Defects of vision and corrective lenses

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.

A human eye has a far point of 6.0 m.

State the name of this defect of vision.

Calculate the power of the correcting lens required for this eye.

An eye with astigmatism requires the following prescription:

−4.00 –0.75 ×30

Which row identifies the meaning of each number?

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

Which would be a correct lens prescription for a person with hypermetropia and astigmatism?

Tick () one box.

A student views an object O and cannot see it clearly unaided. The student is diagnosed with myopia and is prescribed a suitable correcting lens. Using the correcting lens, an intermediate image is formed that can be viewed clearly by the student.

The student states that she can see O more clearly because the intermediate image is enlarged.

Discuss the validity of the student’s statement. In your answer you should:

• describe how myopia affects vision

• draw a labelled ray diagram of the correcting lens, showing how the intermediate image of O is formed

• explain how the correcting lens enables the student to see clearly.

Space for diagram

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.

Explain why the unaided defective eye cannot form a clearly focused image of the object placed 24 cm from the eye.

An object is placed 24 cm from the spectacle lens.

Calculate the distance of the image formed from the spectacle lens. Give your answer to a suitable number of significant figures.

What is the name for the position where the image is formed by the spectacle lens? Tick () the correct answer.

Draw a ray diagram to show how this spectacle lens forms an image of the object placed 24 cm from the spectacle lens. On your diagram clearly label the object, image and a principal focus of the lens. Your diagram does not have to be drawn to scale.