material bounded by two curved surfaces.
Types:
| Lens Type | Description | Example |
|---|---|---|
| Convex | Thicker at the center | Magnifying glass |
| Concave | Thinner at the center | Spectacles (myopia) |
Image Formation by Convex Lens:
| Object Position | Image Position | Nature of Image |
|---|---|---|
| Beyond 2F | Between F and 2F | Real, inverted, diminished |
| At 2F | At 2F | Real, inverted, same size |
| Between F and O | Behind the lens | Virtual, erect, magnified |
🔣 Lens Formula and Mirror Formula
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Mirror Formula: 1/f = 1/v + 1/u
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Lens Formula: 1/f = 1/v – 1/u
Where:
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f = focal length
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v = image distance
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u = object distance
Sign Conventions (New Cartesian System):
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All distances measured from the pole or optical center.
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Left side: negative; right side: positive
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Above principal axis: positive; below: negative
Magnification (m):
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m = height of image / height of object = v/u (for mirrors)
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m = v/u (for lenses)
Example:
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A concave mirror forms an image 10 cm in front of it of an object placed 30 cm away. f = ?
1/f = 1/v + 1/u = 1/(-10) + 1/(-30) = -1/10 -1/30 = -4/30 → f = -7.5 cm