Unit 4: Optics
4.1 Light Energy and Its Sources (sec 10.1 pg 289-293)
1) What is light? You cannot touch it or taste it but you can see it. It does not have any mass but you
can feel its warmth.
2) We define light as a form of energy that can be detected by the eye.
3) Light can reach your eyes in two different ways (Fig.1 p.289):
o Directly – this is when it travels directly from the source to your eye.
o Indirectly (diffused or reflected) – This is when the light hits some object before reflecting up
into your eye.
4) There are two kinds of objects in the world.
o Luminous – objects that emit light.
o Non-luminous – objects that do not emit light but reflect it.
5) Luminous objects must transform some kind of ‘input’ energy into light energy. There are different
ways of doing this.
o Incandescence – This is when electrical energy is transformed into heat and light energy (see
Fig.2 & 3 p.290). In a light bulb, a electricity passing through a fine metal wire makes it so hot
it glows brightly.
o Phosphorescence – This is when a material absorbs light energy and then re-emits the energy
as light afterward (e.g. glow-in-the-dark materials). The colour and length of time depends on
the light used (Fig. 4 p.291)
o Electric discharge – This is when electricity passes through a gas, and causes the gas to emit
light (e.g. lightning, neon gas lights – see Fig.5 p.291)
o Fluorescence – This is a combination of phosphorescence and electric discharge. The electric
discharge causes the gas in the tube to give of UV light. This is absorbed by a powder layer and
it phosphoresces to give off white light (Fig.6 p.292).
o Chemiluminescence – This is when a chemical reaction gives off light (e.g. a glow stick; see
Fig.7 p.292).
o Bioluminescence – This is when a biological creature gives off light, using a chemical reaction
similar to chemiluminescence e.g. angler fish; Fig.8 p.293).
4.2 Getting in Light’s Way (sec 10.3 pg 296-298)
1) Imagine if we could only make building out of wood (without glass). The inside would be really dark.
The fact that some materials allow light to pass through is really useful. Transparency is the
measure of how much light can pass through a material (Fig 1-3 p.296).
o Objects that allow light to pass through easily are called transparent. (glass, plastic wrap, etc)
o Objects that allow some light through but that also reflect some light are called translucent.
They usually do not allow a clear image through. (skin, paper, frosted glass, etc)
o Objects that absorb or reflect all of the light are called opaque. (wood, milk, etc)
2) When light strikes any material some of the light is absorbed, heating it up, while some is reflected
(If the material is transparent or translucent some passes through). The reflected light is what
allows us to see objects.
3) There are three properties that are related to the amount of light that is absorbed or reflected by a
material.
o Colour – Dark colours absorb more light than light colours
o Shininess – Dull material like wood absorbs more energy than shiny materials like aluminum
o Texture- Rough material absorbs more than smooth material.
4.3 The Visible Spectrum (sec 10.4 pg 299 – 300)
1) You know that light tends to travel in straight lines. It reflects, absorbs, and is transmitted, but none
of this explains the phenomena of colour.
2) You have all seen a rainbow before (Fig.1 p.299). The rainbow, which displays the visible spectrum,
gives an important clue about how coloured light works. The visible spectrum is divided into 6 main
colours (Red, Orange, Yellow, Green, Blue, Violet – acronym = ROYGBV). They always appear in the
same order in a rainbow.
3) Scientists used to think that colour was added to white light when it reflected off of an object. Then
in 1666 Isaac Newton passed white light through a prism and it split into the rainbow. White light is
made up of all of the colours.
4) Some people thought that the colours came from inside of the prism so he used another prism to
put the colours back together to make white light again (Fig.2 & 3 p.299).
5) So why does a green object appear green then? This has to do with how light behaves when it hits
an object. It can be reflected, absorbed or transmitted. A green object reflects the green portion of
light (it reflects the green part of the visible spectrum, and absorbs the rest of the visible spectrum).
If an object reflects all colours in the spectrum we see it as white. If it absorbs all colours in the
spectrum then it appears black.
4.4 The Electromagnetic Spectrum (Sec 10.5 pg 301-305)
1) Light is a form of radiant energy that you can see. But think back to the rainbow. The colours were
always in the same order: Red, Orange, Yellow, Green, Blue, Violet. There are more colours than we
can see. You have likely heard of ultraviolet (this is a form of radiant energy that it beyond violet)
and infrared (this is a form of radiant energy that is beyond red). The entire range of all radiant
energy is called the Electromagnetic Spectrum.
2) Light is special because it can travel through the vacuum of space. It travels very fast 300 000 km/s
(to the sun in 8 minutes) it takes about 2.6 seconds for it to travel to the moon and back.
3) The other interesting thing about light is has wave-like properties. In the same way that each wave
on a beach is different, each colour of light has a different wave with different properties.
