Year 9 Science - light

Electromagnetic spectrum

shorter waves = higher frequency, higher energy

longer waves = lower frequency, lower energy

all travel at the same speed

Radio waves

Used to broadcast radio and television

Lowest frequency

Microwaves

Used in cooking, radar, telephone and other signals.

Infrared

Transmits heat from sun, fires, radiators.

Visible light

Makes things able to be seen

Ultraviolet

Absorbed by the skin, used in fluorescent tubes

X - rays

Used to see inside of bodies and objects

Gamma Rays

Used in medicine for killing cancer cells

highest frequency

Colours of the visible spectrum

Red - longest wavelength

Orange

Yellow

Green

Blue

Indigo

Violet - shortest wavelength

Law of Reflection

The angle of incidence = the angle of reflection

Regular reflection

Plane (flat) mirrors reflect light in a regular way (at the same angle).

Diffuse reflection

Most objects reflect irregularly because their surface is NOT perfectly smooth. Light is reflected in all directions.

Wave diagram

Plane diagram

Convex mirror diagram

Concave mirror diagram

How does light pass through different transparent media?

Light refracts in a different direction. This occurs because light travels at different speeds for different media.

Refraction diagram

Refraction

• As light passes through different transparent medium, light bends.

• This is because as light travels from one medium to another it speeds up or slows down

Rules of refraction

When light enters a more dense medium at an angle it is bent towards the normal (because the light slows down).

When light enters a less dense medium at an angle it is bent away from the normal (because the light speeds up).

Refraction in water

Converging lens

Light refracts inwards - biconvex lens

Diverging lens

Light refracts outwards - biconcave lens

Biconvex lens diagram

Biconcave lens diagram

Real images

Will be flipped and smaller. This is because the image is beyond the focal point.

Virtual images

An image between the lens and the focal point will be upright and enlarged.

Why do objects appear white

when they reflect all of the colours of the spectrum

Why do objects appear black

Objects appear black when all of the colours of the spectrum in light are absorbed

Why does an object appear green

An object appears green because it reflects green and absorbs all other colours

Eye diagram

Cornea function

Clear window, refracts light, helps to focus the light

Pupil function

Hole in the iris, which allows light to enter the eye

Iris function

Coloured, changes size to control the amount of light entering the eye

Lens function

Refracts light to focus image on the retina. Focal length adjusted by ciliary muscles

Retina function

Light sensitive cells (rods & cones) which change light energy to nerve impulses.

Optical nerve function

Takes messages from the retina to the brain

Sclera function

The white, tough outer layer that protects the eye.

Liquid humour function

Watery at the front, jelly at the back, helps to keep eye’s shape and provide medium for light to travel through.

What parts of the eye will a light ray pass through to strike the retina?

To reach the retina, light rays first enter through the cornea, then pass through the pupil (its size regulated by the iris), and finally through the lens which focuses the light onto the retina

How does brightness change the eye in light or darkness?

In bright light, the eye's pupil constricts (becomes smaller) to limit the amount of light entering, while in darkness, the pupil dilates (becomes larger) to allow more light in. This is a reflex action controlled by the iris, which contains muscles that adjust the pupil's size.

Rods

Rods are sensitive to light and dark, shape and movement. They only have one type of light-sensitive pigment.

We use mainly rods when trying to see in a dark room – hence why we don’t really see colour in the dark!

Cones

• Cones aren’t as sensitive to light, but they are each sensitive to one of the three primary colours.

• Cones are used for finer details and seeing colour

• People who are colour-blind will either be missing one type of cone or it is weaker.