Structure of the Eye (Oxford AQA IGCSE Physics)

Structure of the Eye

• Our eyes only detect a limited range of electromagnetic waves known as visible light

• Visible light is made up of wavelengths that form the colours of the spectrum:

  • Red

  • Orange

  • Yellow

  • Green

  • Blue

  • Indigo

  • Violet

The electromagnetic spectrum

The structure of the eye

The cornea

• Light rays enter the eye through the cornea

  • The cornea is a transparent layer that covers the front of the eye to protect it

  • It helps to focus light on the retina

• When passing through the cornea light rays are refracted (change direction)

The lens and the ciliary muscles

• The lens focuses light onto the retina

• Light is also refracted as it passes through the lens

• The lens is referred to as a variable focus lens because it can change its shape to focus on objects at different distances away

• The lens is controlled by the ciliary muscles which are attached to the lens by suspensory ligaments

• The muscles are attached to fibres which pull and stretch the lens

  • This changes the thickness of the lens

  • Which controls the eye's focal length

• When the muscles contract the lens becomes thicker and more powerful

  • This occurs when the eye is focusing on an object close by

• When the muscles relax the lens becomes thinner and less powerful

  • It occurs when the eye is focusing on an object close to the far point

The retina

• After passing through the lens the light is focused on the retina

• The retina is made up of light-sensitive cells around the inside of the eye

• The light rays are refracted through the cornea and lens so that they cross within the eye and arrive at the retina the opposite way around

  • Rays from the top of the object are now at the bottom of the retina and vice versa

• The brain interprets the image so it is the correct way up

The inverted image on the retina

The pupil

• The pupil is surrounded by muscles called the iris

• They control the amount of light entering the eye

  • When in a dark room the iris expands allowing the pupil to dilate (widen) so more light can enter the eye

  • When in bright sunlight the iris contracts causing the pupil to get smaller, so less light can enter the eye

Contracted and dilated pupils