In a concave mirror: 1️⃣ A parallel ray reflects through the focus. 2️⃣ A ray through the focus reflects parallel. 3️⃣ A ray through the center reflects back. 4️⃣ An oblique ray reflects at an equal angle.
1️⃣ A Ray Parallel to the Principal Axis
- A ray of light that travels parallel to the principal axis of a concave mirror is reflected in such a way that it passes through the principal focus (F) of the mirror.
- This is because of the parabolic shape of the mirror, which ensures that parallel rays converge at the focus after reflection.Example: Headlights of a car focus light in parallel beams due to the mirror’s properties.
2️⃣ A Ray Passing Through the Principal Focus (F)
- A ray of light that passes through the principal focus (F) of a concave mirror before striking its surface will be reflected parallel to the principal axis.
- This is the reverse of Rule 1, and it follows the principle of reflection due to the mirror’s geometry.Example: A torch uses this principle to direct light.
3️⃣ A Ray Passing Through the Center of Curvature (C)
- A ray of light that passes through the center of curvature (C) of a concave mirror strikes the mirror at 90 degrees (perpendicular) to the surface.
- As it is normally incident, it is reflected back along the same path.Why?
- The center of curvature is the center of the sphere of which the mirror is a part, making the ray strike the surface perpendicularly.
4️⃣ A Ray Incident Obliquely to the Principal Axis
- A ray of light that strikes the concave mirror obliquely, at an angle to the principal axis, is reflected obliquely.
- The angle of incidence equals the angle of reflection (law of reflection).Why?
- The law of reflection governs all types of mirrors: the angle at which the light hits the mirror (angle of incidence) equals the angle at which it bounces off (angle of reflection).
Practical Applications:
- These rules help in constructing ray diagrams to find the position, size, and nature (real or virtual, inverted or upright) of the image formed by a concave mirror.
Additional Notes:
- Principal Focus (F): The point where rays parallel to the principal axis converge after reflection.
- Center of Curvature (C): The center of the sphere of which the mirror is a part.
- Principal Axis: The line passing through the center of curvature and the pole of the mirror.
📝 Tip for Drawing Ray Diagrams:
- Draw neat, labeled lines with a ruler.
- Use at least two rays to find the exact position of the image.
- Remember to follow the reflection laws while extending or reflecting the rays.