Light Reflection – Detailed Notes

        This version removes irrelevant images and gives a fuller explanation of reflection, mirrors, image formation, and ray diagrams.
    

1) Reflection of light

Reflection of light is the bouncing back of light rays into the same medium after they strike a smooth or polished surface. Mirrors are the best examples of reflecting surfaces.

The reflected ray, incident ray, and the normal at the point of incidence always lie in the same plane.

2) Laws of reflection

>The incident ray, reflected ray, and the normal lie in the same plane. >The angle of incidence is equal to the angle of reflection, so \( \angle i = \angle r \).

de>Angle of incidence = Angle of reflection de>∠i = ∠r

3) Mirror terminology

Term	Meaning
Incident ray	The ray that falls on the mirror
Reflected ray	The ray that bounces back from the mirror
Normal	Perpendicular line drawn at point of incidence
Point of incidence	Point where the ray strikes the mirror
Pole (P)	Geometrical center of the mirror
Principal axis	Line passing through pole and centre of curvature
Centre of curvature (C)	Center of the sphere of which the mirror is a part
Radius of curvature (R)	Distance between pole and centre of curvature
Principal focus (F)	Point where rays parallel to the axis meet or appear to meet
Focal length (f)	Distance between pole and focus
Aperture	Effective diameter of the mirror

4) Types of spherical mirrors

Spherical mirrors are mirrors whose reflecting surfaces form a part of a sphere. There are two types: concave mirror and convex mirror.

Concave mirror

A concave mirror has its reflecting surface curved inward. It can form both real and virtual images.

Convex mirror

A convex mirror has its reflecting surface curved outward. It always forms a virtual, erect, and diminished image.

Concave mirror ray diagram and image formation.

Convex mirror ray diagram and image formation.

5) Image formation by concave mirror

The image formed by a concave mirror depends on the position of the object. The image may be real or virtual, inverted or erect, magnified or diminished.

Object position	Image position	Nature of image	Size of image
At infinity	At focus F	Real, inverted	Highly diminished / point-sized
Beyond C	Between C and F	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 the mirror	Virtual, erect	Enlarged

Concave mirror: object at infinity forms image at focus.

Rules for ray diagrams in concave mirror

>A ray parallel to the principal axis passes through the focus after reflection. >A ray passing through the focus reflects parallel to the principal axis. >A ray passing through the centre of curvature returns along the same path. >A ray striking the pole follows the law of reflection.

For a concave mirror, image position changes with object position. This topic is very important for numerical and diagram-based questions.

6) Image formation by convex mirror

A convex mirror always forms the image behind the mirror. The image is always virtual, erect, and smaller than the object.

Object position	Image position	Nature of image	Size of image
At infinity	At focus F behind mirror	Virtual, erect	Highly diminished / very small
At finite distance	Between P and F behind mirror	Virtual, erect	Diminished / small

Convex mirror: object at infinity forms a very small image at focus behind the mirror.

Rules for ray diagrams in convex mirror

>A ray parallel to the principal axis appears to come from the focus after reflection. >A ray directed towards the focus reflects parallel to the principal axis. >A ray directed towards the centre of curvature reflects back on itself. >Reflected rays diverge, so their backward extensions form a virtual image behind the mirror.

A convex mirror never forms a real image. It always gives a diminished image, which is why it is used in vehicle rear-view mirrors.

7) Mirror formula and magnification

The mirror formula relates object distance, image distance, and focal length.

de>1/f = 1/v + 1/u de>m = h' / h = -v / u de>f = R / 2

Magnification tells us the size of the image relative to the object. A negative magnification means the image is inverted, and a positive magnification means the image is erect.

8) Sign convention

Quantity	Sign rule
Distances measured in direction of incident light	Positive
Distances measured opposite to incident light	Negative
Heights above principal axis	Positive
Heights below principal axis	Negative

9) Concave and convex mirror comparison

Concave mirror	Convex mirror
Reflecting surface curves inward	Reflecting surface curves outward
Can form real and virtual images	Always forms virtual image
Image may be enlarged or diminished	Image is always diminished
Used in shaving mirrors, headlights, solar furnaces	Used in rear-view mirrors and security mirrors

10) Quick revision

Reflection is the bouncing back of light into the same medium.

\( \angle i = \angle r \) is the law of reflection.

Concave mirror can give real or virtual images depending on object position.

Convex mirror always gives a virtual, erect, diminished image.

For an object at infinity, the concave mirror forms a very small image at F.

For an object at infinity, the convex mirror forms a very small image behind the mirror.

The mirror formula is \( 1/f = 1/v + 1/u \).

Focal length is half the radius of curvature, \( f = R/2 \).

The most important exam idea here is image formation with respect to object position. Concave mirror is the main one where you must remember all cases separately.