Updated: May 2015
Course: SNC 2D1
Unit: Light and Optics

Lesson 3: Title: REflection

Apparatus needed: heat lamp and socket

Preliminaries:
Review incandescence, fluorescence.
You should know what transparent, translucent, and opaque mean, and be able to give examples of each.

Lesson:

When light strikes matter three things can happen:

  1. absorbed - it is changed into heat energy. Example: sunlight on a dark car heats it up
  2. reflected - it bounces off the surface just like a ball bounces off the floor
    Example: a reflection in a mirror
  3. transmitted - it goes right through the object as if it wasn’t there.
    Example: light going through glass.

DEMO heat lamp - reflected off silver at back, transmitted through glass at front, absorbed by your skin.

You can also have combinations of these. For example, light can be reflected and transmitted by glass. If you look at a window from an angle, you may see a reflection off the glass even while you’re seeing things through the window. Light is absorbed and reflected from coloured objects. A red book reflects red but absorbs all other colours.

Q. Does white paper reflect or absorb light? If it reflects light, then how is it different from a mirror?

There are two types of reflection:

1. Specular reflection2. Diffuse reflection


 this is reflection off of a very smooth surface reflection off of something rough
(metal or a mirror)(paper)
 you can see an image in it it just looks white
 it looks shiny (normally silvery)
 all the reflected rays are still parallel reflected rays bounce off in all directions

The law of reflection:

  1. The angle of reflection is equal to the angle of incidence.
  2. The incident ray, normal, and reflected ray are all in the same plane.

Note that the angle is not measured from the mirror but from the normal!

This law holds true even for reflection off of curved mirrors.

This is why we use the angle from the normal. It doesn’t matter how curved or buckled a surface is, you can always find the Normal.

Q. Why is it hard to trim your own hair using a mirror? Because the mirror switches left and right.

Q. Why does the mirror switch left and right and not up and down?
Is it because our eyes are next to each other and not above each other?

Explain how to draw an image in a plane mirror.

… ok, this is a lot more complicated than I thought.
There’s no easy set of rules as there is for lenses or curved mirrors. Just look at some diagrams to try and get an understanding of it.

Q Have you ever been in a bar or a gym or dance studio where a whole wall is a mirror? It looks like there is a whole other room back there behind the mirror.

A virtual imageis an image located where you cannot put a screen to see the image on. For example, you can’t put a screen (or a piece of white paper) in the wall behind the mirror and see your face on the screen. There’s nothing there. It’s a virtual image.

Virtual images seem to be always in focus.

A real image is one which you can see on a screen. An overhead projector – same with a movie projector – produces a real image, because you see the image (of the object) on the screen.

Real images can only be in focus in one spot – if the screen is in the correct location.

Virtual images are normally (i) behind the mirror or (ii) in front of the lens.

Real images are the opposite.

Virtual images are produced by diverging rays.

 Real images are produced by converging rays.

 If there is enough time, demonstrate real images using convex lenses!

Homework:

Terminology (know these terms)

Define or describe the following terms: (see Martindale: p374, p444, index) (Nelson: p 329, 356)

raythe path taken by light (illustrated by an arrow)
normal
angle of incidence
angle of reflection
law of reflection

parallel raysrays that are parallel and never meet

diverging raysrays that get farther apart

converging raysray that are coming together

objectthe source of the light rays (may be reflected rays - e.g. a pen can be an object)

imagea picture of the object formed by some optical device

Next day: curved mirror mini lab / discovery activity.
explain rules for drawing ray diagrams in curved mirrors