Light and OpticsTopic 2: Reflection

Vocab!

• Transparent – light rays go through it

• Translucent – some light rays go through it, some get reflected or absorbed

• Opaque – light rays get reflected or absorbed

The Ray Model of Light

• The ray model of light is a way of using arrows, lines or rays to show how light acts.

• The ray model tells us that light travels in straight lines.

• Light rays do not bend around an object, they either hit the object OR they go right past it in a straight line.

How do shadows prove the ray

model?

• A shadow has sharp edges because the light does not bend around the object.

• A shadow is at least the same size (or bigger) than the object.

• A shadow's size changes in a predictable way:– the shadow gets bigger as the object moves towards

the light source

The Ray Model of Light

Reflection• Reflection is when light strikes a surface then bounces off

that surface

Law of Reflection

• The law of reflection states that the angle of

reflection will always equal the angle of

incidence for a ray of light that is reflecting off

of a surface

• In other words, when a ray of light strikes a

reflective surface such as a mirror, the light will

bounce back from that surface at the same angle

that it hit the surface.

Reflection diagram

Reflective surface (mirror)

This imaginary line is called the "normal" line = n

(90 degrees from mirror line)

n

Incident ray "I"Angle of incidence

“ i ”

i

Reflec

ted ra

y "R"

r

Angle of reflection

“ r ”

Reflection Diagram

Is the Law of Reflection Always

True?

Why can you see yourself in a mirror or a highly

polished surface such as a window but not see

yourself in the dirt?

A

“smooth”

piece of

paper

Is the Law of Reflection always

true? YES!!

Normal line is different for each ray.

Normal line is the same for each ray.

• The answer is yes! The only difference with a

rough surface is that the normal line will be

different for each ray of light.

• Therefore, despite the fact that each of the rays

How do we see a reflected image in a

mirror?

• Light rays coming from a light source reflect

off of an object and towards the mirror.

• These rays then travel from the mirror to your

eyes in a straight line.

• Since your brain thinks that the rays are

coming from the mirror, it looks like the object

is inside of the mirror!

Regular Reflection

• smooth surfaces allow incoming rays to bounce

off as parallel rays (regular reflection)

• the shinier and smoother the surface, the better the

reflection

Label the diagrams of types of

beams.

divergingdiverging parallelparallel convergingconverging

Curved Mirrors

Concave Mirrors Convex Mirrors

Concave Mirrors

• curve inwards

• reflect light inwards to a central point in front of the mirror.

• reflect light from a small area

• when you are close to the mirror: images are larger/closer (magnified)

• when you are far from the mirror: images are inverted and smaller/farther away

• Example: make-up mirrors.

What is happening?

Convex Mirrors

• curve outward

• reflect light outward away from the outer

edges of the mirror

• reflect light from a large area

• images are smaller/further away but never

inverted

• Example: security mirrors, car mirrors

What is happening?

Concave up

close.

(Not inverted)

(Magnified)

Convex.

(Not Inverted)

(Smaller/farther

away)

Concave from

far away.

(Inverted)

Review