Physics of the eye

Figure 1 shows a simplified diagram of an eye that has no sight defects. The eye is viewing a distant point object.

Complete, on Figure 1, the paths of the two rays to the back of the eye.

Accommodation is the ability of the eye to change its optical power.

Which part of the eye enables accommodation?

Tick (✓) one box.

The accommodation amplitude A, in dioptres, is given by

where

d₁ is the distance, in m, from the eye to its near point d₂ is the distance, in m, from the eye to its far point.

Figure 2 shows how the value of A varies with age for a typical person.

An eye of a typical 30-year-old person has a far point at infinity. The focal length of the eye when viewing an object at its far point is 23 mm.

Calculate the focal length of the eye when viewing an object at its near point.

For a typical person, the far point remains at infinity as the person’s age increases. Some studies suggest that hypermetropia increases with age.

Explain how Figure 2 supports this suggestion.

Electrophoretic screens are used in handheld electronic devices.

The screen contains individual squares known as pixels. Pixels can be changed independently from light to dark to create the shapes of letters and numbers. An external light source is needed in order to read the screen.

Figure 1 shows a letter formed by three electrophoretic screens that have different pixel line densities. Pixel line density is the number of pixels along a 1.0 cm length of the screen.

A particular screen is designed so that two dark pixels separated by one light pixel cannot be resolved as separate images by the eye when viewed from a distance of 0.50 m.

Determine, in pixels per cm, the minimum pixel line density required for this screen.

typical diameter of cones in a human eye at the fovea = 1.5 μm typical length of the human eye = 21 mm

On a different electrophoretic screen, two dark pixels separated by one light pixel can just be resolved at a particular distance when the external light source is bright.

Explain why these pixels cannot be resolved at the same distance when the intensity of the external light source is reduced.

Figure 2 shows the spectral response of the three different types of cone in a human eye.

The eye is illuminated by light of wavelength 603 nm.

Show that the response of a red-sensitive cone is approximately double the response of a green-sensitive cone.

Other types of screen use blue, green and red pixels to produce coloured images.

Table 1 shows the wavelength of the light emitted by each pixel when it is turned on.

On one screen, the blue pixels are turned off. When the green pixels and the red pixels are turned on, they emit light with the same intensity. A human eye that has the spectral response shown in Figure 2 responds to this light.

Determine, in nm, the single wavelength of light that will produce the same response in the same human eye as the light emitted from the green and red pixels.

An eye condition is corrected using a +4.0D lens.

Which eye condition could be corrected by using this lens? Tick () one box.

Calculate the magnification produced by the +4.0D lens when viewing an object 75 cm from this lens.

Figure 1 shows a diagram of an eye.

State the name and primary optical function of X, Y and Z.

Primary optical function of X

Primary optical function of Y

Primary optical function of Z

State and explain two differences between the perceived image of a brightly coloured object in bright light and the perceived image of the same object when viewed in very dark conditions.

In your answer you should refer to the visual receptors in the eye.

According to some legends, in the 17th century a pirate with two healthy eyes covered one eye with a patch to keep the eye in the dark. The patch was removed when going from bright conditions outside to the very dark conditions below decks in an enemy ship.

It was necessary for the pirate to put the patch on about 45 minutes before going into the very dark conditions inside the ship.

What is the name of the process which occurs when the pirate’s eye is covered by the patch? Tick () one box.

Discuss why it was necessary to wear the eye patch for 45 minutes before entering the ship.

Car drivers must be able to

• read a speedometer from a distance of 50 cm

• read a number plate from a distance of 20.5 m.

A driver has an unaided far point of 55 cm and an unaided near point of 25 cm.

Identify the driver’s eye defect.

Figure 1 shows the position of a number plate at a distance of 20.5 m in front of the driver’s unaided eye.

Figure 2 shows the same situation and the position of a corrective lens.

Complete both ray diagrams to show how and where the image of the number plate is formed in each case. Add a suitable lens to Figure 2.

An optician considers the use of three different lenses, A, B and C, for use by the driver when driving.

Power of A = ─2.18D Power of B = ─1.77D Power of C = +1.95D

Deduce which lens is suitable. Support your answer with calculations.

The fovea in a typical human eye consists of cones which have an average diameter

An eye looks directly at two point sources of light which are 12 mm apart at a distance of 61 m from the centre of the eye lens. The fovea is at the centre of the retina a distance of 21 mm behind the centre of the eye lens.

Deduce whether the eye would be able to resolve the two images formed at the fovea.

Three types of cone are present at the fovea.

On Figure 2 sketch and clearly label three curves to show how the relative light absorption of each type of cone varies with wavelength.