PHYSICS

 LENSES AND THE EYE 

REVISION NOTES 

GCSE 9-1 SEPERATE SCIENCE 

There are two types of lens: 

Converging Lens ( Convex) : these are lens which cause parallel light rays to converge to focus , this point is known as the principal focus.

Diverging Lens

 ( Concave): these are lens which cause parallel light rays to diverge away. The point where the light rays appear to come from is known as the principal focus.

REAL VS VIRTUAL IMAGE :

A Real image is when you can see a clear image of th object on a screen , this because all the light rays meet or focus onto the screen . example : an image from a projector.

- always inverted

A Virtual image is formed where the rays appear to come from but the image can only be seen from the lens . example: the image or object seen through a magnifying glass.

- always erect

virtual image - example a mirror

real image - example cinema screen

THE EYE:

Light enters the eye through a tough transparent layer called the cornea. This protects the eye and helps to focus light onto the retina. The retina is a layer of light- sensitive cells inside the back of the eye. The amount of light entering the eye is controlled by the iris, which adjusts the size of pupil- the circular opening at the centre of the iris. In bright light, the iris becomes larger and makes the pupil narrow so less light enters the eye. In dim light the iris becomes smaller and makes the pupil wider so more light enters the eye. The eye lens is converging lens that focuses light to give a sharp image on the retina . Although the image on the retina is inverted , the brain interprets it so that you see it the right way up.

other parts of the eye:

Ciliary muscles : are attached to lens by suspensory ligaments , these muscles change the thickness of the eye lens.

Optic Nerve : carries nerve impulses from the retina to the brain.

key points

- the ciliary muscles alters the thickness of the lens, hence making the eye focus on objects.

- the lens becomes thinner if object is at focus at far distance.

- the lens becomes thicker if object is at focus at closer distance.

- the normal eye has a range of vision from 25 cm to infinity.

CAMERA & THE EYE: