Defects of Vision (AQA A Level Physics): Revision Note

Defects of vision

Short-sightedness

• Short-sightedness is also called myopia

• It happens when the lens is more curved than normal

  • So the cornea and lens are too powerful

  • Myopia can also occur if the eyeball is too long

• This means light rays are refracted too much so the focal point of the image falls in front of the retina (rather than on it)

  • So distant objects appear blurry as people with short-sightedness cannot focus on distant objects

Treatment of short-sightedness

• Short-sightedness can be corrected using contact lenses or glasses with a diverging (concave) lens

  • The lens causes the light rays to diverge before reaching the eye

  • So when they are focused by the eye the image now forms on the retina

• Remember from Converging & Diverging Lenses that:

  • Diverging lenses have a negative focal length and a negative power

• A short-sighted person can remove their glasses if they wish to read or view something close-up because it is easy for them to focus on objects close to the eye

The effect of a diverging lens on a short-sighted eye

 The far point and short-sightedness

• Remember that a "normal-sighted" person has a far point at infinity

  • A short-sighted person, however, has a far point that is less than infinity

• So choosing the correct diverging lens depends on the position of the far point for that person

  • The lens must have its principal focus in the same position as the eye's faulty far point

Astigmatism & prescriptions

Correcting for an astigmatism

• Astigmatism can be corrected with a cylindrical lens that adds power to rays of light coming from one plane but not the plane perpendicular to it

• There are many different combinations of astigmatism

• A few examples are:

  • One plane is short-sighted and the other is long-sighted

  • One plane has correct vision and the other is long-sighted

  • Both planes are short-sighted by different amounts

• The cylindrical power (CYL) of the astigmatism is related to the difference in power between the two points of focus of the two planes of light

  • The greater the difference then the greater the astigmatism 

  • The smaller the difference then the smaller the astigmatism

  • It is measured in Dioptres (D)

The cylindrical power of the astigmatism

• The astigmatism can be present at any angle on the curvature of the eye known as the angle of astigmatism 

  • This is measured in the same way as if a protractor is placed against the front of the eye

  • It is an angle measured from the left-hand side of the protractor between 0° and 180°

Prescription for astigmatism

• The prescription for the cylindrical lens to correct astigmatism will state the following three pieces of information for each eye:

  1. The sphere (SPH) - The focal power needed to correct for either long or short-sightedness.

  2. The cylinder (CYL) - The cylindrical power needed to correct for the astigmatism.

  3. The axis angle - This is the angle needed by the lens to correct for astigmatism.

• In the exam, calculations of the power of a lens needed in a prescription may be asked

• This is calculated for each eye by subtracting CYL from SPH

  • For example, in the image for the left eye this is

  • In the right eye this is

• Remember that one or both of the SPH and CYL values can be negative depending on the vision defect

  • So, you may need to subtract a negative power

  • e.g.

• It is important to note that the angle of astigmatism and the axis angle for the prescription are perpendicular to each other

  • A curved area on the cornea at a specific angle causes astigmatism 

  • This is corrected by an equally curved area in the correcting lens placed perpendicular to it