4) Waves have some important properties.
o Crest – the top of the wave
o Trough – the bottom of the wave
o Wavelength – the distance between 2 crests (or one full cycle).
o Amplitude – The height of the wave above the resting position.
o Frequency (hertz) – the number of waves that pass a point in a second.
5) We use the different wavelengths of the electromagnetic spectrum for different uses: radio
(1kmlong wavelengths), microwaves (about 1cm long), visible (500 000 waves per cm), x-rays (100
million waves into 1 cm). See Table 1 p.304.
6) The higher frequency waves have higher energy levels. That is way ultraviolet light burns your skin
faster than regular light.
4.5 Reflecting Light Off Surfaces (sec 11.2 pg 316 – 318)
1) If you have ever seen a ball bounce off of grass, you know it is somewhat unpredictable because the
ground is uneven. The same thing happens to light when it bounces off of uneven surfaces.
2) Light that reflects off of an uneven surface is called diffuse light and the reflection is called diffuse
reflection.
This kind of light is easier on the eyes, and causes less strain. Rooms are usually designed to
promote diffuse light to make it easier on your eyes.
3) Light that reflects off of a smooth surface is called specular light (direct light) and the reflection is
called specular reflection.
This is the kind of light that bounces off of a mirror, smooth metal or calm water. You can see a
detailed image of the object that reflects on the surface.
4) During Investigation 11.1, we all reflected light off of the mirror we all got the same results. When
scientists get consistent results they create “laws” to describe what is happening.
They have created laws of reflection.
I. The angle of incidence (incoming) equals the angle of reflection (outgoing).
II. The incident ray, normal, and reflected ray all lie in the same plane.
These laws can be used to explain why the eye sees an image in a plane mirror (see Fig.3 p.317)
4.6 Describing Images (sec 11.3 pg 319 – 320)
1) When we use the projector in class, an image appears on the screen. When you look at these
images, an image appears on the retina of your eye. An image is the likeness of an object. Any
device that produces an image is called an optical device.
2) There are two kinds of images.
o Virtual images are those you can see, but only by looking through, or at, an optical device. (The
image in a plane mirror is virtual)
o Real images are those you can see AND that you can “touch”. They appear in front of concave
mirrors or through lenses like a magnifying lens.
3) Images have some characteristics that describe them (Table 1 p.320).
Characteristic
Size
Attitude
Location
Type











Possible Descriptions
Smaller than the object
Larger than the object
Same size as the object
Upright (right side up)
Inverted (upside down)
Behind the mirror
In front of the mirror
Same side of a lens as the object
Opposite side of a lens as the object
Virtual
Real
4.7 Using Curved mirrors (sec 11.6 pg 326)
1) If you pay attention you will start to notice that you see curved mirrors all of the time in everyday
life. There are 3 key words that we use to describe a curved mirror (Fig.1 & 2 p.326):
o Principal axis – The line that goes through the center of the mirror and the principal focus.
o Principal focus – The position where reflected parallel light rays come together.
o Focal length – the distance from the mirror to the principal focus to the middle of the mirror.
2) A concave mirror focuses parallel light rays through the principal focus. Concave mirrors are used in:
o Telescopes (Fig.3 p.326)
o Cosmetic mirrors (Fig.4 p.327)
3) A convex mirror spreads parallel light rays away from the principal focus. They are used in:
o Surveillance
o Bus mirrors (Fig.6 p.327)
4) Table 1 (p.328) provides a summary of image characteristics in different mirrors:
4.8 Refracting Light in Lenses (sec 11.8 pg 331 – 333)
1) Your eyes rely on refraction. The lens in your eye bends the light that hits it to form an image on the
back of your retina. A lens is a curved transparent material that causes light to refract as it passes
through. It can either cause light to converge or diverge.
2) Light bends when it enters a new medium, but why? When light travels from the air into a new
medium it slows down. This slowing down causes the light to bend off course a little. This is similar
to when you ride your bike from the pavement to the dirt (Fig 2 p.331). The front tire slows down a
bit and you bend slightly off course. However, you can turn straight again but light cannot.
3) Refraction of light occurs when light travels from one medium into another medium; light rays will
bend as they either speed up or slow down in the new medium.
4) Lenses are either convex or concave.
o Convex – thicker in the middle (bumps out). The parallel light rays that go through converge at a
focal point (Fig.3 p.332).
o Concave – thinner in the middle (caves in). The parallel light that goes through diverges as if
coming from a focal point (Fig.4 p.333).
5) You can combine different types lenses (convex/concave; thicker/thinner; etc.) to form complex
optical devices like binoculars, microscopes, etc.
