light. reflection of light when light is reflected off of a mirror it forms an image. mirror...

LIGHT

Reflection of light• When light is reflected off of a mirror it forms an image.

Mirror

Incident ray Reflected ray

Normal

Reflection of light• The law of reflection states that:

the angle of incidence = the angle of reflection

i = r

Mirror

Incident ray Reflected ray

Normal

i r

Properties of reflection• An image is formed when light is reflected.• The image is the same size as the object and the same

distance behind the mirror as the object is in front.

Properties of reflection• The image is laterally inverted (left is switched with right)• The image is virtual (it’s not real)

Summary• The normal at a point on a mirror is perpendicular to the

mirror.• The image of an object in a plane mirror is virtual, the

same size, laterally inverted, and the same distance from the mirror.

• For a light ray reflected by a mirror:

the angle of incidence = the angle of reflection• Homework: pg. 117 #1,2

REFRACTION OF LIGHT

Refraction of light• Refraction occurs when light pass from one transparent

medium (substance) to another. For example it happens when light passes from air to glass.

Glass

Air

Changes• Refraction occurs because the speed of the light changes

in different mediums. • When it travels from a fast medium to a slow medium, the

light bends towards the normal.

Glass

Air

Changes• When light travels from a slow medium to a fast medium,

the light bends away from the normal.

Glass

Air

Apparent Depth• Why do things look different when they are under water?

Apparent Depth• Because light waves are refracted when they change

medium, they make object appear differently. They create an image of an object that is not exactly where the object is.

Astronomical Refraction• Light traveling from stars is refracted when it crosses into

the Earth’s atmosphere. A vacuum is a fast medium and air is the slow medium, so the light bends towards the normal.

Summary• Refraction is the change of direction of light when it

travels from one transparent medium to another.• Refraction is towards the normal when it goes from a fast

to slow medium.• Refraction is away from the normal when it goes from a

slow to fast medium.• Classwork: pg. 119 #1,2

REFRACTION INDEX

Refraction• The change of direction of a light ray depends on which

substance to light is traveling in.• Light well bend differently when traveling through water

than it would through glass.

The refractive index• The refractive index of a substance is n.

n = the speed of light in air

the speed of light in the substance

Example• The speed of light is air is 300,000 km/s. The speed of

light in glass is 200,000 km/s. Calculate the refractive index.

n = the speed of light in air

the speed of light in the substance

n = 300,000 km/s

200,000 km/s

n = 1.50

The refractive index• It is a measure of how much a substance changes the

direction of light. • The refractive index for water is 1.33 and glass is 1.50.

Therefore, glass will change light more than water.

The refractive index• How could we find the refractive index if we do not know

the speed? A man named Snell figured out how in 1618.• Snell’s law:

n = sin i

sin r

n – the refractive indexi – the angle of the incident ray from the normalr – the angle of the refracted ray from the normal

The refractive index• Remember that the angle of incidence and angle of

refraction are measured from the normal.

Glass

Airi

r

n = sin i

sin r

Example• A light ray travels through air into water. The angle of

incidence is 50o and the angle of refraction is 35o. Calculate the refractive index.

n = sin i

sin r

n = sin 50

sin 35

n = 1.34

Summary• The refractive index of a substance is

• For a light ray traveling from air into a transparent substance

• Homework: pg. 121 #1,2

n = sin i

sin r

n = the speed of light in air

the speed of light in the substance

TOTAL INTERNAL REFLECTION

Refraction of Light• When light crosses a boundary from a slow medium to a

fast medium, the light moves away from the normal.

Glass

Air

Refraction of Light• The larger the angle of incidence the larger the refracted

angle.

Glass

Air

Refraction of Light• There is an angle where the angle of refraction will be 90o.• This is called the critical angle.

Glass

Air

c

Refraction of Light• If the angle of incidence is larger than the critical angle,

there will be no refracted ray, only a reflected ray.• This is called total internal reflection.

Glass

Air

Optical Fibres• Optical fibres use total internal reflection to transfer

information. The light travels down the wire but does not escape because it is reflected from the boundary.

Summary• When a light ray travels from a slow medium into a fast

medium there is an angle where the refraction is 90o. This is called the critical angle.

• When the incident angle is less than the critical angle, most of the light is refracted.

• When the incident angle is at the critical angle the light is refracted along the surface.

• When the incident angle is greater than the critical angle, all of the light is reflected. This is called total internal reflection.

