Optics Test - Grade 10

3 Properties of Light

1. Light travels at a very high speed (3 × 10^8)

2. Light travels in straight lines

3. Light is an electromagnetic wave - does not require a medium to traval

Visable Light

Visible light are any electromagnetic waves that the human eye can detect. The visible spectrum ranges from 700 to 400 nanometers.

The order from least to most energy can be seen as red (700nm) to violet (400nm) waves

Measuring Light

Wavelength is the physical distance from the top of one wave to the top of the next wave and is measured in nanometers

A nanometer is a billionth of a millimeter

The shorter the wavelength, the more energy it possesses and therefore the more dangerous it can be

Frequency is the number of waves per second

The higher the frequency, the shorter the wavelength, the more energy it possesses and therefore the more dangerous it can be

Electromagnetic Waves: Radio Waves

• highest wavelength

• used to broadcast radio or television

Electromagnetic Waves: Mircrowaves

• comes after radio waves

• used in cooking, radar, telephone, and other signals

Electromagnetic Waves: Infrared Light

• come after microwaves

• transmits heat from sun, fires, radiators

Electromagnetic Waves: Visable Light

• comes after infrared light

• makes humans see

Electromagnetic Waves: UV light

• comes after visable light

• absorbed by skin, used in flourecent tubes

Electromagnetic Waves: X-ray

• come after uv lights

• used as medical screening

Electromagnetic Waves: Gamma rays

• come after x-rays

• used to kill cancer cells

How is light produced

When atoms absorb energy, their electrons become excited (have more energy) and move to a higher orbit

When electrons return to their normal position, this energy is released in the form of

Luminous vs Non Luminous

• Luminous objects are those that can produce light
  (e.g. the sun/stars)

• Non-luminous objects cannot produce light
  (e.g. the moon)

Natural vs Artificial Light

Natural light: light that is produced by nature

Artificial light: light that is produced by people

Incandescent Light

A substance that emits light because it has been heated to a high temperature

(e.g. metal)

Fluorescent Light

Light given off by a substance when exposed to electromagnetic radiation.

For example: Fluorescent Bulbs

Electricity sent through the bulb causes mercury vapours to give off ultraviolet (UV) radiation. This radiation causes the phosphor lining of the bulb to give off light.

These bulbs are 20% efficient at creating light, and last longer than incandescent bulbs.

Other examples include:

fluorescent dyes, animals (e.g. fish) and minerals

Fluorescent lights:

are 4 times more energy efficient produce less heat + use less electricity more expensive but last longer

Phosphorescent Light

Phosphorescence is the ability to store energy from a source of light and then emit it slowly over a long period.

Examples:

Glow in the dark toys, stickers, watch hands

Chemiluminescent Light

Light produced because of a chemical reaction

Little to no heat produced

Examples: glow-sticks and luminol - used in analyzing crime scenes (luminol)

Bioluminescent Light

A type of chemiluminescence

Living organisms produce light due to a chemical reaction between oxygen and luciferin

Triboluminescent Light

Light made from friction, by scratching, crushing or rubbing certain crystals

(e.g. crushing wintergreen candies with a hammer)

Caused by the energy released when chemical bonds are broken

Electric Discharge

Light produced by passing an electric current through a gas

(e.g. neon, air)

Electricity causes the gas to glow

(e.g. neon sign, lightning, plasma ball lights)

Sources of Invisible Light (1)

1. Ultraviolet Light

   • Causes substances to glow or fluoresce

   • Skin exposed to UV light turns brown because of increased melanin production

   • Too much exposure to radiation causes burns and can eventually cause skin cancer

   • Responsible for the formation of vitamin D in animals

Examples: the sun and other stars, UV lamps

Sources of Invisable Light (2)

2. Infrared Light

   • Thermal energy emitted by objects near room temperature

   • Light can be seen through camera lenses 

   • Night vision devices make use of this technology detecting people and animals because of heat radiation given off

Examples: remote control devices, used in wireless communication and military tracking devices such as night vision goggles

Light-Emitting Diode (LED)

LED’s are an electroluminescent light source made out of a material called a semiconductor.

