SPI Physics · Light and Optics

Light and Reflection

Light is a transverse electromagnetic wave that travels in straight lines, and when it bounces off a mirror the angle of reflection always equals the angle of incidence.

All Light and Optics notes

Why this matters

Six questions in the bank, all EASY: what kind of wave light is, what a bundle of rays is called, whether the angle of reflection always equals the angle of incidence, and twice which mirror gives the widest view. Each is one fact, so a short table and the two laws of reflection cover the page.

Concept 1 of 3: What light is

Light needs nothing to travel through: it reaches us from the Sun across empty space. It is an electromagnetic wave whose vibrations are at right angles to its direction of travel, and it moves in straight lines, which is why objects cast sharp shadows.

Definition

Light is an electromagnetic wave, so it needs no medium and can travel through a vacuum.
It is a transverse wave: its electric and magnetic fields vibrate at right angles to the direction it travels.
A ray is a single straight line along which light travels; a beam is a bundle of rays.

Fact	Value
Kind of wave	Electromagnetic
Transverse or longitudinal	Transverse Sound is the longitudinal one; light is not.
Needs a medium	No, it travels through a vacuum
Speed in a vacuum	$3 \times 10^8$ m/s
Path in one medium	Straight lines, so opaque objects cast shadows
Ray	One straight line along which light travels
Beam	A bundle of rays

Radio waves, X-rays and light are all electromagnetic waves; they differ only in wavelength.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 1 · Light and Optics · Light and Reflection

Light is a:

Calling light a radio wave

Radio waves and light belong to the same electromagnetic family, but light is not a radio wave. The family name is 'electromagnetic waves'.

Calling light longitudinal

Light is transverse: its fields vibrate across its direction of travel. A longitudinal wave, like sound, vibrates along it.

Drill 2 more on what light is

Concept 2 of 3: The laws of reflection

A ray of light bounces off a mirror like a ball off a wall: it leaves at the same slope it came in. The slope is always measured from the normal, the line at right angles to the mirror.

Definition

First law: the incident ray, the reflected ray and the normal all lie in one plane.
Second law: the angle of reflection equals the angle of incidence, $\angle r = \angle i$ .
Both angles are measured from the normal, never from the mirror's surface.
The laws hold always: for plane and curved mirrors alike, using the normal at the point where the ray strikes.
A plane mirror gives an image that is virtual, erect, the same size, as far behind the mirror as the object is in front, and laterally inverted (left and right swapped).

Law of reflection

\angle r = \angle i \qquad \angle i = 90^\circ - (\text{angle between the ray and the mirror})

Worked example

A ray strikes a plane mirror making an angle of

35^\circ

with the mirror's surface. Find the angle of reflection and the angle between the incident and reflected rays.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Light and Optics · Light and Reflection

The angle of reflection is equal to the angle of incidence:

Measuring from the mirror

A ray at

30^\circ

to the mirror's surface has an angle of incidence of

60^\circ

, because the angle is taken from the normal.

Thinking the law fails for curved mirrors

The angle of reflection equals the angle of incidence for every mirror, every time. On a curved mirror, use the normal at the point where the ray strikes.

Concept 3 of 3: Plane, concave and convex mirrors and their uses

A mirror's shape decides its image, and its image decides its job. A convex mirror bulges out, so it squeezes a wide scene into a small upright picture; a concave mirror curves in, so it can gather light or magnify a face held close.

Definition

A concave mirror curves inward; a convex mirror bulges outward.
For a spherical mirror the focal length is half the radius of curvature: $f = \dfrac{R}{2}$ .
A convex mirror always gives a virtual, erect, diminished image and the widest field of view.

Mirror	Image	Uses
Plane	Virtual, erect, same size	Looking glass, periscope
Concave, object far away	Real and inverted	Torch and headlight reflectors, solar cookers
Concave, object very close	Virtual, erect, enlarged	Shaving and make-up mirrors, a dentist's mirror
Convex	Always virtual, erect, diminished	Rear-view and side-view mirrors of vehicles, shop security mirrors Widest field of view of the three.

A torch or headlight puts its bulb at the focus of a concave mirror to send out a parallel beam.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 3 · Light and Optics · Light and Reflection

The mirror which is used for side view in vehicles is ____ :-

Choosing a plane mirror for the rear view

A plane mirror shows only an area as big as itself. A convex mirror shows a much wider area, which is why vehicles use it.

Thinking a concave mirror always gives a real image

With the object between the focus and the mirror, a concave mirror gives a virtual, erect, enlarged image. That is how a shaving mirror works.

Drill 1 more on plane, concave and convex mirrors and their uses

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The laws of reflection
Law of reflection
$\angle r = \angle i \qquad \angle i = 90^\circ - (\text{angle between the ray and the mirror})$

Reference tables (2)

What light is7 rows

Fact	Value
Kind of wave	Electromagnetic
Transverse or longitudinal	Transverse Sound is the longitudinal one; light is not.
Needs a medium	No, it travels through a vacuum
Speed in a vacuum	$3 \times 10^8$ m/s
Path in one medium	Straight lines, so opaque objects cast shadows
Ray	One straight line along which light travels
Beam	A bundle of rays

Radio waves, X-rays and light are all electromagnetic waves; they differ only in wavelength.

Plane, concave and convex mirrors and their uses4 rows

Mirror	Image	Uses
Plane	Virtual, erect, same size	Looking glass, periscope
Concave, object far away	Real and inverted	Torch and headlight reflectors, solar cookers
Concave, object very close	Virtual, erect, enlarged	Shaving and make-up mirrors, a dentist's mirror
Convex	Always virtual, erect, diminished	Rear-view and side-view mirrors of vehicles, shop security mirrors Widest field of view of the three.

A torch or headlight puts its bulb at the focus of a concave mirror to send out a parallel beam.

Watch out for (6)

Calling light a radio wave
→ What light is
Calling light longitudinal
→ What light is
Measuring from the mirror
→ The laws of reflection
Thinking the law fails for curved mirrors
→ The laws of reflection
Choosing a plane mirror for the rear view
→ Plane, concave and convex mirrors and their uses
Thinking a concave mirror always gives a real image
→ Plane, concave and convex mirrors and their uses

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