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Class 9–10 · Physics
Reflection of light
Change it and see
Reflection of light
Lessons in this chapter
Lessons
1Light and rays
Light is electromagnetic radiation. Rays show its direction in a uniform transparent medium. Beams may be parallel, converging or diverging. A pinhole camera forms an inverted image because rays from opposite parts cross through the opening.
Key relationship
c ≈ 3×10⁸ m/s in vacuum
Worked example
Upper object point → pinhole → lower screen point
Next lesson →2Laws of reflection
Measure incidence and reflection angles from the normal, perpendicular to the surface at the point of incidence. The angles are equal and both rays and the normal lie in one plane.
Key relationship
i = r
Worked example
Ray makes 30° with surface ⇒ i = 60° ⇒ r = 60°
Next lesson →3Specular and diffuse reflection
Smooth surfaces preserve an orderly reflected beam. Rough surfaces have differently oriented local normals, scattering light in many directions. Each local reflection still obeys i = r. Diffuse reflection makes ordinary pages visible from different directions.
Key relationship
local i = local r
Worked example
Mirror: recognisable image; white paper: scattered light without a clear image
Next lesson →4Plane-mirror images
A plane mirror produces an upright, same-size virtual image as far behind the mirror as the object is in front. Rays only appear to originate there. The reversal is across the mirror plane and is commonly described as lateral inversion.
Key relationship
object distance = image distance; magnification magnitude = 1
Worked example
Object 2 m in front ⇒ image 2 m behind ⇒ separation 4 m
Next lesson →5Spherical mirrors
Concave mirrors reflect from an inner spherical surface and convex mirrors from the outer surface. Identify pole, curvature centre, radius and principal focus. For paraxial rays and a small aperture, focal length is approximately half the curvature radius.
Key relationship
f = R/2
Worked example
R = 40 cm ⇒ focal-length magnitude 20 cm
Next lesson →6Concave-mirror ray diagrams
Use two principal rays. Beyond C gives a smaller inverted real image; at C, equal size; between C and F, enlarged and inverted. At F the ideal image is at infinity; inside F it is enlarged, upright and virtual.
Key relationship
parallel → F; through F → parallel; through C → retraces
Worked example
Object between P and F ⇒ enlarged upright virtual image behind mirror
Next lesson →7Convex mirrors
Diverging reflected rays appear to originate between pole and focus. A real object's image is always smaller, upright and virtual. The wider field of view is useful in vehicle and safety mirrors.
Key relationship
image lies between P and F behind mirror
Worked example
Smaller traffic images allow a wider view, but can complicate distance judgement.
Next lesson →8Mirror equation and magnification
Use the Cartesian convention: incident light travels left to right and right is positive. A real object has negative u; a concave mirror has negative f. Interpret v for location and m for orientation and size.
Key relationship
1/f = 1/u+1/v; m = −v/u
Worked example
f = −10 cm, u = −30 cm ⇒ v = −15 cm; m = −0.5
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Ray is 20° to surface: reflection angle?
i = 90−20 = 70° ⇒ r = 70°
2. Plane mirror, object 1.5 m in front: object-image separation?
1.5 + 1.5 = 3 m
3. Curvature radius 30 cm: focal-length magnitude?
f = R/2 = 15 cm
4. Convex mirror image of a real object?
Upright, smaller, virtual
5. Concave mirror with object between P and F?
Upright, enlarged, virtual
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.