Lecture 3 reflection of light

Q: What is reflection of light?

A: Reflection occurs when light travelling in one medium encounters a boundary and part of it returns into the original medium.

Q: Compare regular and diffuse reflection.

Regular (Specular)

Smooth surface

Parallel rays reflected in one direction

Diffuse

Rough surface

Rays scattered in many directions

Diffuse reflection allows us to see everyday objects.

Q: Define angle of incidence and angle of reflection.

Angle of incidence (i) = angle between incident ray and normal

Angle of reflection (i') = angle between reflected ray and normal

Q: State the two laws of reflection.

Incident ray, reflected ray and normal lie in the same plane.

Angle of incidence = angle of reflection.

i=i′

Q: How is an image formed by a plane mirror?

Reflected rays appear to diverge from a point behind the mirror, creating a virtual image.

Q: What are the characteristics of an image formed by a plane mirror?

Virtual

Upright

Same size as object

Laterally inverted

Same distance behind mirror as object is in front

Q: Why is a plane mirror used in an eye examination room?

A mirror creates a virtual image of the chart so a 6 m testing distance can be achieved in a shorter room. The chart letters must be printed in reverse.

Q: What are the two types of spherical mirror?

Concave mirror

Convex mirror

Q: Define the key parts of a spherical mirror.

A = Pole (vertex)

C = Centre of curvature

AC = Radius of curvature (r)

F = Principal focus

AF = Focal length (f)

AC = Principal axis

Q: What is the principal focus?

The point where reflected rays intersect, or appear to intersect, after reflection from a mirror.

Q: Compare the principal focus of concave and convex mirrors.

Concave mirror

Real focus

Rays actually meet

Convex mirror

Virtual focus

Rays only appear to meet

Q: What is the relationship between radius of curvature and focal length?

r=2f

Q: What are the four principal rays used for a concave mirror?

Parallel to axis → reflected through F

Through F → reflected parallel to axis

Through C → reflected back on itself

Through vertex → reflected with i = i'

Q: What are the principal rays for a convex mirror?

Parallel ray → appears to come from F

Towards C → reflected back on itself

Vertex ray follows law of reflection

Q: How does image formation change with object position in a concave mirror?

Beyond C

Real

Inverted

Between C and F

At C

Real

Inverted

At C

Between C and F

Real

Inverted

Beyond C

Between F and Mirror

Virtual

Upright

Behind mirror

Q: What image is always produced by a convex mirror?

Virtual

Upright

Smaller (diminished)

Q: What is the mirror formula?

f = focal length

l = object distance

l′ = image distance

Q: What is the magnification formula?

h = object height

h′ = image height

Q: What sign convention is used for mirrors?

Along axis

Same direction as light = positive

Opposite direction = negative

Heights

Above axis = positive

Below axis = negative

Mirror values

Concave mirror → f and r are negative

Convex mirror → f and r are positive

Q: Explain reflection and image formation by mirrors.

Reflection occurs when light returns to its original medium after striking a boundary.

Reflection may be specular (regular) or diffuse.

Laws of reflection:

Incident ray, reflected ray and normal lie in one plane.

Angle of incidence equals angle of reflection.

Plane mirrors form images that are:

Virtual

Upright

Same size

Laterally inverted

Same distance behind mirror as object is in front.

Spherical mirrors may be concave or convex.

Key terms:

Pole (A)

Centre of curvature (C)

Radius (r)

Principal focus (F)

Focal length (f)

Relationship:

• Concave mirrors can form real or virtual images depending on object position.

• Convex mirrors always form virtual, upright and diminished images.

• Mirror formula: magnification

• Sign convention:

  • With light = +

  • Against light = -

  • Above axis = +

  • Below axis = -

  • Concave: negative f and r

  • Convex: positive f and r