mirrors

Physics Notes: Mirrors

1. Flat (Plane) Mirrors

Definition

A flat (plane) mirror has a flat reflective surface.

Law of Reflection

  • Angle of incidence = Angle of reflection

  • Measured from the normal (an imaginary line perpendicular to the mirror).

Characteristics of Images Formed

1. Virtual Image

  • Cannot be projected onto a screen.

  • Appears to come from behind the mirror.

  • Image is not actually behind the mirror.

2. Front-to-Back (Lateral) Reversal

  • Left and right appear reversed.

  • Not an up-down reversal.

Example:

  • "AMBULANCE" is written backward so it appears normal in rear-view mirrors.

3. Magnification

  • Magnification = 1

  • Image is exactly the same size as the object.

4. Image Distance

  • Image distance = Object distance

Formula:

di = do

where:

  • di = image distance

  • do = object distance

Example:
If you stand 2 m from the mirror, your image appears 2 m behind it.

Summary of Plane Mirror Images

  • Virtual

  • Upright

  • Same size

  • Same distance behind mirror as object is in front

  • Laterally reversed

2. Concave Mirrors (Converging Mirrors)

Definition

Reflect light from the curved inward surface.

Also called:

  • Converging mirrors

Parts

Vertex (A)

Center of the mirror.

Principal Axis

Imaginary line through the center.

Center of Curvature (C)

Center of the sphere the mirror belongs to.

Radius of Curvature (R)

Distance from:
Vertex → Center of Curvature

Focal Point (F)

Halfway between A and C.

Focal Length (f)

Distance from:
Vertex → Focal Point

Formula:

f = R/2

Reflection Rules

Rule 1

Ray parallel to principal axis

↓

Reflects through focal point (F)

Rule 2

Ray through focal point

↓

Reflects parallel to principal axis

Images Formed by Concave Mirrors

1. Object at Infinity

Image:

  • Real

  • Inverted

  • Smaller

  • At F

2. Object at F

Image:

  • No image

  • Rays become parallel

3. Object at C

Image:

  • Real

  • Inverted

  • Same size

  • At C

4. Object Beyond C

Image:

  • Real

  • Inverted

  • Smaller

  • Between C and F

5. Object Between C and F

Image:

  • Real

  • Inverted

  • Larger

  • Beyond C

6. Object Between F and Mirror

Image:

  • Virtual

  • Upright

  • Larger

  • Behind mirror

This is the only case where a concave mirror forms a virtual image.

Concave Mirror Summary Table

Object Position

Image Type

Orientation

Size

Image Position

Infinity

Real

Inverted

Smaller

At F

At F

None

—

—

—

At C

Real

Inverted

Same

At C

Beyond C

Real

Inverted

Smaller

Between C and F

Between C & F

Real

Inverted

Larger

Beyond C

Between F & Mirror

Virtual

Upright

Larger

Behind Mirror

Uses of Concave Mirrors

Flashlights

  • Bulb at focal point

  • Produces parallel beam

Reflecting Telescopes

  • Newtonian telescope

  • Hubble primary mirror

Solar Cookers

  • Focus sunlight to cook food

Makeup/Shaving Mirrors

  • Produce enlarged upright image when face is inside focal length

3. Convex Mirrors (Diverging Mirrors)

Definition

Reflect light from the outward curved surface.

Also called:

  • Diverging mirrors

Characteristics

Unlike concave mirrors:

  • Focal point is behind mirror.

  • Center of curvature is behind mirror.

  • Focal length is considered negative.

Reflection Rules

Rule 1

Ray parallel to principal axis

↓

Appears to come from focal point after reflection.

Rule 2

Ray directed toward focal point

↓

Reflects parallel to principal axis.

Images Formed by Convex Mirrors

Always:

  • Virtual

  • Upright

  • Smaller (reduced)

  • Behind mirror

No exceptions.

As object moves farther away:

  • Image becomes smaller.

  • Image also moves farther behind the mirror.

Uses of Convex Mirrors

Vehicle Side Mirrors

  • Wider field of view

  • Objects appear smaller and farther away

("Objects in mirror are closer than they appear.")

Security Mirrors

  • Stores

  • Parking garages

  • Hallways

Bus Mirrors

  • Eliminate blind spots

Secondary Mirror in Hubble Telescope

  • Increases effective focal length

  • Produces greater magnification

Key Formulas

Plane Mirror

di = do

Concave Mirror

f = R/2

Mirror Comparison

Property

Plane

Concave

Convex

Shape

Flat

Curves inward

Curves outward

Reflection

Same angle

Converges rays

Diverges rays

Focal Point

None

In front

Behind

Image Type

Virtual

Real or Virtual

Virtual

Orientation

Upright

Upright or Inverted

Upright

Size

Same

Smaller, Same, or Larger

Smaller

Main Uses

Household mirrors

Flashlights, telescopes, makeup mirrors

Car mirrors, security mirrors

Important Facts to Memorize

  • Plane mirrors always produce virtual, upright, same-size images.

  • Concave mirrors are converging mirrors.

  • Convex mirrors are diverging mirrors.

  • Only concave mirrors can produce real images.

  • Only concave mirrors can produce both real and virtual images.

  • Convex mirrors always produce virtual, upright, reduced images.

  • The focal length of a concave mirror is half the radius of curvature.

  • A real image can be projected onto a screen.

  • A virtual image cannot be projected onto a screen.