Consume less energy

Geometric optics

Geometric optics can show us how light behaves when it strikes a mirror or a lens. Mirrors and lenses have been used for a variety of purposes that society has benefited from.

Mirrors

Most mirrors consist of two parts:

A sheet of glass

A thin layer of reflective silver (Ag) or aluminum (Al)

Plane Mirrors

A plane/flat mirror shows how predictable a path of light is when it hits and bounces off (reflects) a mirror.

Laws of Reflection

The incident ray, the reflected ray, and the normal always lie in the same plane

The angle of reflection = angle of incidence

Ray Diagrams

You can show where an image will form by drawing light rays that strike the mirror and reflect into your eyes.

1. The incident light rays are reflected from the object onto the reflective surface (mirror)

2. At the mirror, the angle of incidence = the angle of reflection

3. The eye receives the reflected rays and the brain projects the reflected rays behind the mirror. 

4. This results in your brain thinking there is a light source behind the mirror. This image is called a virtual image (not real).

Image Characteristics

All images in plane (flat) mirrors are:

Same size

Upright (not flipped)

Behind the mirror

Virtual (not real)

Concave Mirrors

If the reflective surface is the inner surface of the sphere/curved surface, the mirror is concave.

These are also called converging mirrors.

‘Converge’ means to come together (meet) at a common point.

Any light rays that are parallel to the principal axis will be reflected off the mirror through a single point.

The light that is reflected “converges”.

The FOCUS is on the SAME SIDE as the object.

Applications: car headlights, flashlights, makeup/shaving mirrors

Convex Mirrors

If the reflective surface is the outer surface of the sphere/curved surface, the mirror is convex.

These are also called diverging mirrors.

Diverge means to spread apart. 

Any light rays that are parallel to the principal axis are reflected away from the virtual focus, which is behind the mirror.

Diverging mirrors ALWAYS produce a smaller, upright, virtual image.

The FOCUS is BEHIND the mirror.

Examples: security mirrors, vehicle ‘side-view’ mirrors

Curved Mirror Terminology

The centre of curvature (C): The centre of the sphere where the surface has been used to make the mirror.

If you were to stand at the centre of curvature of a sphere and throw a ball, the ball would bounce back to you at the centre of curvature.

The principal axis: The line through the centre of curvature to the midpoint of the mirror

The vertex (V): The point where the principal axis meets the mirror

The focus or ‘focal point’ (F): The point at which light rays parallel to  the principal axis converge when they are reflected off a concave mirror (½ way between C & V)

Ray Rules (Converging Mirrors)

Ray Rules (Diverging Mirrors)

Refraction

When light moves from one medium to another, it’s speed changes! We can visualize this change as a ‘bending’ of the light rays.

Refraction: the directional change of light as it moves from one medium to another.

Bending Light

Depending on the medium, light will either slow down OR speed up. We visualize this as the ray of light bending either toward or away from the normal.

For example:

• Light bends towards the normal when the 2nd medium slows light down

• Light bends away from the normal when the 2nd medium speeds light up

Index of Refraction (n)

The Index of Refraction is a number that represents how much slower light passes through a medium, compared to the speed of light in air.

For example:

The index of refraction from air to acrylic is 1.49. This means that light moves 49% slower in acrylic than it does in air.

The EQUATION for Index of Refraction is:

c = speed of light in air (3.0 x 108m/s)

v = speed of light in 2nd medium

Visualizing Refraction

Critical Angle

Light bends away from the normal when it speeds up at the boundary of two media. (An example of this is when light travels from acrylic into air.) In this situation, the angle of refraction is always larger than the angle of incidence. In fact, the angle of refraction continues to increase as the angle of incidence increases. Eventually, the angle of refraction will become 90°. The angle of incidence at this point is called the critical angle. The critical angle is the angle of incidence that produces a refracted angle of 90°.

total internal reflection

If you increase the angle of incidence past the critical angle, the refracted ray will no longer exit the medium. Instead, it will reflect back into the medium. In other words, the refracted ray disappears; only a reflected ray is total internal reflection the situation visible. This phenomenon is called total internal reflection

When does total internal reflection occur

Total internal refl ection occurs when these two conditions are met:

1. Light is travelling more slowly in the first medium than in the second.

2. The angle of incidence is large enough that no refraction occurs in the second medium. Instead, the ray is reflected back into the first medium.

