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light

🔦 What is Light?

Light is a form of energy that causes the sensation of vision.
It travels in a straight line (called rectilinear propagation of light) in a homogenous medium.
Light does not require a material medium to travel—it can even travel through vacuum.
The speed of light in vacuum:
c = 3 × 10⁸ m/s

🌟 Reflection of Light

🔁 Definition:

The phenomenon in which a ray of light bounces back into the same medium after striking a smooth surface (like a mirror) is called reflection.

🔗 Laws of Reflection

There are two laws of reflection, and they apply to all types of reflecting surfaces:

Angle of incidence (∠i) = Angle of reflection (∠r)
→ These angles are measured with respect to the normal.
The incident ray, the reflected ray, and the normal to the surface all lie in the same plane.

🔄 Types of Reflection

Type	Description	Surface	Image Formed
Regular Reflection	Parallel incident rays reflect as parallel rays	Smooth and polished (like a plane mirror)	Clear image
Diffuse Reflection	Parallel rays reflect in different directions	Rough surface (like wall, paper)	No clear image

💡 Note: Even in diffuse reflection, laws of reflection are obeyed—irregularities in the surface cause different normals.

Spherical Mirrors

🧩 Definition:

A mirror whose reflecting surface is a part of a sphere is called a spherical mirror.

🌀 Types of Spherical Mirrors:

Concave Mirror:
- Inner surface is reflecting.
- Also called a converging mirror.
- Parallel rays converge (meet) after reflection.
Convex Mirror:
- Outer surface is reflecting.
- Also called a diverging mirror.
- Parallel rays diverge after reflection and appear to come from a point.

🔎 Key Terms in Spherical Mirrors

Term	Description
Pole (P)	The central point of the mirror's surface.
Center of Curvature (C)	Center of the sphere from which the mirror is cut.
Radius of Curvature (R)	Distance between P and C (i.e., PC).
Principal Axis	An imaginary straight line passing through C and P.
Focus (F)	Point on the principal axis where light rays parallel to the axis converge (concave) or appear to diverge from (convex).
Focal Length (f)	Distance between P and F.
➤ f = R/2
Aperture	Width of the reflecting surface of the mirror.

🎯 Ray Diagrams – Rules

To locate the image formed by spherical mirrors, use at least two rays among these:

📌 Standard Rays for Ray Diagrams

Incident Ray	Behavior After Reflection
Parallel to principal axis	Passes through Focus (F) (concave) / Appears to diverge from F (convex)
Through Focus (F)	Reflects parallel to principal axis
Through Center of Curvature (C)	Returns along the same path
Striking the Pole (P)	Reflects at the same angle as incidence (obeys ∠i = ∠r)

🖼 Image Formation by Spherical Mirrors

✳ Concave Mirror

Forms real/inverted or virtual/erect images depending on object distance.

Object Position	Image Position	Size	Nature
At ∞	At F	Point-sized	Real, Inverted
Beyond C	Between F and C	Smaller	Real, Inverted
At C	At C	Same size	Real, Inverted
Between C and F	Beyond C	Enlarged	Real, Inverted
At F	At ∞	Highly enlarged	Real, Inverted
Between F and P	Behind mirror	Enlarged	Virtual, Erect