4.9 The Human Eye and a Camera (sec 12.1 page 341-345)
1) The human eye is an amazingly complex optical device that allows us to see images both near and
far, in bright and dim light.
o Cornea – The eye is covered in a tough white layer called the sclera. 6 muscles attach to it to
turn it almost any direction. The front of the sclera is a clear part called the cornea. It allows
light to enter into the eye. (camera = 1st lens)
o Lens – The cornea refracts incoming light initially, but the lens focuses light into a fine image on
the retina. The lens is convex and can change its focal length. (camera = 2 nd lens)
o Iris – This is the colored part of the eye that surrounds the pupil. It can dilate to allow more light
into the eye in the dark. It can also contract to allow very little light in. (camera = diaphragm)
o Ciliary muscles – These muscles pull on the lens to make it thinner (far sight) or slacken to make
it thicker (near sight).
o Retina – This is the light-sensitive layer in the back of the eye. Made of light receptors called
rods and cones. There about 120 million rods which see the world in gray scale (black and
white). There are about 6 million cones which can see colour (red, green, blue). The image
appears upside down on the retina.
o Optic Nerve – The rods and cones send electric signals to the brain that it interprets as an
image. The part of the retina that has the optic nerve behind it has almost no rods or cones
above it. This is called the blind spot.
The Human Eye and Camera Handout (also see Fig.7 p.345)
4.10 Vision and Vision Problems (sec 12.2 pg 346 – 349)
1) If you have ever noticed something seems blurry or hard to read it is likely because you have some
sort of vision defect. A vision defect is when the eye does not function exactly like a normal eye.
2) Normal vision has traditionally been measured using something called the Snellen Eye Chart (Fig.1
p.346). It compares your vision at a 20 foot distance against a normal person. You have probably
heard of 20/20 vision. The top number is the distance that you see and the bottom number is the
normal distance. 20/20 means you see at 20 feet, what ‘normal’ people see at 20 feet.
3) A normal eye produces a sharp image on the retina. There are 2 main kinds of refractive vision
problems:
o Myopia (near-sightedness) occurs as a result of a slightly elongated eyeball, and the image is
produced in front of the retina (Fig.3 p.347). It affects approximately 1/3 of the population.
o Hyperopia (far-sightedness) occurs as a result of a slightly shortened (squashed) eyeball, and the
image is produced behind the retina (Fig.4 p.347). Affects approximately 1/4 of the population.
4) Another rarer type of refractive vision problem is called astigmatism. This occurs when the cornea is
not rounded like a basketball, but is oval like the side of a football. This causes the eye to focus the
light into 2 images on the retina, and images at all distances seem blurry.
5) As people age, it is relatively common for their lens and cornea harden, and lose the ability to focus
light as well. This is called presbyopia.
6) So….how is vision corrected?
o Myopia is corrected with a concave lens (Fig.5 p.349).
o Hyperopia is corrected with a convex lens (Fig.6 p.349).
o Astigmatism requires a specially shaped lens to correct for the defect.
o Presbyopia requires both a concave and a convex lens for either near or far sight.
o Most can be done with contacts.
o Some can be corrected by laser surgery (you should read ‘Tech CONNECT’ on pg.350).
4.11 Colour vision (sec 12.4 pg 353-355)
1) There are only three types of cones in the retina of your eyes – one is sensitive to red light, one to
blue, and a third is sensitive to green light (Fig.1 p.353). Our eyes combine signals from these cones
to construct all other colors!
2) The process of adding colors together to make other colors is called additive color mixing.
3) Primary light colors are the three colors our cones detect (red, blue, green). When our cones
combine two primary colors, secondary light colors are produced (cyan, yellow, magenta).
See Fig.2 p.354 in your text!
4) Complementary light colors are any two colors of light that make white light when added together
– for example, adding magenta and green produces white light (Fig.2 p.354).
5) Check out Fig.4 p.355 to see if you’re color blind – affects about 8% of male population. Some of the
cones in color blind people don’t respond to certain incoming colors of light.
4.12 A telescope for every wave (sec 12.6 pg 358-360)
1) A lens bends the light that we are able to see. Just as we can build a telescope that magnifies the
light we can see, we can build a telescope that can magnify the light that we cannot see. Some kinds
of light that we cannot see include microwaves, x-rays, radio waves, etc.
2) A telescope that bends x-rays, radio waves, etc uses the same rules of optics that visible light does.
3) The first telescopes were made in ancient Greece but the first modern telescopes were made in the
early 1600s.
4) To make really big telescopes this way would require huge lens that would crumble under their own
weight. To get around this a telescope that did not rely on a lens had to be built. This led to the first
reflecting telescope being made. It uses a concave mirror instead to reflect the light.
5) A radio telescope is like a reflecting telescope in design. A radio telescope uses a large radio
reflector to focus all of the radio noise onto a small detector placed at the focal point. Different
objects in space emit radio waves and by studying them we can learn more about them.
6) X-ray telescopes and gamma-ray telescopes have to be designed with special reflectors that the rays
cannot travel through. Even then images are often blurry.
7) Many telescopes are placed away from cities to avoid the light and often where the air is dry to
avoid twinkling. Some telescope have even been put into orbit to see more clearly.