Fibre Optics

Fibre optics is a technology that uses light to transmit information along a glass cable. The light must not escape as it travels along the cable. To achieve this, the cable must have a small critical angle so that light entering it will have an angle of incidence greater than the critical angle. Substances that have a small critical angle include high-purity glass and special types of plastics, such as Lucite

apparent depth

the depth that an object appears to be at due to the refraction of light in a transparent medium

mirage

a virtual image that forms as a result of refraction and total internal reflection in Earth’s atmosphere

dispersion

the separation of white light into its constituent colours

Cornea

The outermost lens of the eye

Can heal itself from small scratches

Focuses (refracts) most of the light entering the eye

Pupil

The black spot you see when you look at someones eye

A hole in the iris that allows light into the eye.

Changes in size are controlled by involuntary muscles called the iris

Iris

Contains pigment that gives the eye its color.

Circular band of muscle surrounding the pupil

Controls the size of the pupil and therefore, the amount of light that enters the eye

In dim light: Iris dilates and pupil gets bigger to let in more light

In bright light: Iris contracts and pupil shrinks so that less light enters the eye

Lens

The lens focuses the light the pupil takes in to create clear images on the retina.

It is held in place by the ciliary muscles which controls its shape to focus the light and create clear images of objects that are positioned at various distances.

A convex lens collects light and directs it to a focal point

Your lens allows you to change your focus so that you can see an object clearly regardless of whether it is right in front of you or further out.

Sclera

The tough, opaque outer layer that protects the rest of the eye.

The sclera is commonly referred to as the "whites" of the eye.

Retina

The inner lining at the back of the eye that acts as a projection screen for the light entering the eye. The image formed is upside down but the brain interprets it as rightside up

The retina contains photoreceptors called rods and cones. These convert light into neural signals which are sent to the brain.

Rod and Cone Cells

Rod Cells: Detect shapes and movement

Cone Cells: Detect colours red, green or blue.

Aqueous & Vitreous Humour

Fluid filling the eye.

A.H. - watery substance that fills the front part of the eye.

V.H. - gel located in the back of the eye

Help to maintain the shape of the eye.

Optic nerve

Connects the eye to the brain (by transmitting neural signals) where the image is interpreted.

Humans have a blind spot where the optic nerve attaches to the retina.

You do not notice your blind spot because your brain fills in that spot with whatever colours are nearby in what you are looking at.

Eye

How the eye works

The cornea and lens BOTH refract the light entering the eye to focus it on the retina.

The retina initiates nerve impulses and sends the messages to the optic nerve.

The optic nerve sends the messages to the brain for interpretation.

cornea + lens → retina → optic nerve → brain

The lens is a converging lens, and can change shape depending on the size and location of the object. The relaxed normal eye lens focuses a distant object correctly on the retina.

The ciliary muscles make the lens shorter and thicker when an object is closer to change the location of the focal point, focusing the object properly on the retina.

The image does get formed upside down” on the retina, but the brain interprets the information in an upright position.

Converging Lens

Thickest at the middle and thinnest at the edge

Causes incident rays that are parallel to converge (or come together) through a single point (F) after refraction

The principal focus is on the opposite side of the lens as the incident rays

Diverging Lens

Thinnest at the middle and thickest at the edge

Causes incident rays that are parallel to diverge (or spread apart) after refraction so that it looks as if they have come from a virtual focus

The principal focus is on the same side of the lens as the incident rays

Lens Landmarks:

Ray Rules for Converging LENSES