• Homework pg. 123 #1,2

THE CONVERGING LENS

Converging lens• A converging lens changes the direction of light passing

through it so it is directed to one point called the focal point.

• The following objects use converging lens:

Converging Lens• The following is when rays travel parallel to the principal

axis of the lens.

Ray Diagrams• When completing ray diagrams there are three important rays to

consider.

1. The ray traveling parallel to the principal axis is refracted through the focal point.

2. The ray traveling through the center of the lens is not refracted.

3. The ray traveling through the focal point is refracted parallel to the principal axis

Applications of Lens• The camera lens is used to make a smaller, inverted, real

image on a film.• The distance between the lens and the sensor must be

changed depending on the distance of the object.

Applications of Lens• A projector is used to give a larger, inverted, real image.

The Magnifying Glass• The magnifying glass is used to create an image that is

virtual, upright and magnified.

Summary• The principal focus of a converging lens is the point where

parallel rays directed straight at the lens converge.• The focal length of a converging lens is the distance from

the lens to the principal focus of the lens.• A real image is formed by a converging lens if the object is

further away than the principal focus of the lens.• Copy the table at the bottom of page 127 into your

notebook.• Homework pg. 125 #1,2 and pg. 127 #1,2

ELECTROMAGNETIC WAVES

Review• The index of refraction is:

• The wave equation is:

v = fλ

n = the speed of light in air

the speed of light in the substance

Definition: Spectrum• A spectrum is a way of comparing objects by ordering

them. The following is an “economic spectrum”:

Poor Rich

• We can use spectra to compare things instead of using charts or graphs.

Middle

The Light Spectrum• Light from a lamp or the Sun is called white light. It

contains all of the colours on the visible spectrum.• Light that is only one colour is called monochromatic.• The following is a picture of all the colours:

“Roy G Biv”

Different colours• Different colours have a different wavelength (λ) and a

different frequency (f).• Red has a longer wavelength and a lower frequency.• Violet has a shorter wavelength and a higher frequency.

Dispersion• The different colours of light travel at different speeds in

glass.• If white light is passed through a glass prism, the colours

will be refracted differently.• This is called dispersion.

The Electromagnetic Spectrum• The visible light spectrum (what we can see) is a small

part of a larger spectrum called the electromagnetic spectrum.

• Electromagnetic waves (EM waves) are disturbances that transfer energy from one place to another.

• The following are some examples of EM waves: radio waves, microwaves, infra-red, ultraviolet, x-rays

The Electromagnetic Spectrum

Properties of EM Waves• EM waves can have different frequencies and different

wavelengths.• All EM waves have a speed of 300,000,000 m/s in a

vacuum.

Radio Waves• Wavelength ≈ 1km• Uses: communications (radio, TV, mobile phones)

Microwaves• Wavelength ≈ 1m• Uses: communications (satellites), heating food

Infra-red Radiation• Wavelength ≈ 1mm• Uses: cookers and heaters, cameras, optical fibre, TV remote

controls.

Visible Light• Wavelength ≈ 1μm (micrometer 0.000001m)• Uses: optical instruments, photography, seeing

Ultraviolet Radiation• Wavelength ≈ 1nm (nanometer 0.000000001m)• Uses: UV sun tan lamps, security markers, sterilizing

equipment

X-rays and Gamma Radiation• Wavelength ≈ 1pm (picometer 0.000000000001m)• Uses: X-radiography, destroying cancer cells

Problems

1) Which types of electromagnetic waves are transverse and which are longitudinal?

All transverse.

2) How could x-rays give us images of bones?

They have short wavelengths so they are able to pass through flesh. However, they are reflected by bones.

3) Which types of electromagnetic waves could harm humans?

X-rays and gamma rays can kills cells or cause cancerous cells. UV rays can damage your eyes or skin. Radio waves and microwaves can have a heating effect on the body.

Problems

Summary• Dispersion is the splitting of white light into the colours of

the spectrum. Light of one colour is called monochromatic.

• All electromagnetic waves travel at 300,000 km/s through space.

• The parts of the electromagnetic spectrum starting from the largest wavelength are: radio waves, microwaves, infra-red radiation, light, ultraviolet radiation, X-rays, gamma rays.

• Glue electromagnetic spectrum in your notebook.• Classwork – pg. 129 #1,2 and pg. 131 #1,2